Add cancellation to asynchronous security APIs.

18 files changed, 709 insertions, 410 deletions

diff --git a/src/core/lib/security/credentials/composite/composite_credentials.c b/src/core/lib/security/credentials/composite/composite_credentials.c
index 77d7b04627..09fd60a12c 100644
--- a/src/core/lib/security/credentials/composite/composite_credentials.c
+++ b/src/core/lib/security/credentials/composite/composite_credentials.c
@@ -32,88 +32,98 @@
 typedef struct {
   grpc_composite_call_credentials *composite_creds;
   size_t creds_index;
-  grpc_credentials_md_store *md_elems;
-  grpc_auth_metadata_context auth_md_context;
-  void *user_data;
   grpc_polling_entity *pollent;
-  grpc_credentials_metadata_cb cb;
+  grpc_auth_metadata_context auth_md_context;
+  grpc_credentials_mdelem_array *md_array;
+  grpc_closure *on_request_metadata;
+  grpc_closure internal_on_request_metadata;
 } grpc_composite_call_credentials_metadata_context;
 
 static void composite_call_destruct(grpc_exec_ctx *exec_ctx,
                                     grpc_call_credentials *creds) {
   grpc_composite_call_credentials *c = (grpc_composite_call_credentials *)creds;
-  size_t i;
-  for (i = 0; i < c->inner.num_creds; i++) {
+  for (size_t i = 0; i < c->inner.num_creds; i++) {
     grpc_call_credentials_unref(exec_ctx, c->inner.creds_array[i]);
   }
   gpr_free(c->inner.creds_array);
 }
 
-static void composite_call_md_context_destroy(
-    grpc_exec_ctx *exec_ctx,
-    grpc_composite_call_credentials_metadata_context *ctx) {
-  grpc_credentials_md_store_unref(exec_ctx, ctx->md_elems);
-  gpr_free(ctx);
-}
-
-static void composite_call_metadata_cb(grpc_exec_ctx *exec_ctx, void *user_data,
-                                       grpc_credentials_md *md_elems,
-                                       size_t num_md,
-                                       grpc_credentials_status status,
-                                       const char *error_details) {
+static void composite_call_metadata_cb(grpc_exec_ctx *exec_ctx, void *arg,
+                                       grpc_error *error) {
   grpc_composite_call_credentials_metadata_context *ctx =
-      (grpc_composite_call_credentials_metadata_context *)user_data;
-  if (status != GRPC_CREDENTIALS_OK) {
-    ctx->cb(exec_ctx, ctx->user_data, NULL, 0, status, error_details);
-    return;
-  }
-
-  /* Copy the metadata in the context. */
-  if (num_md > 0) {
-    size_t i;
-    for (i = 0; i < num_md; i++) {
-      grpc_credentials_md_store_add(ctx->md_elems, md_elems[i].key,
-                                    md_elems[i].value);
+      (grpc_composite_call_credentials_metadata_context *)arg;
+  if (error == GRPC_ERROR_NONE) {
+    /* See if we need to get some more metadata. */
+    if (ctx->creds_index < ctx->composite_creds->inner.num_creds) {
+      grpc_call_credentials *inner_creds =
+          ctx->composite_creds->inner.creds_array[ctx->creds_index++];
+      if (grpc_call_credentials_get_request_metadata(
+              exec_ctx, inner_creds, ctx->pollent, ctx->auth_md_context,
+              ctx->md_array, &ctx->internal_on_request_metadata, &error)) {
+        // Synchronous response, so call ourselves recursively.
+        composite_call_metadata_cb(exec_ctx, arg, error);
+        GRPC_ERROR_UNREF(error);
+      }
+      return;
     }
+    // We're done!
   }
-
-  /* See if we need to get some more metadata. */
-  if (ctx->creds_index < ctx->composite_creds->inner.num_creds) {
-    grpc_call_credentials *inner_creds =
-        ctx->composite_creds->inner.creds_array[ctx->creds_index++];
-    grpc_call_credentials_get_request_metadata(
-        exec_ctx, inner_creds, ctx->pollent, ctx->auth_md_context,
-        composite_call_metadata_cb, ctx);
-    return;
-  }
-
-  /* We're done!. */
-  ctx->cb(exec_ctx, ctx->user_data, ctx->md_elems->entries,
-          ctx->md_elems->num_entries, GRPC_CREDENTIALS_OK, NULL);
-  composite_call_md_context_destroy(exec_ctx, ctx);
+  GRPC_CLOSURE_SCHED(exec_ctx, ctx->on_request_metadata, GRPC_ERROR_REF(error));
+  gpr_free(ctx);
 }
 
-static void composite_call_get_request_metadata(
+static bool composite_call_get_request_metadata(
     grpc_exec_ctx *exec_ctx, grpc_call_credentials *creds,
     grpc_polling_entity *pollent, grpc_auth_metadata_context auth_md_context,
-    grpc_credentials_metadata_cb cb, void *user_data) {
+    grpc_credentials_mdelem_array *md_array, grpc_closure *on_request_metadata,
+    grpc_error **error) {
   grpc_composite_call_credentials *c = (grpc_composite_call_credentials *)creds;
   grpc_composite_call_credentials_metadata_context *ctx;
-
   ctx = gpr_zalloc(sizeof(grpc_composite_call_credentials_metadata_context));
-  ctx->auth_md_context = auth_md_context;
-  ctx->user_data = user_data;
-  ctx->cb = cb;
   ctx->composite_creds = c;
   ctx->pollent = pollent;
-  ctx->md_elems = grpc_credentials_md_store_create(c->inner.num_creds);
-  grpc_call_credentials_get_request_metadata(
-      exec_ctx, c->inner.creds_array[ctx->creds_index++], ctx->pollent,
-      auth_md_context, composite_call_metadata_cb, ctx);
+  ctx->auth_md_context = auth_md_context;
+  ctx->md_array = md_array;
+  ctx->on_request_metadata = on_request_metadata;
+  GRPC_CLOSURE_INIT(&ctx->internal_on_request_metadata,
+                    composite_call_metadata_cb, ctx, grpc_schedule_on_exec_ctx);
+  while (ctx->creds_index < ctx->composite_creds->inner.num_creds) {
+    grpc_call_credentials *inner_creds =
+        ctx->composite_creds->inner.creds_array[ctx->creds_index++];
+    if (grpc_call_credentials_get_request_metadata(
+            exec_ctx, inner_creds, ctx->pollent, ctx->auth_md_context,
+            ctx->md_array, &ctx->internal_on_request_metadata, error)) {
+      if (*error != GRPC_ERROR_NONE) break;
+    } else {
+      break;
+    }
+  }
+  // If we got through all creds synchronously or we got a synchronous
+  // error on one of them, return synchronously.
+  if (ctx->creds_index == ctx->composite_creds->inner.num_creds ||
+      *error != GRPC_ERROR_NONE) {
+    gpr_free(ctx);
+    return true;
+  }
+  // At least one inner cred is returning asynchronously, so we'll
+  // return asynchronously as well.
+  return false;
+}
+
+static void composite_call_cancel_get_request_metadata(
+    grpc_exec_ctx *exec_ctx, grpc_call_credentials *creds,
+    grpc_credentials_mdelem_array *md_array, grpc_error *error) {
+  grpc_composite_call_credentials *c = (grpc_composite_call_credentials *)creds;
+  for (size_t i = 0; i < c->inner.num_creds; ++i) {
+    grpc_call_credentials_cancel_get_request_metadata(
+        exec_ctx, c->inner.creds_array[i], md_array, GRPC_ERROR_REF(error));
+  }
+  GRPC_ERROR_UNREF(error);
 }
 
 static grpc_call_credentials_vtable composite_call_credentials_vtable = {
-    composite_call_destruct, composite_call_get_request_metadata};
+    composite_call_destruct, composite_call_get_request_metadata,
+    composite_call_cancel_get_request_metadata};
 
 static grpc_call_credentials_array get_creds_array(
     grpc_call_credentials **creds_addr) {
diff --git a/src/core/lib/security/credentials/credentials.c b/src/core/lib/security/credentials/credentials.c
index b1f1e82076..8a67c9865b 100644
--- a/src/core/lib/security/credentials/credentials.c
+++ b/src/core/lib/security/credentials/credentials.c
@@ -38,13 +38,10 @@
 /* -- Common. -- */
 
 grpc_credentials_metadata_request *grpc_credentials_metadata_request_create(
-    grpc_call_credentials *creds, grpc_credentials_metadata_cb cb,
-    void *user_data) {
+    grpc_call_credentials *creds) {
   grpc_credentials_metadata_request *r =
       gpr_zalloc(sizeof(grpc_credentials_metadata_request));
   r->creds = grpc_call_credentials_ref(creds);
-  r->cb = cb;
-  r->user_data = user_data;
   return r;
 }
 
@@ -104,18 +101,25 @@ void grpc_call_credentials_release(grpc_call_credentials *creds) {
   grpc_exec_ctx_finish(&exec_ctx);
 }
 
-void grpc_call_credentials_get_request_metadata(
+bool grpc_call_credentials_get_request_metadata(
     grpc_exec_ctx *exec_ctx, grpc_call_credentials *creds,
     grpc_polling_entity *pollent, grpc_auth_metadata_context context,
-    grpc_credentials_metadata_cb cb, void *user_data) {
+    grpc_credentials_mdelem_array *md_array, grpc_closure *on_request_metadata,
+    grpc_error **error) {
   if (creds == NULL || creds->vtable->get_request_metadata == NULL) {
-    if (cb != NULL) {
-      cb(exec_ctx, user_data, NULL, 0, GRPC_CREDENTIALS_OK, NULL);
-    }
+    return true;
+  }
+  return creds->vtable->get_request_metadata(
+      exec_ctx, creds, pollent, context, md_array, on_request_metadata, error);
+}
+
+void grpc_call_credentials_cancel_get_request_metadata(
+    grpc_exec_ctx *exec_ctx, grpc_call_credentials *creds,
+    grpc_credentials_mdelem_array *md_array, grpc_error *error) {
+  if (creds == NULL || creds->vtable->cancel_get_request_metadata == NULL) {
     return;
   }
-  creds->vtable->get_request_metadata(exec_ctx, creds, pollent, context, cb,
-                                      user_data);
+  creds->vtable->cancel_get_request_metadata(exec_ctx, creds, md_array, error);
 }
 
 grpc_security_status grpc_channel_credentials_create_security_connector(
diff --git a/src/core/lib/security/credentials/credentials.h b/src/core/lib/security/credentials/credentials.h
index c45c2e957c..04a54b0ca8 100644
--- a/src/core/lib/security/credentials/credentials.h
+++ b/src/core/lib/security/credentials/credentials.h
@@ -138,48 +138,39 @@ grpc_channel_credentials *grpc_channel_credentials_from_arg(
 grpc_channel_credentials *grpc_channel_credentials_find_in_args(
     const grpc_channel_args *args);
 
-/* --- grpc_credentials_md. --- */
+/* --- grpc_credentials_mdelem_array. --- */
 
 typedef struct {
-  grpc_slice key;
-  grpc_slice value;
-} grpc_credentials_md;
+  grpc_mdelem *md;
+  size_t size;
+} grpc_credentials_mdelem_array;
 
-typedef struct {
-  grpc_credentials_md *entries;
-  size_t num_entries;
-  size_t allocated;
-  gpr_refcount refcount;
-} grpc_credentials_md_store;
+/// Takes a new ref to \a md.
+void grpc_credentials_mdelem_array_add(grpc_credentials_mdelem_array *list,
+                                       grpc_mdelem md);
 
-grpc_credentials_md_store *grpc_credentials_md_store_create(
-    size_t initial_capacity);
+/// Appends all elements from \a src to \a dst, taking a new ref to each one.
+void grpc_credentials_mdelem_array_append(grpc_credentials_mdelem_array *dst,
+                                          grpc_credentials_mdelem_array *src);
 
-/* Will ref key and value. */
-void grpc_credentials_md_store_add(grpc_credentials_md_store *store,
-                                   grpc_slice key, grpc_slice value);
-void grpc_credentials_md_store_add_cstrings(grpc_credentials_md_store *store,
-                                            const char *key, const char *value);
-grpc_credentials_md_store *grpc_credentials_md_store_ref(
-    grpc_credentials_md_store *store);
-void grpc_credentials_md_store_unref(grpc_exec_ctx *exec_ctx,
-                                     grpc_credentials_md_store *store);
+void grpc_credentials_mdelem_array_destroy(grpc_exec_ctx *exec_ctx,
+                                           grpc_credentials_mdelem_array *list);
 
 /* --- grpc_call_credentials. --- */
 
-/* error_details must be NULL if status is GRPC_CREDENTIALS_OK. */
-typedef void (*grpc_credentials_metadata_cb)(
-    grpc_exec_ctx *exec_ctx, void *user_data, grpc_credentials_md *md_elems,
-    size_t num_md, grpc_credentials_status status, const char *error_details);
-
 typedef struct {
   void (*destruct)(grpc_exec_ctx *exec_ctx, grpc_call_credentials *c);
-  void (*get_request_metadata)(grpc_exec_ctx *exec_ctx,
+  bool (*get_request_metadata)(grpc_exec_ctx *exec_ctx,
                                grpc_call_credentials *c,
                                grpc_polling_entity *pollent,
                                grpc_auth_metadata_context context,
-                               grpc_credentials_metadata_cb cb,
-                               void *user_data);
+                               grpc_credentials_mdelem_array *md_array,
+                               grpc_closure *on_request_metadata,
+                               grpc_error **error);
+  void (*cancel_get_request_metadata)(grpc_exec_ctx *exec_ctx,
+                                      grpc_call_credentials *c,
+                                      grpc_credentials_mdelem_array *md_array,
+                                      grpc_error *error);
 } grpc_call_credentials_vtable;
 
 struct grpc_call_credentials {
@@ -191,15 +182,29 @@ struct grpc_call_credentials {
 grpc_call_credentials *grpc_call_credentials_ref(grpc_call_credentials *creds);
 void grpc_call_credentials_unref(grpc_exec_ctx *exec_ctx,
                                  grpc_call_credentials *creds);
-void grpc_call_credentials_get_request_metadata(
+
+/// Returns true if completed synchronously, in which case \a error will
+/// be set to indicate the result.  Otherwise, \a on_request_metadata will
+/// be invoked asynchronously when complete.  \a md_array will be populated
+/// with the resulting metadata once complete.
+bool grpc_call_credentials_get_request_metadata(
     grpc_exec_ctx *exec_ctx, grpc_call_credentials *creds,
     grpc_polling_entity *pollent, grpc_auth_metadata_context context,
-    grpc_credentials_metadata_cb cb, void *user_data);
+    grpc_credentials_mdelem_array *md_array, grpc_closure *on_request_metadata,
+    grpc_error **error);
+
+/// Cancels a pending asynchronous operation started by
+/// grpc_call_credentials_get_request_metadata() with the corresponding
+/// value of \a md_array.
+void grpc_call_credentials_cancel_get_request_metadata(
+    grpc_exec_ctx *exec_ctx, grpc_call_credentials *c,
+    grpc_credentials_mdelem_array *md_array, grpc_error *error);
 
 /* Metadata-only credentials with the specified key and value where
    asynchronicity can be simulated for testing. */
 grpc_call_credentials *grpc_md_only_test_credentials_create(
-    const char *md_key, const char *md_value, int is_async);
+    grpc_exec_ctx *exec_ctx, const char *md_key, const char *md_value,
+    bool is_async);
 
 /* --- grpc_server_credentials. --- */
 
@@ -238,14 +243,11 @@ grpc_server_credentials *grpc_find_server_credentials_in_args(
 
 typedef struct {
   grpc_call_credentials *creds;
-  grpc_credentials_metadata_cb cb;
   grpc_http_response response;
-  void *user_data;
 } grpc_credentials_metadata_request;
 
 grpc_credentials_metadata_request *grpc_credentials_metadata_request_create(
-    grpc_call_credentials *creds, grpc_credentials_metadata_cb cb,
-    void *user_data);
+    grpc_call_credentials *creds);
 
 void grpc_credentials_metadata_request_destroy(
     grpc_exec_ctx *exec_ctx, grpc_credentials_metadata_request *r);
diff --git a/src/core/lib/security/credentials/credentials_metadata.c b/src/core/lib/security/credentials/credentials_metadata.c
index fcfdd52d20..ccd39e610f 100644
--- a/src/core/lib/security/credentials/credentials_metadata.c
+++ b/src/core/lib/security/credentials/credentials_metadata.c
@@ -24,65 +24,36 @@
 
 #include "src/core/lib/slice/slice_internal.h"
 
-static void store_ensure_capacity(grpc_credentials_md_store *store) {
-  if (store->num_entries == store->allocated) {
-    store->allocated = (store->allocated == 0) ? 1 : store->allocated * 2;
-    store->entries = gpr_realloc(
-        store->entries, store->allocated * sizeof(grpc_credentials_md));
+static void mdelem_list_ensure_capacity(grpc_credentials_mdelem_array *list,
+                                        size_t additional_space_needed) {
+  size_t target_size = list->size + additional_space_needed;
+  // Find the next power of two greater than the target size (i.e.,
+  // whenever we add more space, we double what we already have).
+  size_t new_size = 2;
+  while (new_size < target_size) {
+    new_size *= 2;
   }
+  list->md = gpr_realloc(list->md, sizeof(grpc_mdelem) * new_size);
 }
 
-grpc_credentials_md_store *grpc_credentials_md_store_create(
-    size_t initial_capacity) {
-  grpc_credentials_md_store *store =
-      gpr_zalloc(sizeof(grpc_credentials_md_store));
-  if (initial_capacity > 0) {
-    store->entries = gpr_malloc(initial_capacity * sizeof(grpc_credentials_md));
-    store->allocated = initial_capacity;
-  }
-  gpr_ref_init(&store->refcount, 1);
-  return store;
-}
-
-void grpc_credentials_md_store_add(grpc_credentials_md_store *store,
-                                   grpc_slice key, grpc_slice value) {
-  if (store == NULL) return;
-  store_ensure_capacity(store);
-  store->entries[store->num_entries].key = grpc_slice_ref_internal(key);
-  store->entries[store->num_entries].value = grpc_slice_ref_internal(value);
-  store->num_entries++;
+void grpc_credentials_mdelem_array_add(grpc_credentials_mdelem_array *list,
+                                       grpc_mdelem md) {
+  mdelem_list_ensure_capacity(list, 1);
+  list->md[list->size++] = GRPC_MDELEM_REF(md);
 }
 
-void grpc_credentials_md_store_add_cstrings(grpc_credentials_md_store *store,
-                                            const char *key,
-                                            const char *value) {
-  if (store == NULL) return;
-  store_ensure_capacity(store);
-  store->entries[store->num_entries].key = grpc_slice_from_copied_string(key);
-  store->entries[store->num_entries].value =
-      grpc_slice_from_copied_string(value);
-  store->num_entries++;
-}
-
-grpc_credentials_md_store *grpc_credentials_md_store_ref(
-    grpc_credentials_md_store *store) {
-  if (store == NULL) return NULL;
-  gpr_ref(&store->refcount);
-  return store;
+void grpc_credentials_mdelem_array_append(grpc_credentials_mdelem_array *dst,
+                                          grpc_credentials_mdelem_array *src) {
+  mdelem_list_ensure_capacity(dst, src->size);
+  for (size_t i = 0; i < src->size; ++i) {
+    dst->md[dst->size++] = GRPC_MDELEM_REF(src->md[i]);
+  }
 }
 
-void grpc_credentials_md_store_unref(grpc_exec_ctx *exec_ctx,
-                                     grpc_credentials_md_store *store) {
-  if (store == NULL) return;
-  if (gpr_unref(&store->refcount)) {
-    if (store->entries != NULL) {
-      size_t i;
-      for (i = 0; i < store->num_entries; i++) {
-        grpc_slice_unref_internal(exec_ctx, store->entries[i].key);
-        grpc_slice_unref_internal(exec_ctx, store->entries[i].value);
-      }
-      gpr_free(store->entries);
-    }
-    gpr_free(store);
+void grpc_credentials_mdelem_array_destroy(
+    grpc_exec_ctx *exec_ctx, grpc_credentials_mdelem_array *list) {
+  for (size_t i = 0; i < list->size; ++i) {
+    GRPC_MDELEM_UNREF(exec_ctx, list->md[i]);
   }
+  gpr_free(list->md);
 }
diff --git a/src/core/lib/security/credentials/fake/fake_credentials.c b/src/core/lib/security/credentials/fake/fake_credentials.c
index 67e74f7b92..ac9017850f 100644
--- a/src/core/lib/security/credentials/fake/fake_credentials.c
+++ b/src/core/lib/security/credentials/fake/fake_credentials.c
@@ -98,49 +98,44 @@ const char *grpc_fake_transport_get_expected_targets(
 static void md_only_test_destruct(grpc_exec_ctx *exec_ctx,
                                   grpc_call_credentials *creds) {
   grpc_md_only_test_credentials *c = (grpc_md_only_test_credentials *)creds;
-  grpc_credentials_md_store_unref(exec_ctx, c->md_store);
+  GRPC_MDELEM_UNREF(exec_ctx, c->md);
 }
 
-static void on_simulated_token_fetch_done(grpc_exec_ctx *exec_ctx,
-                                          void *user_data, grpc_error *error) {
-  grpc_credentials_metadata_request *r =
-      (grpc_credentials_metadata_request *)user_data;
-  grpc_md_only_test_credentials *c = (grpc_md_only_test_credentials *)r->creds;
-  r->cb(exec_ctx, r->user_data, c->md_store->entries, c->md_store->num_entries,
-        GRPC_CREDENTIALS_OK, NULL);
-  grpc_credentials_metadata_request_destroy(exec_ctx, r);
-}
-
-static void md_only_test_get_request_metadata(
+static bool md_only_test_get_request_metadata(
     grpc_exec_ctx *exec_ctx, grpc_call_credentials *creds,
     grpc_polling_entity *pollent, grpc_auth_metadata_context context,
-    grpc_credentials_metadata_cb cb, void *user_data) {
+    grpc_credentials_mdelem_array *md_array, grpc_closure *on_request_metadata,
+    grpc_error **error) {
   grpc_md_only_test_credentials *c = (grpc_md_only_test_credentials *)creds;
-
+  grpc_credentials_mdelem_array_add(md_array, c->md);
   if (c->is_async) {
-    grpc_credentials_metadata_request *cb_arg =
-        grpc_credentials_metadata_request_create(creds, cb, user_data);
-    GRPC_CLOSURE_SCHED(exec_ctx,
-                       GRPC_CLOSURE_CREATE(on_simulated_token_fetch_done,
-                                           cb_arg, grpc_executor_scheduler),
-                       GRPC_ERROR_NONE);
-  } else {
-    cb(exec_ctx, user_data, c->md_store->entries, 1, GRPC_CREDENTIALS_OK, NULL);
+    GRPC_CLOSURE_SCHED(exec_ctx, on_request_metadata, GRPC_ERROR_NONE);
+    return false;
   }
+  return true;
+}
+
+static void md_only_test_cancel_get_request_metadata(
+    grpc_exec_ctx *exec_ctx, grpc_call_credentials *c,
+    grpc_credentials_mdelem_array *md_array, grpc_error *error) {
+  GRPC_ERROR_UNREF(error);
 }
 
 static grpc_call_credentials_vtable md_only_test_vtable = {
-    md_only_test_destruct, md_only_test_get_request_metadata};
+    md_only_test_destruct, md_only_test_get_request_metadata,
+    md_only_test_cancel_get_request_metadata};
 
 grpc_call_credentials *grpc_md_only_test_credentials_create(
-    const char *md_key, const char *md_value, int is_async) {
+    grpc_exec_ctx *exec_ctx, const char *md_key, const char *md_value,
+    bool is_async) {
   grpc_md_only_test_credentials *c =
       gpr_zalloc(sizeof(grpc_md_only_test_credentials));
   c->base.type = GRPC_CALL_CREDENTIALS_TYPE_OAUTH2;
   c->base.vtable = &md_only_test_vtable;
   gpr_ref_init(&c->base.refcount, 1);
-  c->md_store = grpc_credentials_md_store_create(1);
-  grpc_credentials_md_store_add_cstrings(c->md_store, md_key, md_value);
+  c->md =
+      grpc_mdelem_from_slices(exec_ctx, grpc_slice_from_copied_string(md_key),
+                              grpc_slice_from_copied_string(md_value));
   c->is_async = is_async;
   return &c->base;
 }
diff --git a/src/core/lib/security/credentials/fake/fake_credentials.h b/src/core/lib/security/credentials/fake/fake_credentials.h
index fae7a6a9c4..aa0f3b6e20 100644
--- a/src/core/lib/security/credentials/fake/fake_credentials.h
+++ b/src/core/lib/security/credentials/fake/fake_credentials.h
@@ -52,8 +52,8 @@ const char *grpc_fake_transport_get_expected_targets(
 
 typedef struct {
   grpc_call_credentials base;
-  grpc_credentials_md_store *md_store;
-  int is_async;
+  grpc_mdelem md;
+  bool is_async;
 } grpc_md_only_test_credentials;
 
 #endif /* GRPC_CORE_LIB_SECURITY_CREDENTIALS_FAKE_FAKE_CREDENTIALS_H */
diff --git a/src/core/lib/security/credentials/iam/iam_credentials.c b/src/core/lib/security/credentials/iam/iam_credentials.c
index 4b32c5a047..3de8319d98 100644
--- a/src/core/lib/security/credentials/iam/iam_credentials.c
+++ b/src/core/lib/security/credentials/iam/iam_credentials.c
@@ -30,26 +30,33 @@
 static void iam_destruct(grpc_exec_ctx *exec_ctx,
                          grpc_call_credentials *creds) {
   grpc_google_iam_credentials *c = (grpc_google_iam_credentials *)creds;
-  grpc_credentials_md_store_unref(exec_ctx, c->iam_md);
+  grpc_credentials_mdelem_array_destroy(exec_ctx, &c->md_array);
 }
 
-static void iam_get_request_metadata(grpc_exec_ctx *exec_ctx,
+static bool iam_get_request_metadata(grpc_exec_ctx *exec_ctx,
                                      grpc_call_credentials *creds,
                                      grpc_polling_entity *pollent,
                                      grpc_auth_metadata_context context,
-                                     grpc_credentials_metadata_cb cb,
-                                     void *user_data) {
+                                     grpc_credentials_mdelem_array *md_array,
+                                     grpc_closure *on_request_metadata,
+                                     grpc_error **error) {
   grpc_google_iam_credentials *c = (grpc_google_iam_credentials *)creds;
-  cb(exec_ctx, user_data, c->iam_md->entries, c->iam_md->num_entries,
-     GRPC_CREDENTIALS_OK, NULL);
+  grpc_credentials_mdelem_array_append(md_array, &c->md_array);
+  return true;
 }
 
-static grpc_call_credentials_vtable iam_vtable = {iam_destruct,
-                                                  iam_get_request_metadata};
+static void iam_cancel_get_request_metadata(
+    grpc_exec_ctx *exec_ctx, grpc_call_credentials *c,
+    grpc_credentials_mdelem_array *md_array, grpc_error *error) {
+  GRPC_ERROR_UNREF(error);
+}
+
+static grpc_call_credentials_vtable iam_vtable = {
+    iam_destruct, iam_get_request_metadata, iam_cancel_get_request_metadata};
 
 grpc_call_credentials *grpc_google_iam_credentials_create(
     const char *token, const char *authority_selector, void *reserved) {
-  grpc_google_iam_credentials *c;
+  grpc_exec_ctx exec_ctx = GRPC_EXEC_CTX_INIT;
   GRPC_API_TRACE(
       "grpc_iam_credentials_create(token=%s, authority_selector=%s, "
       "reserved=%p)",
@@ -57,14 +64,22 @@ grpc_call_credentials *grpc_google_iam_credentials_create(
   GPR_ASSERT(reserved == NULL);
   GPR_ASSERT(token != NULL);
   GPR_ASSERT(authority_selector != NULL);
-  c = gpr_zalloc(sizeof(grpc_google_iam_credentials));
+  grpc_google_iam_credentials *c = gpr_zalloc(sizeof(*c));
   c->base.type = GRPC_CALL_CREDENTIALS_TYPE_IAM;
   c->base.vtable = &iam_vtable;
   gpr_ref_init(&c->base.refcount, 1);
-  c->iam_md = grpc_credentials_md_store_create(2);
-  grpc_credentials_md_store_add_cstrings(
-      c->iam_md, GRPC_IAM_AUTHORIZATION_TOKEN_METADATA_KEY, token);
-  grpc_credentials_md_store_add_cstrings(
-      c->iam_md, GRPC_IAM_AUTHORITY_SELECTOR_METADATA_KEY, authority_selector);
+  grpc_mdelem md = grpc_mdelem_from_slices(
+      &exec_ctx,
+      grpc_slice_from_static_string(GRPC_IAM_AUTHORIZATION_TOKEN_METADATA_KEY),
+      grpc_slice_from_copied_string(token));
+  grpc_credentials_mdelem_array_add(&c->md_array, md);
+  GRPC_MDELEM_UNREF(&exec_ctx, md);
+  md = grpc_mdelem_from_slices(
+      &exec_ctx,
+      grpc_slice_from_static_string(GRPC_IAM_AUTHORITY_SELECTOR_METADATA_KEY),
+      grpc_slice_from_copied_string(authority_selector));
+  grpc_credentials_mdelem_array_add(&c->md_array, md);
+  GRPC_MDELEM_UNREF(&exec_ctx, md);
+  grpc_exec_ctx_finish(&exec_ctx);
   return &c->base;
 }
diff --git a/src/core/lib/security/credentials/iam/iam_credentials.h b/src/core/lib/security/credentials/iam/iam_credentials.h
index fff3c6d2ca..5e3cf65bad 100644
--- a/src/core/lib/security/credentials/iam/iam_credentials.h
+++ b/src/core/lib/security/credentials/iam/iam_credentials.h
@@ -23,7 +23,7 @@
 
 typedef struct {
   grpc_call_credentials base;
-  grpc_credentials_md_store *iam_md;
+  grpc_credentials_mdelem_array md_array;
 } grpc_google_iam_credentials;
 
 #endif /* GRPC_CORE_LIB_SECURITY_CREDENTIALS_IAM_IAM_CREDENTIALS_H */
diff --git a/src/core/lib/security/credentials/jwt/jwt_credentials.c b/src/core/lib/security/credentials/jwt/jwt_credentials.c
index 4357657def..02c82e99ba 100644
--- a/src/core/lib/security/credentials/jwt/jwt_credentials.c
+++ b/src/core/lib/security/credentials/jwt/jwt_credentials.c
@@ -29,10 +29,8 @@
 
 static void jwt_reset_cache(grpc_exec_ctx *exec_ctx,
                             grpc_service_account_jwt_access_credentials *c) {
-  if (c->cached.jwt_md != NULL) {
-    grpc_credentials_md_store_unref(exec_ctx, c->cached.jwt_md);
-    c->cached.jwt_md = NULL;
-  }
+  GRPC_MDELEM_UNREF(exec_ctx, c->cached.jwt_md);
+  c->cached.jwt_md = GRPC_MDNULL;
   if (c->cached.service_url != NULL) {
     gpr_free(c->cached.service_url);
     c->cached.service_url = NULL;
@@ -49,33 +47,34 @@ static void jwt_destruct(grpc_exec_ctx *exec_ctx,
   gpr_mu_destroy(&c->cache_mu);
 }
 
-static void jwt_get_request_metadata(grpc_exec_ctx *exec_ctx,
+static bool jwt_get_request_metadata(grpc_exec_ctx *exec_ctx,
                                      grpc_call_credentials *creds,
                                      grpc_polling_entity *pollent,
                                      grpc_auth_metadata_context context,
-                                     grpc_credentials_metadata_cb cb,
-                                     void *user_data) {
+                                     grpc_credentials_mdelem_array *md_array,
+                                     grpc_closure *on_request_metadata,
+                                     grpc_error **error) {
   grpc_service_account_jwt_access_credentials *c =
       (grpc_service_account_jwt_access_credentials *)creds;
   gpr_timespec refresh_threshold = gpr_time_from_seconds(
       GRPC_SECURE_TOKEN_REFRESH_THRESHOLD_SECS, GPR_TIMESPAN);
 
   /* See if we can return a cached jwt. */
-  grpc_credentials_md_store *jwt_md = NULL;
+  grpc_mdelem jwt_md = GRPC_MDNULL;
   {
     gpr_mu_lock(&c->cache_mu);
     if (c->cached.service_url != NULL &&
         strcmp(c->cached.service_url, context.service_url) == 0 &&
-        c->cached.jwt_md != NULL &&
+        !GRPC_MDISNULL(c->cached.jwt_md) &&
         (gpr_time_cmp(gpr_time_sub(c->cached.jwt_expiration,
                                    gpr_now(GPR_CLOCK_REALTIME)),
                       refresh_threshold) > 0)) {
-      jwt_md = grpc_credentials_md_store_ref(c->cached.jwt_md);
+      jwt_md = GRPC_MDELEM_REF(c->cached.jwt_md);
     }
     gpr_mu_unlock(&c->cache_mu);
   }
 
-  if (jwt_md == NULL) {
+  if (GRPC_MDISNULL(jwt_md)) {
     char *jwt = NULL;
     /* Generate a new jwt. */
     gpr_mu_lock(&c->cache_mu);
@@ -89,27 +88,33 @@ static void jwt_get_request_metadata(grpc_exec_ctx *exec_ctx,
       c->cached.jwt_expiration =
           gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), c->jwt_lifetime);
       c->cached.service_url = gpr_strdup(context.service_url);
-      c->cached.jwt_md = grpc_credentials_md_store_create(1);
-      grpc_credentials_md_store_add_cstrings(
-          c->cached.jwt_md, GRPC_AUTHORIZATION_METADATA_KEY, md_value);
+      c->cached.jwt_md = grpc_mdelem_from_slices(
+          exec_ctx,
+          grpc_slice_from_static_string(GRPC_AUTHORIZATION_METADATA_KEY),
+          grpc_slice_from_copied_string(md_value));
       gpr_free(md_value);
-      jwt_md = grpc_credentials_md_store_ref(c->cached.jwt_md);
+      jwt_md = GRPC_MDELEM_REF(c->cached.jwt_md);
     }
     gpr_mu_unlock(&c->cache_mu);
   }
 
-  if (jwt_md != NULL) {
-    cb(exec_ctx, user_data, jwt_md->entries, jwt_md->num_entries,
-       GRPC_CREDENTIALS_OK, NULL);
-    grpc_credentials_md_store_unref(exec_ctx, jwt_md);
+  if (!GRPC_MDISNULL(jwt_md)) {
+    grpc_credentials_mdelem_array_add(md_array, jwt_md);
+    GRPC_MDELEM_UNREF(exec_ctx, jwt_md);
   } else {
-    cb(exec_ctx, user_data, NULL, 0, GRPC_CREDENTIALS_ERROR,
-       "Could not generate JWT.");
+    *error = GRPC_ERROR_CREATE_FROM_STATIC_STRING("Could not generate JWT.");
   }
+  return true;
+}
+
+static void jwt_cancel_get_request_metadata(
+    grpc_exec_ctx *exec_ctx, grpc_call_credentials *c,
+    grpc_credentials_mdelem_array *md_array, grpc_error *error) {
+  GRPC_ERROR_UNREF(error);
 }
 
-static grpc_call_credentials_vtable jwt_vtable = {jwt_destruct,
-                                                  jwt_get_request_metadata};
+static grpc_call_credentials_vtable jwt_vtable = {
+    jwt_destruct, jwt_get_request_metadata, jwt_cancel_get_request_metadata};
 
 grpc_call_credentials *
 grpc_service_account_jwt_access_credentials_create_from_auth_json_key(
diff --git a/src/core/lib/security/credentials/jwt/jwt_credentials.h b/src/core/lib/security/credentials/jwt/jwt_credentials.h
index 6e461f1715..07f4022669 100644
--- a/src/core/lib/security/credentials/jwt/jwt_credentials.h
+++ b/src/core/lib/security/credentials/jwt/jwt_credentials.h
@@ -29,7 +29,7 @@ typedef struct {
   // the service_url for a more sophisticated one.
   gpr_mu cache_mu;
   struct {
-    grpc_credentials_md_store *jwt_md;
+    grpc_mdelem jwt_md;
     char *service_url;
     gpr_timespec jwt_expiration;
   } cached;
diff --git a/src/core/lib/security/credentials/oauth2/oauth2_credentials.c b/src/core/lib/security/credentials/oauth2/oauth2_credentials.c
index 9de561b310..ffa941bb9e 100644
--- a/src/core/lib/security/credentials/oauth2/oauth2_credentials.c
+++ b/src/core/lib/security/credentials/oauth2/oauth2_credentials.c
@@ -107,7 +107,7 @@ static void oauth2_token_fetcher_destruct(grpc_exec_ctx *exec_ctx,
                                           grpc_call_credentials *creds) {
   grpc_oauth2_token_fetcher_credentials *c =
       (grpc_oauth2_token_fetcher_credentials *)creds;
-  grpc_credentials_md_store_unref(exec_ctx, c->access_token_md);
+  GRPC_MDELEM_UNREF(exec_ctx, c->access_token_md);
   gpr_mu_destroy(&c->mu);
   grpc_httpcli_context_destroy(exec_ctx, &c->httpcli_context);
 }
@@ -115,7 +115,7 @@ static void oauth2_token_fetcher_destruct(grpc_exec_ctx *exec_ctx,
 grpc_credentials_status
 grpc_oauth2_token_fetcher_credentials_parse_server_response(
     grpc_exec_ctx *exec_ctx, const grpc_http_response *response,
-    grpc_credentials_md_store **token_md, gpr_timespec *token_lifetime) {
+    grpc_mdelem *token_md, gpr_timespec *token_lifetime) {
   char *null_terminated_body = NULL;
   char *new_access_token = NULL;
   grpc_credentials_status status = GRPC_CREDENTIALS_OK;
@@ -184,17 +184,18 @@ grpc_oauth2_token_fetcher_credentials_parse_server_response(
     token_lifetime->tv_sec = strtol(expires_in->value, NULL, 10);
     token_lifetime->tv_nsec = 0;
     token_lifetime->clock_type = GPR_TIMESPAN;
-    if (*token_md != NULL) grpc_credentials_md_store_unref(exec_ctx, *token_md);
-    *token_md = grpc_credentials_md_store_create(1);
-    grpc_credentials_md_store_add_cstrings(
-        *token_md, GRPC_AUTHORIZATION_METADATA_KEY, new_access_token);
+    if (!GRPC_MDISNULL(*token_md)) GRPC_MDELEM_UNREF(exec_ctx, *token_md);
+    *token_md = grpc_mdelem_from_slices(
+        exec_ctx,
+        grpc_slice_from_static_string(GRPC_AUTHORIZATION_METADATA_KEY),
+        grpc_slice_from_copied_string(new_access_token));
     status = GRPC_CREDENTIALS_OK;
   }
 
 end:
-  if (status != GRPC_CREDENTIALS_OK && (*token_md != NULL)) {
-    grpc_credentials_md_store_unref(exec_ctx, *token_md);
-    *token_md = NULL;
+  if (status != GRPC_CREDENTIALS_OK && !GRPC_MDISNULL(*token_md)) {
+    GRPC_MDELEM_UNREF(exec_ctx, *token_md);
+    *token_md = GRPC_MDNULL;
   }
   if (null_terminated_body != NULL) gpr_free(null_terminated_body);
   if (new_access_token != NULL) gpr_free(new_access_token);
@@ -205,63 +206,124 @@ end:
 static void on_oauth2_token_fetcher_http_response(grpc_exec_ctx *exec_ctx,
                                                   void *user_data,
                                                   grpc_error *error) {
+  GRPC_LOG_IF_ERROR("oauth_fetch", GRPC_ERROR_REF(error));
   grpc_credentials_metadata_request *r =
       (grpc_credentials_metadata_request *)user_data;
   grpc_oauth2_token_fetcher_credentials *c =
       (grpc_oauth2_token_fetcher_credentials *)r->creds;
+  grpc_mdelem access_token_md = GRPC_MDNULL;
   gpr_timespec token_lifetime;
-  grpc_credentials_status status;
-
-  GRPC_LOG_IF_ERROR("oauth_fetch", GRPC_ERROR_REF(error));
-
+  grpc_credentials_status status =
+      grpc_oauth2_token_fetcher_credentials_parse_server_response(
+          exec_ctx, &r->response, &access_token_md, &token_lifetime);
+  // Update cache and grab list of pending requests.
   gpr_mu_lock(&c->mu);
-  status = grpc_oauth2_token_fetcher_credentials_parse_server_response(
-      exec_ctx, &r->response, &c->access_token_md, &token_lifetime);
-  if (status == GRPC_CREDENTIALS_OK) {
-    c->token_expiration =
-        gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), token_lifetime);
-    r->cb(exec_ctx, r->user_data, c->access_token_md->entries,
-          c->access_token_md->num_entries, GRPC_CREDENTIALS_OK, NULL);
-  } else {
-    c->token_expiration = gpr_inf_past(GPR_CLOCK_REALTIME);
-    r->cb(exec_ctx, r->user_data, NULL, 0, status,
-          "Error occured when fetching oauth2 token.");
-  }
+  c->token_fetch_pending = false;
+  c->access_token_md = GRPC_MDELEM_REF(access_token_md);
+  c->token_expiration =
+      status == GRPC_CREDENTIALS_OK
+          ? gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), token_lifetime)
+          : gpr_inf_past(GPR_CLOCK_REALTIME);
+  grpc_oauth2_pending_get_request_metadata *pending_request =
+      c->pending_requests;
+  c->pending_requests = NULL;
   gpr_mu_unlock(&c->mu);
+  // Invoke callbacks for all pending requests.
+  while (pending_request != NULL) {
+    if (status == GRPC_CREDENTIALS_OK) {
+      grpc_credentials_mdelem_array_add(pending_request->md_array,
+                                        access_token_md);
+    } else {
+      error = GRPC_ERROR_CREATE_REFERENCING_FROM_STATIC_STRING(
+          "Error occured when fetching oauth2 token.", &error, 1);
+    }
+    GRPC_CLOSURE_SCHED(exec_ctx, pending_request->on_request_metadata, error);
+    grpc_oauth2_pending_get_request_metadata *prev = pending_request;
+    pending_request = pending_request->next;
+    gpr_free(prev);
+  }
+  GRPC_MDELEM_UNREF(exec_ctx, access_token_md);
+  grpc_call_credentials_unref(exec_ctx, r->creds);
   grpc_credentials_metadata_request_destroy(exec_ctx, r);
 }
 
-static void oauth2_token_fetcher_get_request_metadata(
+static bool oauth2_token_fetcher_get_request_metadata(
     grpc_exec_ctx *exec_ctx, grpc_call_credentials *creds,
     grpc_polling_entity *pollent, grpc_auth_metadata_context context,
-    grpc_credentials_metadata_cb cb, void *user_data) {
+    grpc_credentials_mdelem_array *md_array, grpc_closure *on_request_metadata,
+    grpc_error **error) {
   grpc_oauth2_token_fetcher_credentials *c =
       (grpc_oauth2_token_fetcher_credentials *)creds;
+  // Check if we can use the cached token.
   gpr_timespec refresh_threshold = gpr_time_from_seconds(
       GRPC_SECURE_TOKEN_REFRESH_THRESHOLD_SECS, GPR_TIMESPAN);
-  grpc_credentials_md_store *cached_access_token_md = NULL;
-  {
-    gpr_mu_lock(&c->mu);
-    if (c->access_token_md != NULL &&
-        (gpr_time_cmp(
-             gpr_time_sub(c->token_expiration, gpr_now(GPR_CLOCK_REALTIME)),
-             refresh_threshold) > 0)) {
-      cached_access_token_md =
-          grpc_credentials_md_store_ref(c->access_token_md);
-    }
+  grpc_mdelem cached_access_token_md = GRPC_MDNULL;
+  gpr_mu_lock(&c->mu);
+  if (!GRPC_MDISNULL(c->access_token_md) &&
+      (gpr_time_cmp(
+           gpr_time_sub(c->token_expiration, gpr_now(GPR_CLOCK_REALTIME)),
+           refresh_threshold) > 0)) {
+    cached_access_token_md = GRPC_MDELEM_REF(c->access_token_md);
+  }
+  if (!GRPC_MDISNULL(cached_access_token_md)) {
     gpr_mu_unlock(&c->mu);
+    grpc_credentials_mdelem_array_add(md_array, cached_access_token_md);
+    GRPC_MDELEM_UNREF(exec_ctx, cached_access_token_md);
+    return true;
   }
-  if (cached_access_token_md != NULL) {
-    cb(exec_ctx, user_data, cached_access_token_md->entries,
-       cached_access_token_md->num_entries, GRPC_CREDENTIALS_OK, NULL);
-    grpc_credentials_md_store_unref(exec_ctx, cached_access_token_md);
-  } else {
-    c->fetch_func(
-        exec_ctx,
-        grpc_credentials_metadata_request_create(creds, cb, user_data),
-        &c->httpcli_context, pollent, on_oauth2_token_fetcher_http_response,
-        gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), refresh_threshold));
+  // Couldn't get the token from the cache.
+  // Add request to c->pending_requests and start a new fetch if needed.
+  grpc_oauth2_pending_get_request_metadata *pending_request =
+      (grpc_oauth2_pending_get_request_metadata *)gpr_malloc(
+          sizeof(*pending_request));
+  pending_request->md_array = md_array;
+  pending_request->on_request_metadata = on_request_metadata;
+  pending_request->next = c->pending_requests;
+  c->pending_requests = pending_request;
+  bool start_fetch = false;
+  if (!c->token_fetch_pending) {
+    c->token_fetch_pending = true;
+    start_fetch = true;
+  }
+  gpr_mu_unlock(&c->mu);
+  if (start_fetch) {
+    grpc_call_credentials_ref(creds);
+    c->fetch_func(exec_ctx, grpc_credentials_metadata_request_create(creds),
+                  &c->httpcli_context, pollent,
+                  on_oauth2_token_fetcher_http_response,
+                  gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), refresh_threshold));
   }
+  return false;
+}
+
+static void oauth2_token_fetcher_cancel_get_request_metadata(
+    grpc_exec_ctx *exec_ctx, grpc_call_credentials *creds,
+    grpc_credentials_mdelem_array *md_array, grpc_error *error) {
+  grpc_oauth2_token_fetcher_credentials *c =
+      (grpc_oauth2_token_fetcher_credentials *)creds;
+  gpr_mu_lock(&c->mu);
+  grpc_oauth2_pending_get_request_metadata *prev = NULL;
+  grpc_oauth2_pending_get_request_metadata *pending_request =
+      c->pending_requests;
+  while (pending_request != NULL) {
+    if (pending_request->md_array == md_array) {
+      // Remove matching pending request from the list.
+      if (prev != NULL) {
+        prev->next = pending_request->next;
+      } else {
+        c->pending_requests = pending_request->next;
+      }
+      // Invoke the callback immediately with an error.
+      GRPC_CLOSURE_SCHED(exec_ctx, pending_request->on_request_metadata,
+                         GRPC_ERROR_REF(error));
+      gpr_free(pending_request);
+      break;
+    }
+    prev = pending_request;
+    pending_request = pending_request->next;
+  }
+  gpr_mu_unlock(&c->mu);
+  GRPC_ERROR_UNREF(error);
 }
 
 static void init_oauth2_token_fetcher(grpc_oauth2_token_fetcher_credentials *c,
@@ -280,7 +342,8 @@ static void init_oauth2_token_fetcher(grpc_oauth2_token_fetcher_credentials *c,
 //
 
 static grpc_call_credentials_vtable compute_engine_vtable = {
-    oauth2_token_fetcher_destruct, oauth2_token_fetcher_get_request_metadata};
+    oauth2_token_fetcher_destruct, oauth2_token_fetcher_get_request_metadata,
+    oauth2_token_fetcher_cancel_get_request_metadata};
 
 static void compute_engine_fetch_oauth2(
     grpc_exec_ctx *exec_ctx, grpc_credentials_metadata_request *metadata_req,
@@ -301,7 +364,6 @@ static void compute_engine_fetch_oauth2(
   grpc_httpcli_get(
       exec_ctx, httpcli_context, pollent, resource_quota, &request, deadline,
       GRPC_CLOSURE_CREATE(response_cb, metadata_req, grpc_schedule_on_exec_ctx),
-
       &metadata_req->response);
   grpc_resource_quota_unref_internal(exec_ctx, resource_quota);
 }
@@ -331,7 +393,8 @@ static void refresh_token_destruct(grpc_exec_ctx *exec_ctx,
 }
 
 static grpc_call_credentials_vtable refresh_token_vtable = {
-    refresh_token_destruct, oauth2_token_fetcher_get_request_metadata};
+    refresh_token_destruct, oauth2_token_fetcher_get_request_metadata,
+    oauth2_token_fetcher_cancel_get_request_metadata};
 
 static void refresh_token_fetch_oauth2(
     grpc_exec_ctx *exec_ctx, grpc_credentials_metadata_request *metadata_req,
@@ -416,26 +479,33 @@ grpc_call_credentials *grpc_google_refresh_token_credentials_create(
 static void access_token_destruct(grpc_exec_ctx *exec_ctx,
                                   grpc_call_credentials *creds) {
   grpc_access_token_credentials *c = (grpc_access_token_credentials *)creds;
-  grpc_credentials_md_store_unref(exec_ctx, c->access_token_md);
+  GRPC_MDELEM_UNREF(exec_ctx, c->access_token_md);
 }
 
-static void access_token_get_request_metadata(
+static bool access_token_get_request_metadata(
     grpc_exec_ctx *exec_ctx, grpc_call_credentials *creds,
     grpc_polling_entity *pollent, grpc_auth_metadata_context context,
-    grpc_credentials_metadata_cb cb, void *user_data) {
+    grpc_credentials_mdelem_array *md_array, grpc_closure *on_request_metadata,
+    grpc_error **error) {
   grpc_access_token_credentials *c = (grpc_access_token_credentials *)creds;
-  cb(exec_ctx, user_data, c->access_token_md->entries, 1, GRPC_CREDENTIALS_OK,
-     NULL);
+  grpc_credentials_mdelem_array_add(md_array, c->access_token_md);
+  return true;
+}
+
+static void access_token_cancel_get_request_metadata(
+    grpc_exec_ctx *exec_ctx, grpc_call_credentials *c,
+    grpc_credentials_mdelem_array *md_array, grpc_error *error) {
+  GRPC_ERROR_UNREF(error);
 }
 
 static grpc_call_credentials_vtable access_token_vtable = {
-    access_token_destruct, access_token_get_request_metadata};
+    access_token_destruct, access_token_get_request_metadata,
+    access_token_cancel_get_request_metadata};
 
 grpc_call_credentials *grpc_access_token_credentials_create(
     const char *access_token, void *reserved) {
   grpc_access_token_credentials *c =
       gpr_zalloc(sizeof(grpc_access_token_credentials));
-  char *token_md_value;
   GRPC_API_TRACE(
       "grpc_access_token_credentials_create(access_token=<redacted>, "
       "reserved=%p)",
@@ -444,10 +514,13 @@ grpc_call_credentials *grpc_access_token_credentials_create(
   c->base.type = GRPC_CALL_CREDENTIALS_TYPE_OAUTH2;
   c->base.vtable = &access_token_vtable;
   gpr_ref_init(&c->base.refcount, 1);
-  c->access_token_md = grpc_credentials_md_store_create(1);
+  char *token_md_value;
   gpr_asprintf(&token_md_value, "Bearer %s", access_token);
-  grpc_credentials_md_store_add_cstrings(
-      c->access_token_md, GRPC_AUTHORIZATION_METADATA_KEY, token_md_value);
+  grpc_exec_ctx exec_ctx = GRPC_EXEC_CTX_INIT;
+  c->access_token_md = grpc_mdelem_from_slices(
+      &exec_ctx, grpc_slice_from_static_string(GRPC_AUTHORIZATION_METADATA_KEY),
+      grpc_slice_from_copied_string(token_md_value));
+  grpc_exec_ctx_finish(&exec_ctx);
   gpr_free(token_md_value);
   return &c->base;
 }
diff --git a/src/core/lib/security/credentials/oauth2/oauth2_credentials.h b/src/core/lib/security/credentials/oauth2/oauth2_credentials.h
index 72093afe9e..9d041a20ea 100644
--- a/src/core/lib/security/credentials/oauth2/oauth2_credentials.h
+++ b/src/core/lib/security/credentials/oauth2/oauth2_credentials.h
@@ -58,11 +58,20 @@ typedef void (*grpc_fetch_oauth2_func)(grpc_exec_ctx *exec_ctx,
                                        grpc_polling_entity *pollent,
                                        grpc_iomgr_cb_func cb,
                                        gpr_timespec deadline);
+
+typedef struct grpc_oauth2_pending_get_request_metadata {
+  grpc_credentials_mdelem_array *md_array;
+  grpc_closure *on_request_metadata;
+  struct grpc_oauth2_pending_get_request_metadata *next;
+} grpc_oauth2_pending_get_request_metadata;
+
 typedef struct {
   grpc_call_credentials base;
   gpr_mu mu;
-  grpc_credentials_md_store *access_token_md;
+  grpc_mdelem access_token_md;
   gpr_timespec token_expiration;
+  bool token_fetch_pending;
+  grpc_oauth2_pending_get_request_metadata *pending_requests;
   grpc_httpcli_context httpcli_context;
   grpc_fetch_oauth2_func fetch_func;
 } grpc_oauth2_token_fetcher_credentials;
@@ -76,7 +85,7 @@ typedef struct {
 // Access token credentials.
 typedef struct {
   grpc_call_credentials base;
-  grpc_credentials_md_store *access_token_md;
+  grpc_mdelem access_token_md;
 } grpc_access_token_credentials;
 
 // Private constructor for refresh token credentials from an already parsed
@@ -89,6 +98,6 @@ grpc_refresh_token_credentials_create_from_auth_refresh_token(
 grpc_credentials_status
 grpc_oauth2_token_fetcher_credentials_parse_server_response(
     grpc_exec_ctx *exec_ctx, const struct grpc_http_response *response,
-    grpc_credentials_md_store **token_md, gpr_timespec *token_lifetime);
+    grpc_mdelem *token_md, gpr_timespec *token_lifetime);
 
 #endif /* GRPC_CORE_LIB_SECURITY_CREDENTIALS_OAUTH2_OAUTH2_CREDENTIALS_H */
diff --git a/src/core/lib/security/credentials/plugin/plugin_credentials.c b/src/core/lib/security/credentials/plugin/plugin_credentials.c
index 96ebfb4d0d..73e0c23e0f 100644
--- a/src/core/lib/security/credentials/plugin/plugin_credentials.c
+++ b/src/core/lib/security/credentials/plugin/plugin_credentials.c
@@ -31,19 +31,28 @@
 #include "src/core/lib/surface/api_trace.h"
 #include "src/core/lib/surface/validate_metadata.h"
 
-typedef struct {
-  void *user_data;
-  grpc_credentials_metadata_cb cb;
-} grpc_metadata_plugin_request;
-
 static void plugin_destruct(grpc_exec_ctx *exec_ctx,
                             grpc_call_credentials *creds) {
   grpc_plugin_credentials *c = (grpc_plugin_credentials *)creds;
+  gpr_mu_destroy(&c->mu);
   if (c->plugin.state != NULL && c->plugin.destroy != NULL) {
     c->plugin.destroy(c->plugin.state);
   }
 }
 
+static void pending_request_remove_locked(
+    grpc_plugin_credentials *c,
+    grpc_plugin_credentials_pending_request *pending_request) {
+  if (pending_request->prev == NULL) {
+    c->pending_requests = pending_request->next;
+  } else {
+    pending_request->prev->next = pending_request->next;
+  }
+  if (pending_request->next != NULL) {
+    pending_request->next->prev = pending_request->prev;
+  }
+}
+
 static void plugin_md_request_metadata_ready(void *request,
                                              const grpc_metadata *md,
                                              size_t num_md,
@@ -53,76 +62,117 @@ static void plugin_md_request_metadata_ready(void *request,
   grpc_exec_ctx exec_ctx = GRPC_EXEC_CTX_INITIALIZER(
       GRPC_EXEC_CTX_FLAG_IS_FINISHED | GRPC_EXEC_CTX_FLAG_THREAD_RESOURCE_LOOP,
       NULL, NULL);
-  grpc_metadata_plugin_request *r = (grpc_metadata_plugin_request *)request;
-  if (status != GRPC_STATUS_OK) {
-    if (error_details != NULL) {
-      gpr_log(GPR_ERROR, "Getting metadata from plugin failed with error: %s",
-              error_details);
-    }
-    r->cb(&exec_ctx, r->user_data, NULL, 0, GRPC_CREDENTIALS_ERROR,
-          error_details);
-  } else {
-    size_t i;
-    bool seen_illegal_header = false;
-    grpc_credentials_md *md_array = NULL;
-    for (i = 0; i < num_md; i++) {
-      if (!GRPC_LOG_IF_ERROR("validate_metadata_from_plugin",
-                             grpc_validate_header_key_is_legal(md[i].key))) {
-        seen_illegal_header = true;
-        break;
-      } else if (!grpc_is_binary_header(md[i].key) &&
-                 !GRPC_LOG_IF_ERROR(
-                     "validate_metadata_from_plugin",
-                     grpc_validate_header_nonbin_value_is_legal(md[i].value))) {
-        gpr_log(GPR_ERROR, "Plugin added invalid metadata value.");
-        seen_illegal_header = true;
-        break;
+  grpc_plugin_credentials_pending_request *r =
+      (grpc_plugin_credentials_pending_request *)request;
+  // Check if the request has been cancelled.
+  // If not, remove it from the pending list, so that it cannot be
+  // cancelled out from under us.
+  gpr_mu_lock(&r->creds->mu);
+  if (!r->cancelled) pending_request_remove_locked(r->creds, r);
+  gpr_mu_unlock(&r->creds->mu);
+  grpc_call_credentials_unref(&exec_ctx, &r->creds->base);
+  // If it has not been cancelled, process it.
+  if (!r->cancelled) {
+    if (status != GRPC_STATUS_OK) {
+      char *msg;
+      gpr_asprintf(&msg, "Getting metadata from plugin failed with error: %s",
+                   error_details);
+      GRPC_CLOSURE_SCHED(&exec_ctx, r->on_request_metadata,
+                         GRPC_ERROR_CREATE_FROM_COPIED_STRING(msg));
+      gpr_free(msg);
+    } else {
+      bool seen_illegal_header = false;
+      for (size_t i = 0; i < num_md; ++i) {
+        if (!GRPC_LOG_IF_ERROR("validate_metadata_from_plugin",
+                               grpc_validate_header_key_is_legal(md[i].key))) {
+          seen_illegal_header = true;
+          break;
+        } else if (!grpc_is_binary_header(md[i].key) &&
+                   !GRPC_LOG_IF_ERROR(
+                       "validate_metadata_from_plugin",
+                       grpc_validate_header_nonbin_value_is_legal(
+                           md[i].value))) {
+          gpr_log(GPR_ERROR, "Plugin added invalid metadata value.");
+          seen_illegal_header = true;
+          break;
+        }
       }
-    }
-    if (seen_illegal_header) {
-      r->cb(&exec_ctx, r->user_data, NULL, 0, GRPC_CREDENTIALS_ERROR,
-            "Illegal metadata");
-    } else if (num_md > 0) {
-      md_array = gpr_malloc(num_md * sizeof(grpc_credentials_md));
-      for (i = 0; i < num_md; i++) {
-        md_array[i].key = grpc_slice_ref_internal(md[i].key);
-        md_array[i].value = grpc_slice_ref_internal(md[i].value);
+      if (seen_illegal_header) {
+        GRPC_CLOSURE_SCHED(
+            &exec_ctx, r->on_request_metadata,
+            GRPC_ERROR_CREATE_FROM_STATIC_STRING("Illegal metadata"));
+      } else {
+        for (size_t i = 0; i < num_md; ++i) {
+          grpc_mdelem mdelem = grpc_mdelem_from_slices(
+              &exec_ctx, grpc_slice_ref_internal(md[i].key),
+              grpc_slice_ref_internal(md[i].value));
+          grpc_credentials_mdelem_array_add(r->md_array, mdelem);
+          GRPC_MDELEM_UNREF(&exec_ctx, mdelem);
+        }
+        GRPC_CLOSURE_SCHED(&exec_ctx, r->on_request_metadata, GRPC_ERROR_NONE);
       }
-      r->cb(&exec_ctx, r->user_data, md_array, num_md, GRPC_CREDENTIALS_OK,
-            NULL);
-      for (i = 0; i < num_md; i++) {
-        grpc_slice_unref_internal(&exec_ctx, md_array[i].key);
-        grpc_slice_unref_internal(&exec_ctx, md_array[i].value);
-      }
-      gpr_free(md_array);
-    } else if (num_md == 0) {
-      r->cb(&exec_ctx, r->user_data, NULL, 0, GRPC_CREDENTIALS_OK, NULL);
     }
   }
   gpr_free(r);
   grpc_exec_ctx_finish(&exec_ctx);
 }
 
-static void plugin_get_request_metadata(grpc_exec_ctx *exec_ctx,
+static bool plugin_get_request_metadata(grpc_exec_ctx *exec_ctx,
                                         grpc_call_credentials *creds,
                                         grpc_polling_entity *pollent,
                                         grpc_auth_metadata_context context,
-                                        grpc_credentials_metadata_cb cb,
-                                        void *user_data) {
+                                        grpc_credentials_mdelem_array *md_array,
+                                        grpc_closure *on_request_metadata,
+                                        grpc_error **error) {
   grpc_plugin_credentials *c = (grpc_plugin_credentials *)creds;
   if (c->plugin.get_metadata != NULL) {
-    grpc_metadata_plugin_request *request = gpr_zalloc(sizeof(*request));
-    request->user_data = user_data;
-    request->cb = cb;
+    // Create pending_request object.
+    grpc_plugin_credentials_pending_request *pending_request =
+        (grpc_plugin_credentials_pending_request *)gpr_zalloc(
+            sizeof(*pending_request));
+    pending_request->creds = c;
+    pending_request->md_array = md_array;
+    pending_request->on_request_metadata = on_request_metadata;
+    // Add it to the pending list.
+    gpr_mu_lock(&c->mu);
+    if (c->pending_requests != NULL) {
+      c->pending_requests->prev = pending_request;
+    }
+    pending_request->next = c->pending_requests;
+    c->pending_requests = pending_request;
+    gpr_mu_unlock(&c->mu);
+    // Invoke the plugin.  The callback holds a ref to us.
+    grpc_call_credentials_ref(creds);
     c->plugin.get_metadata(c->plugin.state, context,
-                           plugin_md_request_metadata_ready, request);
-  } else {
-    cb(exec_ctx, user_data, NULL, 0, GRPC_CREDENTIALS_OK, NULL);
+                           plugin_md_request_metadata_ready, pending_request);
+    return false;
+  }
+  return true;
+}
+
+static void plugin_cancel_get_request_metadata(
+    grpc_exec_ctx *exec_ctx, grpc_call_credentials *creds,
+    grpc_credentials_mdelem_array *md_array, grpc_error *error) {
+  grpc_plugin_credentials *c = (grpc_plugin_credentials *)creds;
+  gpr_mu_lock(&c->mu);
+  for (grpc_plugin_credentials_pending_request *pending_request =
+           c->pending_requests;
+       pending_request != NULL; pending_request = pending_request->next) {
+    if (pending_request->md_array == md_array) {
+      pending_request->cancelled = true;
+      GRPC_CLOSURE_SCHED(exec_ctx, pending_request->on_request_metadata,
+                         GRPC_ERROR_REF(error));
+      pending_request_remove_locked(c, pending_request);
+      break;
+    }
   }
+  gpr_mu_unlock(&c->mu);
+  GRPC_ERROR_UNREF(error);
 }
 
 static grpc_call_credentials_vtable plugin_vtable = {
-    plugin_destruct, plugin_get_request_metadata};
+    plugin_destruct, plugin_get_request_metadata,
+    plugin_cancel_get_request_metadata};
 
 grpc_call_credentials *grpc_metadata_credentials_create_from_plugin(
     grpc_metadata_credentials_plugin plugin, void *reserved) {
@@ -134,5 +184,6 @@ grpc_call_credentials *grpc_metadata_credentials_create_from_plugin(
   c->base.vtable = &plugin_vtable;
   gpr_ref_init(&c->base.refcount, 1);
   c->plugin = plugin;
+  gpr_mu_init(&c->mu);
   return &c->base;
 }
diff --git a/src/core/lib/security/credentials/plugin/plugin_credentials.h b/src/core/lib/security/credentials/plugin/plugin_credentials.h
index ba3dd76859..57266d589a 100644
--- a/src/core/lib/security/credentials/plugin/plugin_credentials.h
+++ b/src/core/lib/security/credentials/plugin/plugin_credentials.h
@@ -21,10 +21,22 @@
 
 #include "src/core/lib/security/credentials/credentials.h"
 
-typedef struct {
+struct grpc_plugin_credentials;
+
+typedef struct grpc_plugin_credentials_pending_request {
+  bool cancelled;
+  struct grpc_plugin_credentials *creds;
+  grpc_credentials_mdelem_array *md_array;
+  grpc_closure *on_request_metadata;
+  struct grpc_plugin_credentials_pending_request *prev;
+  struct grpc_plugin_credentials_pending_request *next;
+} grpc_plugin_credentials_pending_request;
+
+typedef struct grpc_plugin_credentials {
   grpc_call_credentials base;
   grpc_metadata_credentials_plugin plugin;
-  grpc_credentials_md_store *plugin_md;
+  gpr_mu mu;
+  grpc_plugin_credentials_pending_request *pending_requests;
 } grpc_plugin_credentials;
 
 #endif /* GRPC_CORE_LIB_SECURITY_CREDENTIALS_PLUGIN_PLUGIN_CREDENTIALS_H */
diff --git a/src/core/lib/security/transport/client_auth_filter.c b/src/core/lib/security/transport/client_auth_filter.c
index 50a51b31cd..531a88434f 100644
--- a/src/core/lib/security/transport/client_auth_filter.c
+++ b/src/core/lib/security/transport/client_auth_filter.c
@@ -51,8 +51,15 @@ typedef struct {
   grpc_polling_entity *pollent;
   gpr_atm security_context_set;
   gpr_mu security_context_mu;
+  grpc_credentials_mdelem_array md_array;
   grpc_linked_mdelem md_links[MAX_CREDENTIALS_METADATA_COUNT];
   grpc_auth_metadata_context auth_md_context;
+  grpc_closure closure;
+  // Either 0 (no cancellation and no async operation in flight),
+  // a grpc_closure* (if the lowest bit is 0),
+  // or a grpc_error* (if the lowest bit is 1).
+  gpr_atm cancellation_state;
+  grpc_closure cancel_closure;
 } call_data;
 
 /* We can have a per-channel credentials. */
@@ -61,6 +68,43 @@ typedef struct {
   grpc_auth_context *auth_context;
 } channel_data;
 
+static void decode_cancel_state(gpr_atm cancel_state, grpc_closure **func,
+                                grpc_error **error) {
+  // If the lowest bit is 1, the value is a grpc_error*.
+  // Otherwise, if non-zdero, the value is a grpc_closure*.
+  if (cancel_state & 1) {
+    *error = (grpc_error *)(cancel_state & ~(gpr_atm)1);
+  } else if (cancel_state != 0) {
+    *func = (grpc_closure *)cancel_state;
+  }
+}
+
+static gpr_atm encode_cancel_state_error(grpc_error *error) {
+  // Set the lowest bit to 1 to indicate that it's an error.
+  return (gpr_atm)1 | (gpr_atm)error;
+}
+
+// Returns an error if the call has been cancelled.  Otherwise, sets the
+// cancellation function to be called upon cancellation.
+static grpc_error *set_cancel_func(grpc_call_element *elem,
+                                   grpc_iomgr_cb_func func) {
+  call_data *calld = (call_data *)elem->call_data;
+  // Decode original state.
+  gpr_atm original_state = gpr_atm_acq_load(&calld->cancellation_state);
+  grpc_error *original_error = GRPC_ERROR_NONE;
+  grpc_closure *original_func = NULL;
+  decode_cancel_state(original_state, &original_func, &original_error);
+  // If error is set, return it.
+  if (original_error != GRPC_ERROR_NONE) return GRPC_ERROR_REF(original_error);
+  // Otherwise, store func.
+  GRPC_CLOSURE_INIT(&calld->cancel_closure, func, elem,
+                    grpc_schedule_on_exec_ctx);
+  GPR_ASSERT(((gpr_atm)&calld->cancel_closure & (gpr_atm)1) == 0);
+  gpr_atm_rel_store(&calld->cancellation_state,
+                    (gpr_atm)&calld->cancel_closure);
+  return GRPC_ERROR_NONE;
+}
+
 static void reset_auth_metadata_context(
     grpc_auth_metadata_context *auth_md_context) {
   if (auth_md_context->service_url != NULL) {
@@ -86,41 +130,29 @@ static void add_error(grpc_error **combined, grpc_error *error) {
   *combined = grpc_error_add_child(*combined, error);
 }
 
-static void on_credentials_metadata(grpc_exec_ctx *exec_ctx, void *user_data,
-                                    grpc_credentials_md *md_elems,
-                                    size_t num_md,
-                                    grpc_credentials_status status,
-                                    const char *error_details) {
-  grpc_transport_stream_op_batch *batch =
-      (grpc_transport_stream_op_batch *)user_data;
+static void on_credentials_metadata(grpc_exec_ctx *exec_ctx, void *arg,
+                                    grpc_error *input_error) {
+  grpc_transport_stream_op_batch *batch = (grpc_transport_stream_op_batch *)arg;
   grpc_call_element *elem = batch->handler_private.extra_arg;
   call_data *calld = elem->call_data;
   reset_auth_metadata_context(&calld->auth_md_context);
-  grpc_error *error = GRPC_ERROR_NONE;
-  if (status != GRPC_CREDENTIALS_OK) {
-    error = grpc_error_set_int(
-        GRPC_ERROR_CREATE_FROM_COPIED_STRING(
-            error_details != NULL && strlen(error_details) > 0
-                ? error_details
-                : "Credentials failed to get metadata."),
-        GRPC_ERROR_INT_GRPC_STATUS, GRPC_STATUS_UNAUTHENTICATED);
-  } else {
-    GPR_ASSERT(num_md <= MAX_CREDENTIALS_METADATA_COUNT);
+  grpc_error *error = GRPC_ERROR_REF(input_error);
+  if (error == GRPC_ERROR_NONE) {
+    GPR_ASSERT(calld->md_array.size <= MAX_CREDENTIALS_METADATA_COUNT);
     GPR_ASSERT(batch->send_initial_metadata);
     grpc_metadata_batch *mdb =
         batch->payload->send_initial_metadata.send_initial_metadata;
-    for (size_t i = 0; i < num_md; i++) {
-      add_error(&error,
-                grpc_metadata_batch_add_tail(
-                    exec_ctx, mdb, &calld->md_links[i],
-                    grpc_mdelem_from_slices(
-                        exec_ctx, grpc_slice_ref_internal(md_elems[i].key),
-                        grpc_slice_ref_internal(md_elems[i].value))));
+    for (size_t i = 0; i < calld->md_array.size; ++i) {
+      add_error(&error, grpc_metadata_batch_add_tail(
+                            exec_ctx, mdb, &calld->md_links[i],
+                            GRPC_MDELEM_REF(calld->md_array.md[i])));
     }
   }
   if (error == GRPC_ERROR_NONE) {
     grpc_call_next_op(exec_ctx, elem, batch);
   } else {
+    error = grpc_error_set_int(error, GRPC_ERROR_INT_GRPC_STATUS,
+                               GRPC_STATUS_UNAUTHENTICATED);
     grpc_transport_stream_op_batch_finish_with_failure(exec_ctx, batch, error);
   }
 }
@@ -155,6 +187,14 @@ void build_auth_metadata_context(grpc_security_connector *sc,
   gpr_free(host);
 }
 
+static void cancel_get_request_metadata(grpc_exec_ctx *exec_ctx, void *arg,
+                                        grpc_error *error) {
+  grpc_call_element *elem = (grpc_call_element *)arg;
+  call_data *calld = (call_data *)elem->call_data;
+  grpc_call_credentials_cancel_get_request_metadata(
+      exec_ctx, calld->creds, &calld->md_array, GRPC_ERROR_REF(error));
+}
+
 static void send_security_metadata(grpc_exec_ctx *exec_ctx,
                                    grpc_call_element *elem,
                                    grpc_transport_stream_op_batch *batch) {
@@ -193,20 +233,33 @@ static void send_security_metadata(grpc_exec_ctx *exec_ctx,
 
   build_auth_metadata_context(&chand->security_connector->base,
                               chand->auth_context, calld);
+
+  grpc_error *cancel_error = set_cancel_func(elem, cancel_get_request_metadata);
+  if (cancel_error != GRPC_ERROR_NONE) {
+    grpc_transport_stream_op_batch_finish_with_failure(exec_ctx, batch,
+                                                       cancel_error);
+    return;
+  }
   GPR_ASSERT(calld->pollent != NULL);
-  grpc_call_credentials_get_request_metadata(
-      exec_ctx, calld->creds, calld->pollent, calld->auth_md_context,
-      on_credentials_metadata, batch);
+  GRPC_CLOSURE_INIT(&calld->closure, on_credentials_metadata, batch,
+                    grpc_schedule_on_exec_ctx);
+  grpc_error *error = GRPC_ERROR_NONE;
+  if (grpc_call_credentials_get_request_metadata(
+          exec_ctx, calld->creds, calld->pollent, calld->auth_md_context,
+          &calld->md_array, &calld->closure, &error)) {
+    // Synchronous return; invoke on_credentials_metadata() directly.
+    on_credentials_metadata(exec_ctx, batch, error);
+    GRPC_ERROR_UNREF(error);
+  }
 }
 
-static void on_host_checked(grpc_exec_ctx *exec_ctx, void *user_data,
-                            grpc_security_status status) {
-  grpc_transport_stream_op_batch *batch =
-      (grpc_transport_stream_op_batch *)user_data;
+static void on_host_checked(grpc_exec_ctx *exec_ctx, void *arg,
+                            grpc_error *error) {
+  grpc_transport_stream_op_batch *batch = (grpc_transport_stream_op_batch *)arg;
   grpc_call_element *elem = batch->handler_private.extra_arg;
   call_data *calld = elem->call_data;
 
-  if (status == GRPC_SECURITY_OK) {
+  if (error == GRPC_ERROR_NONE) {
     send_security_metadata(exec_ctx, elem, batch);
   } else {
     char *error_msg;
@@ -223,6 +276,16 @@ static void on_host_checked(grpc_exec_ctx *exec_ctx, void *user_data,
   }
 }
 
+static void cancel_check_call_host(grpc_exec_ctx *exec_ctx, void *arg,
+                                   grpc_error *error) {
+  grpc_call_element *elem = (grpc_call_element *)arg;
+  call_data *calld = (call_data *)elem->call_data;
+  channel_data *chand = (channel_data *)elem->channel_data;
+  grpc_channel_security_connector_cancel_check_call_host(
+      exec_ctx, chand->security_connector, &calld->closure,
+      GRPC_ERROR_REF(error));
+}
+
 static void auth_start_transport_stream_op_batch(
     grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
     grpc_transport_stream_op_batch *batch) {
@@ -232,7 +295,32 @@ static void auth_start_transport_stream_op_batch(
   call_data *calld = elem->call_data;
   channel_data *chand = elem->channel_data;
 
-  if (!batch->cancel_stream) {
+  if (batch->cancel_stream) {
+    while (true) {
+      // Decode the original cancellation state.
+      gpr_atm original_state = gpr_atm_acq_load(&calld->cancellation_state);
+      grpc_error *cancel_error = GRPC_ERROR_NONE;
+      grpc_closure *func = NULL;
+      decode_cancel_state(original_state, &func, &cancel_error);
+      // If we had already set a cancellation error, there's nothing
+      // more to do.
+      if (cancel_error != GRPC_ERROR_NONE) break;
+      // If there's a cancel func, call it.
+      // Note that even if the cancel func has been changed by some
+      // other thread between when we decoded it and now, it will just
+      // be a no-op.
+      cancel_error = GRPC_ERROR_REF(batch->payload->cancel_stream.cancel_error);
+      if (func != NULL) {
+        GRPC_CLOSURE_SCHED(exec_ctx, func, GRPC_ERROR_REF(cancel_error));
+      }
+      // Encode the new error into cancellation state.
+      if (gpr_atm_full_cas(&calld->cancellation_state, original_state,
+                           encode_cancel_state_error(cancel_error))) {
+        break;  // Success.
+      }
+      // The cas failed, so try again.
+    }
+  } else {
     /* double checked lock over security context to ensure it's set once */
     if (gpr_atm_acq_load(&calld->security_context_set) == 0) {
       gpr_mu_lock(&calld->security_context_mu);
@@ -277,12 +365,26 @@ static void auth_start_transport_stream_op_batch(
       }
     }
     if (calld->have_host) {
-      char *call_host = grpc_slice_to_c_string(calld->host);
-      batch->handler_private.extra_arg = elem;
-      grpc_channel_security_connector_check_call_host(
-          exec_ctx, chand->security_connector, call_host, chand->auth_context,
-          on_host_checked, batch);
-      gpr_free(call_host);
+      grpc_error *cancel_error = set_cancel_func(elem, cancel_check_call_host);
+      if (cancel_error != GRPC_ERROR_NONE) {
+        grpc_transport_stream_op_batch_finish_with_failure(exec_ctx, batch,
+                                                           cancel_error);
+      } else {
+        char *call_host = grpc_slice_to_c_string(calld->host);
+        batch->handler_private.extra_arg = elem;
+        grpc_error *error = GRPC_ERROR_NONE;
+        if (grpc_channel_security_connector_check_call_host(
+                exec_ctx, chand->security_connector, call_host,
+                chand->auth_context,
+                GRPC_CLOSURE_INIT(&calld->closure, on_host_checked, batch,
+                                  grpc_schedule_on_exec_ctx),
+                &error)) {
+          // Synchronous return; invoke on_host_checked() directly.
+          on_host_checked(exec_ctx, batch, error);
+          GRPC_ERROR_UNREF(error);
+        }
+        gpr_free(call_host);
+      }
       GPR_TIMER_END("auth_start_transport_stream_op_batch", 0);
       return; /* early exit */
     }
@@ -315,6 +417,7 @@ static void destroy_call_elem(grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
                               const grpc_call_final_info *final_info,
                               grpc_closure *ignored) {
   call_data *calld = elem->call_data;
+  grpc_credentials_mdelem_array_destroy(exec_ctx, &calld->md_array);
   grpc_call_credentials_unref(exec_ctx, calld->creds);
   if (calld->have_host) {
     grpc_slice_unref_internal(exec_ctx, calld->host);
@@ -324,6 +427,11 @@ static void destroy_call_elem(grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
   }
   reset_auth_metadata_context(&calld->auth_md_context);
   gpr_mu_destroy(&calld->security_context_mu);
+  gpr_atm cancel_state = gpr_atm_acq_load(&calld->cancellation_state);
+  grpc_error *cancel_error = GRPC_ERROR_NONE;
+  grpc_closure *cancel_func = NULL;
+  decode_cancel_state(cancel_state, &cancel_func, &cancel_error);
+  GRPC_ERROR_UNREF(cancel_error);
 }
 
 /* Constructor for channel_data */
diff --git a/src/core/lib/security/transport/security_connector.c b/src/core/lib/security/transport/security_connector.c
index 6788126769..a7568b995f 100644
--- a/src/core/lib/security/transport/security_connector.c
+++ b/src/core/lib/security/transport/security_connector.c
@@ -136,15 +136,27 @@ void grpc_security_connector_check_peer(grpc_exec_ctx *exec_ctx,
   }
 }
 
-void grpc_channel_security_connector_check_call_host(
+bool grpc_channel_security_connector_check_call_host(
     grpc_exec_ctx *exec_ctx, grpc_channel_security_connector *sc,
     const char *host, grpc_auth_context *auth_context,
-    grpc_security_call_host_check_cb cb, void *user_data) {
+    grpc_closure *on_call_host_checked, grpc_error **error) {
   if (sc == NULL || sc->check_call_host == NULL) {
-    cb(exec_ctx, user_data, GRPC_SECURITY_ERROR);
-  } else {
-    sc->check_call_host(exec_ctx, sc, host, auth_context, cb, user_data);
+    *error = GRPC_ERROR_CREATE_FROM_STATIC_STRING(
+        "cannot check call host -- no security connector");
+    return true;
   }
+  return sc->check_call_host(exec_ctx, sc, host, auth_context,
+                             on_call_host_checked, error);
+}
+
+void grpc_channel_security_connector_cancel_check_call_host(
+    grpc_exec_ctx *exec_ctx, grpc_channel_security_connector *sc,
+    grpc_closure *on_call_host_checked, grpc_error *error) {
+  if (sc == NULL || sc->cancel_check_call_host == NULL) {
+    GRPC_ERROR_UNREF(error);
+    return;
+  }
+  sc->cancel_check_call_host(exec_ctx, sc, on_call_host_checked, error);
 }
 
 #ifndef NDEBUG
@@ -368,13 +380,19 @@ static void fake_server_check_peer(grpc_exec_ctx *exec_ctx,
   fake_check_peer(exec_ctx, sc, peer, auth_context, on_peer_checked);
 }
 
-static void fake_channel_check_call_host(grpc_exec_ctx *exec_ctx,
+static bool fake_channel_check_call_host(grpc_exec_ctx *exec_ctx,
                                          grpc_channel_security_connector *sc,
                                          const char *host,
                                          grpc_auth_context *auth_context,
-                                         grpc_security_call_host_check_cb cb,
-                                         void *user_data) {
-  cb(exec_ctx, user_data, GRPC_SECURITY_OK);
+                                         grpc_closure *on_call_host_checked,
+                                         grpc_error **error) {
+  return true;
+}
+
+static void fake_channel_cancel_check_call_host(
+    grpc_exec_ctx *exec_ctx, grpc_channel_security_connector *sc,
+    grpc_closure *on_call_host_checked, grpc_error *error) {
+  GRPC_ERROR_UNREF(error);
 }
 
 static void fake_channel_add_handshakers(
@@ -413,6 +431,7 @@ grpc_channel_security_connector *grpc_fake_channel_security_connector_create(
   c->base.request_metadata_creds =
       grpc_call_credentials_ref(request_metadata_creds);
   c->base.check_call_host = fake_channel_check_call_host;
+  c->base.cancel_check_call_host = fake_channel_cancel_check_call_host;
   c->base.add_handshakers = fake_channel_add_handshakers;
   c->target = gpr_strdup(target);
   const char *expected_targets = grpc_fake_transport_get_expected_targets(args);
@@ -663,26 +682,35 @@ void tsi_shallow_peer_destruct(tsi_peer *peer) {
   if (peer->properties != NULL) gpr_free(peer->properties);
 }
 
-static void ssl_channel_check_call_host(grpc_exec_ctx *exec_ctx,
+static bool ssl_channel_check_call_host(grpc_exec_ctx *exec_ctx,
                                         grpc_channel_security_connector *sc,
                                         const char *host,
                                         grpc_auth_context *auth_context,
-                                        grpc_security_call_host_check_cb cb,
-                                        void *user_data) {
+                                        grpc_closure *on_call_host_checked,
+                                        grpc_error **error) {
   grpc_ssl_channel_security_connector *c =
       (grpc_ssl_channel_security_connector *)sc;
   grpc_security_status status = GRPC_SECURITY_ERROR;
   tsi_peer peer = tsi_shallow_peer_from_ssl_auth_context(auth_context);
   if (ssl_host_matches_name(&peer, host)) status = GRPC_SECURITY_OK;
-
   /* If the target name was overridden, then the original target_name was
      'checked' transitively during the previous peer check at the end of the
      handshake. */
   if (c->overridden_target_name != NULL && strcmp(host, c->target_name) == 0) {
     status = GRPC_SECURITY_OK;
   }
-  cb(exec_ctx, user_data, status);
+  if (status != GRPC_SECURITY_OK) {
+    *error = GRPC_ERROR_CREATE_FROM_STATIC_STRING(
+        "call host does not match SSL server name");
+  }
   tsi_shallow_peer_destruct(&peer);
+  return true;
+}
+
+static void ssl_channel_cancel_check_call_host(
+    grpc_exec_ctx *exec_ctx, grpc_channel_security_connector *sc,
+    grpc_closure *on_call_host_checked, grpc_error *error) {
+  GRPC_ERROR_UNREF(error);
 }
 
 static grpc_security_connector_vtable ssl_channel_vtable = {
@@ -811,6 +839,7 @@ grpc_security_status grpc_ssl_channel_security_connector_create(
   c->base.request_metadata_creds =
       grpc_call_credentials_ref(request_metadata_creds);
   c->base.check_call_host = ssl_channel_check_call_host;
+  c->base.cancel_check_call_host = ssl_channel_cancel_check_call_host;
   c->base.add_handshakers = ssl_channel_add_handshakers;
   gpr_split_host_port(target_name, &c->target_name, &port);
   gpr_free(port);
diff --git a/src/core/lib/security/transport/security_connector.h b/src/core/lib/security/transport/security_connector.h
index 1c0fe40045..4f9b63ad20 100644
--- a/src/core/lib/security/transport/security_connector.h
+++ b/src/core/lib/security/transport/security_connector.h
@@ -117,27 +117,38 @@ grpc_security_connector *grpc_security_connector_find_in_args(
 
 typedef struct grpc_channel_security_connector grpc_channel_security_connector;
 
-typedef void (*grpc_security_call_host_check_cb)(grpc_exec_ctx *exec_ctx,
-                                                 void *user_data,
-                                                 grpc_security_status status);
-
 struct grpc_channel_security_connector {
   grpc_security_connector base;
   grpc_call_credentials *request_metadata_creds;
-  void (*check_call_host)(grpc_exec_ctx *exec_ctx,
+  bool (*check_call_host)(grpc_exec_ctx *exec_ctx,
                           grpc_channel_security_connector *sc, const char *host,
                           grpc_auth_context *auth_context,
-                          grpc_security_call_host_check_cb cb, void *user_data);
+                          grpc_closure *on_call_host_checked,
+                          grpc_error **error);
+  void (*cancel_check_call_host)(grpc_exec_ctx *exec_ctx,
+                                 grpc_channel_security_connector *sc,
+                                 grpc_closure *on_call_host_checked,
+                                 grpc_error *error);
   void (*add_handshakers)(grpc_exec_ctx *exec_ctx,
                           grpc_channel_security_connector *sc,
                           grpc_handshake_manager *handshake_mgr);
 };
 
-/* Checks that the host that will be set for a call is acceptable. */
-void grpc_channel_security_connector_check_call_host(
+/// Checks that the host that will be set for a call is acceptable.
+/// Returns true if completed synchronously, in which case \a error will
+/// be set to indicate the result.  Otherwise, \a on_call_host_checked
+/// will be invoked when complete.
+bool grpc_channel_security_connector_check_call_host(
     grpc_exec_ctx *exec_ctx, grpc_channel_security_connector *sc,
     const char *host, grpc_auth_context *auth_context,
-    grpc_security_call_host_check_cb cb, void *user_data);
+    grpc_closure *on_call_host_checked, grpc_error **error);
+
+/// Cancels a pending asychronous call to
+/// grpc_channel_security_connector_check_call_host() with
+/// \a on_call_host_checked as its callback.
+void grpc_channel_security_connector_cancel_check_call_host(
+    grpc_exec_ctx *exec_ctx, grpc_channel_security_connector *sc,
+    grpc_closure *on_call_host_checked, grpc_error *error);
 
 /* Registers handshakers with \a handshake_mgr. */
 void grpc_channel_security_connector_add_handshakers(
diff --git a/src/node/test/credentials_test.js b/src/node/test/credentials_test.js
index 3688f03512..0ff838e7d0 100644
--- a/src/node/test/credentials_test.js
+++ b/src/node/test/credentials_test.js
@@ -319,7 +319,9 @@ describe('client credentials', function() {
                             client_options);
     client.unary({}, function(err, data) {
       assert(err);
-      assert.strictEqual(err.message, 'Authentication error');
+      assert.strictEqual(err.message,
+                         'Getting metadata from plugin failed with error: ' +
+                         'Authentication error');
       assert.strictEqual(err.code, grpc.status.UNAUTHENTICATED);
       done();
     });
@@ -367,7 +369,9 @@ describe('client credentials', function() {
                             client_options);
     client.unary({}, function(err, data) {
       assert(err);
-      assert.strictEqual(err.message, 'Authentication failure');
+      assert.strictEqual(err.message,
+                         'Getting metadata from plugin failed with error: ' +
+                         'Authentication failure');
       done();
     });
   });