Merge branch 'chttp2_timer' into timer

13 files changed, 445 insertions, 426 deletions

diff --git a/src/core/ext/filters/client_channel/lb_policy/grpclb/grpclb.cc b/src/core/ext/filters/client_channel/lb_policy/grpclb/grpclb.cc
index 53fa0fff04..ffd58129c6 100644
--- a/src/core/ext/filters/client_channel/lb_policy/grpclb/grpclb.cc
+++ b/src/core/ext/filters/client_channel/lb_policy/grpclb/grpclb.cc
@@ -345,9 +345,6 @@ typedef struct glb_lb_policy {
   /** are we currently updating lb_call? */
   bool updating_lb_call;
 
-  /** are we currently updating lb_channel? */
-  bool updating_lb_channel;
-
   /** are we already watching the LB channel's connectivity? */
   bool watching_lb_channel;
 
@@ -360,9 +357,6 @@ typedef struct glb_lb_policy {
   /** called upon changes to the LB channel's connectivity. */
   grpc_closure lb_channel_on_connectivity_changed;
 
-  /** args from the latest update received while already updating, or NULL */
-  grpc_lb_policy_args *pending_update_args;
-
   /************************************************************/
   /*  client data associated with the LB server communication */
   /************************************************************/
@@ -982,10 +976,6 @@ static void glb_destroy(grpc_exec_ctx *exec_ctx, grpc_lb_policy *pol) {
   }
   grpc_fake_resolver_response_generator_unref(glb_policy->response_generator);
   grpc_subchannel_index_unref();
-  if (glb_policy->pending_update_args != NULL) {
-    grpc_channel_args_destroy(exec_ctx, glb_policy->pending_update_args->args);
-    gpr_free(glb_policy->pending_update_args);
-  }
   gpr_free(glb_policy);
 }
 
@@ -1752,45 +1742,22 @@ static void glb_update_locked(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy,
   }
   const grpc_lb_addresses *addresses =
       (const grpc_lb_addresses *)arg->value.pointer.p;
-
+  // If a non-empty serverlist hasn't been received from the balancer,
+  // propagate the update to fallback_backend_addresses.
   if (glb_policy->serverlist == NULL) {
-    // If a non-empty serverlist hasn't been received from the balancer,
-    // propagate the update to fallback_backend_addresses.
     fallback_update_locked(exec_ctx, glb_policy, addresses);
-  } else if (glb_policy->updating_lb_channel) {
-    // If we have recieved serverlist from the balancer, we need to defer update
-    // when there is an in-progress one.
-    if (GRPC_TRACER_ON(grpc_lb_glb_trace)) {
-      gpr_log(GPR_INFO,
-              "Update already in progress for grpclb %p. Deferring update.",
-              (void *)glb_policy);
-    }
-    if (glb_policy->pending_update_args != NULL) {
-      grpc_channel_args_destroy(exec_ctx,
-                                glb_policy->pending_update_args->args);
-      gpr_free(glb_policy->pending_update_args);
-    }
-    glb_policy->pending_update_args = (grpc_lb_policy_args *)gpr_zalloc(
-        sizeof(*glb_policy->pending_update_args));
-    glb_policy->pending_update_args->client_channel_factory =
-        args->client_channel_factory;
-    glb_policy->pending_update_args->args = grpc_channel_args_copy(args->args);
-    glb_policy->pending_update_args->combiner = args->combiner;
-    return;
   }
-
-  glb_policy->updating_lb_channel = true;
   GPR_ASSERT(glb_policy->lb_channel != NULL);
+  // Propagate updates to the LB channel (pick_first) through the fake
+  // resolver.
   grpc_channel_args *lb_channel_args = build_lb_channel_args(
       exec_ctx, addresses, glb_policy->response_generator, args->args);
-  /* Propagate updates to the LB channel (pick first) through the fake resolver
-   */
   grpc_fake_resolver_response_generator_set_response(
       exec_ctx, glb_policy->response_generator, lb_channel_args);
   grpc_channel_args_destroy(exec_ctx, lb_channel_args);
-
+  // Start watching the LB channel connectivity for connection, if not
+  // already doing so.
   if (!glb_policy->watching_lb_channel) {
-    // Watch the LB channel connectivity for connection.
     glb_policy->lb_channel_connectivity = grpc_channel_check_connectivity_state(
         glb_policy->lb_channel, true /* try to connect */);
     grpc_channel_element *client_channel_elem = grpc_channel_stack_last_element(
@@ -1842,18 +1809,10 @@ static void glb_lb_channel_on_connectivity_changed_cb(grpc_exec_ctx *exec_ctx,
     /* fallthrough */
     case GRPC_CHANNEL_READY:
       if (glb_policy->lb_call != NULL) {
-        glb_policy->updating_lb_channel = false;
         glb_policy->updating_lb_call = true;
         grpc_call_cancel(glb_policy->lb_call, NULL);
-        // lb_on_server_status_received will pick up the cancel and reinit
+        // lb_on_server_status_received() will pick up the cancel and reinit
         // lb_call.
-        if (glb_policy->pending_update_args != NULL) {
-          grpc_lb_policy_args *args = glb_policy->pending_update_args;
-          glb_policy->pending_update_args = NULL;
-          glb_update_locked(exec_ctx, &glb_policy->base, args);
-          grpc_channel_args_destroy(exec_ctx, args->args);
-          gpr_free(args);
-        }
       } else if (glb_policy->started_picking && !glb_policy->shutting_down) {
         if (glb_policy->retry_timer_active) {
           grpc_timer_cancel(exec_ctx, &glb_policy->lb_call_retry_timer);
diff --git a/src/core/ext/transport/inproc/inproc_transport.cc b/src/core/ext/transport/inproc/inproc_transport.cc
index 1001d74c22..67a8358927 100644
--- a/src/core/ext/transport/inproc/inproc_transport.cc
+++ b/src/core/ext/transport/inproc/inproc_transport.cc
@@ -62,96 +62,22 @@ typedef struct inproc_transport {
   struct inproc_stream *stream_list;
 } inproc_transport;
 
-typedef struct sb_list_entry {
-  grpc_slice_buffer sb;
-  struct sb_list_entry *next;
-} sb_list_entry;
-
-// Specialize grpc_byte_stream for our use case
-typedef struct {
-  grpc_byte_stream base;
-  sb_list_entry *le;
-  grpc_error *shutdown_error;
-} inproc_slice_byte_stream;
-
-typedef struct {
-  // TODO (vjpai): Add some inlined elements to avoid alloc in simple cases
-  sb_list_entry *head;
-  sb_list_entry *tail;
-} slice_buffer_list;
-
-static void slice_buffer_list_init(slice_buffer_list *l) {
-  l->head = NULL;
-  l->tail = NULL;
-}
-
-static void sb_list_entry_destroy(grpc_exec_ctx *exec_ctx, sb_list_entry *le) {
-  grpc_slice_buffer_destroy_internal(exec_ctx, &le->sb);
-  gpr_free(le);
-}
-
-static void slice_buffer_list_destroy(grpc_exec_ctx *exec_ctx,
-                                      slice_buffer_list *l) {
-  sb_list_entry *curr = l->head;
-  while (curr != NULL) {
-    sb_list_entry *le = curr;
-    curr = curr->next;
-    sb_list_entry_destroy(exec_ctx, le);
-  }
-  l->head = NULL;
-  l->tail = NULL;
-}
-
-static bool slice_buffer_list_empty(slice_buffer_list *l) {
-  return l->head == NULL;
-}
-
-static void slice_buffer_list_append_entry(slice_buffer_list *l,
-                                           sb_list_entry *next) {
-  next->next = NULL;
-  if (l->tail) {
-    l->tail->next = next;
-    l->tail = next;
-  } else {
-    l->head = next;
-    l->tail = next;
-  }
-}
-
-static grpc_slice_buffer *slice_buffer_list_append(slice_buffer_list *l) {
-  sb_list_entry *next = (sb_list_entry *)gpr_malloc(sizeof(*next));
-  grpc_slice_buffer_init(&next->sb);
-  slice_buffer_list_append_entry(l, next);
-  return &next->sb;
-}
-
-static sb_list_entry *slice_buffer_list_pophead(slice_buffer_list *l) {
-  sb_list_entry *ret = l->head;
-  l->head = l->head->next;
-  if (l->head == NULL) {
-    l->tail = NULL;
-  }
-  return ret;
-}
-
 typedef struct inproc_stream {
   inproc_transport *t;
   grpc_metadata_batch to_read_initial_md;
   uint32_t to_read_initial_md_flags;
   bool to_read_initial_md_filled;
-  slice_buffer_list to_read_message;
   grpc_metadata_batch to_read_trailing_md;
   bool to_read_trailing_md_filled;
-  bool reads_needed;
-  bool read_closure_scheduled;
-  grpc_closure read_closure;
+  bool ops_needed;
+  bool op_closure_scheduled;
+  grpc_closure op_closure;
   // Write buffer used only during gap at init time when client-side
   // stream is set up but server side stream is not yet set up
   grpc_metadata_batch write_buffer_initial_md;
   bool write_buffer_initial_md_filled;
   uint32_t write_buffer_initial_md_flags;
   grpc_millis write_buffer_deadline;
-  slice_buffer_list write_buffer_message;
   grpc_metadata_batch write_buffer_trailing_md;
   bool write_buffer_trailing_md_filled;
   grpc_error *write_buffer_cancel_error;
@@ -164,11 +90,15 @@ typedef struct inproc_stream {
 
   gpr_arena *arena;
 
+  grpc_transport_stream_op_batch *send_message_op;
+  grpc_transport_stream_op_batch *send_trailing_md_op;
   grpc_transport_stream_op_batch *recv_initial_md_op;
   grpc_transport_stream_op_batch *recv_message_op;
   grpc_transport_stream_op_batch *recv_trailing_md_op;
 
-  inproc_slice_byte_stream recv_message_stream;
+  grpc_slice_buffer recv_message;
+  grpc_slice_buffer_stream recv_stream;
+  bool recv_inited;
 
   bool initial_md_sent;
   bool trailing_md_sent;
@@ -187,54 +117,11 @@ typedef struct inproc_stream {
   struct inproc_stream *stream_list_next;
 } inproc_stream;
 
-static bool inproc_slice_byte_stream_next(grpc_exec_ctx *exec_ctx,
-                                          grpc_byte_stream *bs, size_t max,
-                                          grpc_closure *on_complete) {
-  // Because inproc transport always provides the entire message atomically,
-  // the byte stream always has data available when this function is called.
-  // Thus, this function always returns true (unlike other transports) and
-  // there is never any need to schedule a closure
-  return true;
-}
-
-static grpc_error *inproc_slice_byte_stream_pull(grpc_exec_ctx *exec_ctx,
-                                                 grpc_byte_stream *bs,
-                                                 grpc_slice *slice) {
-  inproc_slice_byte_stream *stream = (inproc_slice_byte_stream *)bs;
-  if (stream->shutdown_error != GRPC_ERROR_NONE) {
-    return GRPC_ERROR_REF(stream->shutdown_error);
-  }
-  *slice = grpc_slice_buffer_take_first(&stream->le->sb);
-  return GRPC_ERROR_NONE;
-}
-
-static void inproc_slice_byte_stream_shutdown(grpc_exec_ctx *exec_ctx,
-                                              grpc_byte_stream *bs,
-                                              grpc_error *error) {
-  inproc_slice_byte_stream *stream = (inproc_slice_byte_stream *)bs;
-  GRPC_ERROR_UNREF(stream->shutdown_error);
-  stream->shutdown_error = error;
-}
-
-static void inproc_slice_byte_stream_destroy(grpc_exec_ctx *exec_ctx,
-                                             grpc_byte_stream *bs) {
-  inproc_slice_byte_stream *stream = (inproc_slice_byte_stream *)bs;
-  sb_list_entry_destroy(exec_ctx, stream->le);
-  GRPC_ERROR_UNREF(stream->shutdown_error);
-}
-
-static const grpc_byte_stream_vtable inproc_slice_byte_stream_vtable = {
-    inproc_slice_byte_stream_next, inproc_slice_byte_stream_pull,
-    inproc_slice_byte_stream_shutdown, inproc_slice_byte_stream_destroy};
-
-void inproc_slice_byte_stream_init(inproc_slice_byte_stream *s,
-                                   sb_list_entry *le) {
-  s->base.length = (uint32_t)le->sb.length;
-  s->base.flags = 0;
-  s->base.vtable = &inproc_slice_byte_stream_vtable;
-  s->le = le;
-  s->shutdown_error = GRPC_ERROR_NONE;
-}
+static grpc_closure do_nothing_closure;
+static bool cancel_stream_locked(grpc_exec_ctx *exec_ctx, inproc_stream *s,
+                                 grpc_error *error);
+static void op_state_machine(grpc_exec_ctx *exec_ctx, void *arg,
+                             grpc_error *error);
 
 static void ref_transport(inproc_transport *t) {
   INPROC_LOG(GPR_DEBUG, "ref_transport %p", t);
@@ -280,12 +167,14 @@ static void unref_stream(grpc_exec_ctx *exec_ctx, inproc_stream *s,
 static void really_destroy_stream(grpc_exec_ctx *exec_ctx, inproc_stream *s) {
   INPROC_LOG(GPR_DEBUG, "really_destroy_stream %p", s);
 
-  slice_buffer_list_destroy(exec_ctx, &s->to_read_message);
-  slice_buffer_list_destroy(exec_ctx, &s->write_buffer_message);
   GRPC_ERROR_UNREF(s->write_buffer_cancel_error);
   GRPC_ERROR_UNREF(s->cancel_self_error);
   GRPC_ERROR_UNREF(s->cancel_other_error);
 
+  if (s->recv_inited) {
+    grpc_slice_buffer_destroy_internal(exec_ctx, &s->recv_message);
+  }
+
   unref_transport(exec_ctx, s->t);
 
   if (s->closure_at_destroy) {
@@ -293,9 +182,6 @@ static void really_destroy_stream(grpc_exec_ctx *exec_ctx, inproc_stream *s) {
   }
 }
 
-static void read_state_machine(grpc_exec_ctx *exec_ctx, void *arg,
-                               grpc_error *error);
-
 static void log_metadata(const grpc_metadata_batch *md_batch, bool is_client,
                          bool is_initial) {
   for (grpc_linked_mdelem *md = md_batch->list.head; md != NULL;
@@ -359,11 +245,9 @@ static int init_stream(grpc_exec_ctx *exec_ctx, grpc_transport *gt,
   s->write_buffer_initial_md_filled = false;
   grpc_metadata_batch_init(&s->write_buffer_trailing_md);
   s->write_buffer_trailing_md_filled = false;
-  slice_buffer_list_init(&s->to_read_message);
-  slice_buffer_list_init(&s->write_buffer_message);
-  s->reads_needed = false;
-  s->read_closure_scheduled = false;
-  GRPC_CLOSURE_INIT(&s->read_closure, read_state_machine, s,
+  s->ops_needed = false;
+  s->op_closure_scheduled = false;
+  GRPC_CLOSURE_INIT(&s->op_closure, op_state_machine, s,
                     grpc_schedule_on_exec_ctx);
   s->t = t;
   s->closure_at_destroy = NULL;
@@ -425,11 +309,6 @@ static int init_stream(grpc_exec_ctx *exec_ctx, grpc_transport *gt,
       grpc_metadata_batch_clear(exec_ctx, &cs->write_buffer_initial_md);
       cs->write_buffer_initial_md_filled = false;
     }
-    while (!slice_buffer_list_empty(&cs->write_buffer_message)) {
-      slice_buffer_list_append_entry(
-          &s->to_read_message,
-          slice_buffer_list_pophead(&cs->write_buffer_message));
-    }
     if (cs->write_buffer_trailing_md_filled) {
       fill_in_metadata(exec_ctx, s, &cs->write_buffer_trailing_md, 0,
                        &s->to_read_trailing_md, NULL,
@@ -488,9 +367,39 @@ static void close_other_side_locked(grpc_exec_ctx *exec_ctx, inproc_stream *s,
   }
 }
 
+// Call the on_complete closure associated with this stream_op_batch if
+// this stream_op_batch is only one of the pending operations for this
+// stream. This is called when one of the pending operations for the stream
+// is done and about to be NULLed out
+static void complete_if_batch_end_locked(grpc_exec_ctx *exec_ctx,
+                                         inproc_stream *s, grpc_error *error,
+                                         grpc_transport_stream_op_batch *op,
+                                         const char *msg) {
+  int is_sm = (int)(op == s->send_message_op);
+  int is_stm = (int)(op == s->send_trailing_md_op);
+  int is_rim = (int)(op == s->recv_initial_md_op);
+  int is_rm = (int)(op == s->recv_message_op);
+  int is_rtm = (int)(op == s->recv_trailing_md_op);
+
+  if ((is_sm + is_stm + is_rim + is_rm + is_rtm) == 1) {
+    INPROC_LOG(GPR_DEBUG, "%s %p %p %p", msg, s, op, error);
+    GRPC_CLOSURE_SCHED(exec_ctx, op->on_complete, GRPC_ERROR_REF(error));
+  }
+}
+
+static void maybe_schedule_op_closure_locked(grpc_exec_ctx *exec_ctx,
+                                             inproc_stream *s,
+                                             grpc_error *error) {
+  if (s && s->ops_needed && !s->op_closure_scheduled) {
+    GRPC_CLOSURE_SCHED(exec_ctx, &s->op_closure, GRPC_ERROR_REF(error));
+    s->op_closure_scheduled = true;
+    s->ops_needed = false;
+  }
+}
+
 static void fail_helper_locked(grpc_exec_ctx *exec_ctx, inproc_stream *s,
                                grpc_error *error) {
-  INPROC_LOG(GPR_DEBUG, "read_state_machine %p fail_helper", s);
+  INPROC_LOG(GPR_DEBUG, "op_state_machine %p fail_helper", s);
   // If we're failing this side, we need to make sure that
   // we also send or have already sent trailing metadata
   if (!s->trailing_md_sent) {
@@ -512,14 +421,7 @@ static void fail_helper_locked(grpc_exec_ctx *exec_ctx, inproc_stream *s,
       if (other->cancel_other_error == GRPC_ERROR_NONE) {
         other->cancel_other_error = GRPC_ERROR_REF(error);
       }
-      if (other->reads_needed) {
-        if (!other->read_closure_scheduled) {
-          GRPC_CLOSURE_SCHED(exec_ctx, &other->read_closure,
-                             GRPC_ERROR_REF(error));
-          other->read_closure_scheduled = true;
-        }
-        other->reads_needed = false;
-      }
+      maybe_schedule_op_closure_locked(exec_ctx, other, error);
     } else if (s->write_buffer_cancel_error == GRPC_ERROR_NONE) {
       s->write_buffer_cancel_error = GRPC_ERROR_REF(error);
     }
@@ -564,14 +466,9 @@ static void fail_helper_locked(grpc_exec_ctx *exec_ctx, inproc_stream *s,
                        err);
     // Last use of err so no need to REF and then UNREF it
 
-    if ((s->recv_initial_md_op != s->recv_message_op) &&
-        (s->recv_initial_md_op != s->recv_trailing_md_op)) {
-      INPROC_LOG(GPR_DEBUG,
-                 "fail_helper %p scheduling initial-metadata-on-complete %p",
-                 error, s);
-      GRPC_CLOSURE_SCHED(exec_ctx, s->recv_initial_md_op->on_complete,
-                         GRPC_ERROR_REF(error));
-    }
+    complete_if_batch_end_locked(
+        exec_ctx, s, error, s->recv_initial_md_op,
+        "fail_helper scheduling recv-initial-metadata-on-complete");
     s->recv_initial_md_op = NULL;
   }
   if (s->recv_message_op) {
@@ -580,20 +477,30 @@ static void fail_helper_locked(grpc_exec_ctx *exec_ctx, inproc_stream *s,
     GRPC_CLOSURE_SCHED(
         exec_ctx, s->recv_message_op->payload->recv_message.recv_message_ready,
         GRPC_ERROR_REF(error));
-    if (s->recv_message_op != s->recv_trailing_md_op) {
-      INPROC_LOG(GPR_DEBUG, "fail_helper %p scheduling message-on-complete %p",
-                 s, error);
-      GRPC_CLOSURE_SCHED(exec_ctx, s->recv_message_op->on_complete,
-                         GRPC_ERROR_REF(error));
-    }
+    complete_if_batch_end_locked(
+        exec_ctx, s, error, s->recv_message_op,
+        "fail_helper scheduling recv-message-on-complete");
     s->recv_message_op = NULL;
   }
+  if (s->send_message_op) {
+    complete_if_batch_end_locked(
+        exec_ctx, s, error, s->send_message_op,
+        "fail_helper scheduling send-message-on-complete");
+    s->send_message_op = NULL;
+  }
+  if (s->send_trailing_md_op) {
+    complete_if_batch_end_locked(
+        exec_ctx, s, error, s->send_trailing_md_op,
+        "fail_helper scheduling send-trailng-md-on-complete");
+    s->send_trailing_md_op = NULL;
+  }
   if (s->recv_trailing_md_op) {
     INPROC_LOG(GPR_DEBUG,
                "fail_helper %p scheduling trailing-md-on-complete %p", s,
                error);
-    GRPC_CLOSURE_SCHED(exec_ctx, s->recv_trailing_md_op->on_complete,
-                       GRPC_ERROR_REF(error));
+    complete_if_batch_end_locked(
+        exec_ctx, s, error, s->recv_trailing_md_op,
+        "fail_helper scheduling recv-trailing-metadata-on-complete");
     s->recv_trailing_md_op = NULL;
   }
   close_other_side_locked(exec_ctx, s, "fail_helper:other_side");
@@ -602,12 +509,61 @@ static void fail_helper_locked(grpc_exec_ctx *exec_ctx, inproc_stream *s,
   GRPC_ERROR_UNREF(error);
 }
 
-static void read_state_machine(grpc_exec_ctx *exec_ctx, void *arg,
-                               grpc_error *error) {
+static void message_transfer_locked(grpc_exec_ctx *exec_ctx,
+                                    inproc_stream *sender,
+                                    inproc_stream *receiver) {
+  size_t remaining =
+      sender->send_message_op->payload->send_message.send_message->length;
+  if (receiver->recv_inited) {
+    grpc_slice_buffer_destroy_internal(exec_ctx, &receiver->recv_message);
+  }
+  grpc_slice_buffer_init(&receiver->recv_message);
+  receiver->recv_inited = true;
+  do {
+    grpc_slice message_slice;
+    grpc_closure unused;
+    GPR_ASSERT(grpc_byte_stream_next(
+        exec_ctx, sender->send_message_op->payload->send_message.send_message,
+        SIZE_MAX, &unused));
+    grpc_error *error = grpc_byte_stream_pull(
+        exec_ctx, sender->send_message_op->payload->send_message.send_message,
+        &message_slice);
+    if (error != GRPC_ERROR_NONE) {
+      cancel_stream_locked(exec_ctx, sender, GRPC_ERROR_REF(error));
+      break;
+    }
+    GPR_ASSERT(error == GRPC_ERROR_NONE);
+    remaining -= GRPC_SLICE_LENGTH(message_slice);
+    grpc_slice_buffer_add(&receiver->recv_message, message_slice);
+  } while (remaining > 0);
+
+  grpc_slice_buffer_stream_init(&receiver->recv_stream, &receiver->recv_message,
+                                0);
+  *receiver->recv_message_op->payload->recv_message.recv_message =
+      &receiver->recv_stream.base;
+  INPROC_LOG(GPR_DEBUG, "message_transfer_locked %p scheduling message-ready",
+             receiver);
+  GRPC_CLOSURE_SCHED(
+      exec_ctx,
+      receiver->recv_message_op->payload->recv_message.recv_message_ready,
+      GRPC_ERROR_NONE);
+  complete_if_batch_end_locked(
+      exec_ctx, sender, GRPC_ERROR_NONE, sender->send_message_op,
+      "message_transfer scheduling sender on_complete");
+  complete_if_batch_end_locked(
+      exec_ctx, receiver, GRPC_ERROR_NONE, receiver->recv_message_op,
+      "message_transfer scheduling receiver on_complete");
+
+  receiver->recv_message_op = NULL;
+  sender->send_message_op = NULL;
+}
+
+static void op_state_machine(grpc_exec_ctx *exec_ctx, void *arg,
+                             grpc_error *error) {
   // This function gets called when we have contents in the unprocessed reads
   // Get what we want based on our ops wanted
   // Schedule our appropriate closures
-  // and then return to reads_needed state if still needed
+  // and then return to ops_needed state if still needed
 
   // Since this is a closure directly invoked by the combiner, it should not
   // unref the error parameter explicitly; the combiner will do that implicitly
@@ -615,12 +571,14 @@ static void read_state_machine(grpc_exec_ctx *exec_ctx, void *arg,
 
   bool needs_close = false;
 
-  INPROC_LOG(GPR_DEBUG, "read_state_machine %p", arg);
+  INPROC_LOG(GPR_DEBUG, "op_state_machine %p", arg);
   inproc_stream *s = (inproc_stream *)arg;
   gpr_mu *mu = &s->t->mu->mu;  // keep aside in case s gets closed
   gpr_mu_lock(mu);
-  s->read_closure_scheduled = false;
+  s->op_closure_scheduled = false;
   // cancellation takes precedence
+  inproc_stream *other = s->other_side;
+
   if (s->cancel_self_error != GRPC_ERROR_NONE) {
     fail_helper_locked(exec_ctx, s, GRPC_ERROR_REF(s->cancel_self_error));
     goto done;
@@ -632,89 +590,116 @@ static void read_state_machine(grpc_exec_ctx *exec_ctx, void *arg,
     goto done;
   }
 
-  if (s->recv_initial_md_op) {
-    if (!s->to_read_initial_md_filled) {
-      // We entered the state machine on some other kind of read even though
-      // we still haven't satisfied initial md . That's an error.
-      new_err =
-          GRPC_ERROR_CREATE_FROM_STATIC_STRING("Unexpected frame sequencing");
-      INPROC_LOG(GPR_DEBUG,
-                 "read_state_machine %p scheduling on_complete errors for no "
-                 "initial md %p",
-                 s, new_err);
+  if (s->send_message_op && other) {
+    if (other->recv_message_op) {
+      message_transfer_locked(exec_ctx, s, other);
+      maybe_schedule_op_closure_locked(exec_ctx, other, GRPC_ERROR_NONE);
+    } else if (!s->t->is_client &&
+               (s->trailing_md_sent || other->recv_trailing_md_op)) {
+      // A server send will never be matched if the client is waiting
+      // for trailing metadata already
+      complete_if_batch_end_locked(
+          exec_ctx, s, GRPC_ERROR_NONE, s->send_message_op,
+          "op_state_machine scheduling send-message-on-complete");
+      s->send_message_op = NULL;
+    }
+  }
+  // Pause a send trailing metadata if there is still an outstanding
+  // send message unless we know that the send message will never get
+  // matched to a receive. This happens on the client if the server has
+  // already sent status.
+  if (s->send_trailing_md_op &&
+      (!s->send_message_op ||
+       (s->t->is_client &&
+        (s->trailing_md_recvd || s->to_read_trailing_md_filled)))) {
+    grpc_metadata_batch *dest = (other == NULL) ? &s->write_buffer_trailing_md
+                                                : &other->to_read_trailing_md;
+    bool *destfilled = (other == NULL) ? &s->write_buffer_trailing_md_filled
+                                       : &other->to_read_trailing_md_filled;
+    if (*destfilled || s->trailing_md_sent) {
+      // The buffer is already in use; that's an error!
+      INPROC_LOG(GPR_DEBUG, "Extra trailing metadata %p", s);
+      new_err = GRPC_ERROR_CREATE_FROM_STATIC_STRING("Extra trailing metadata");
       fail_helper_locked(exec_ctx, s, GRPC_ERROR_REF(new_err));
       goto done;
-    } else if (s->initial_md_recvd) {
+    } else {
+      if (other && !other->closed) {
+        fill_in_metadata(exec_ctx, s,
+                         s->send_trailing_md_op->payload->send_trailing_metadata
+                             .send_trailing_metadata,
+                         0, dest, NULL, destfilled);
+      }
+      s->trailing_md_sent = true;
+      if (!s->t->is_client && s->trailing_md_recvd && s->recv_trailing_md_op) {
+        INPROC_LOG(GPR_DEBUG,
+                   "op_state_machine %p scheduling trailing-md-on-complete", s);
+        GRPC_CLOSURE_SCHED(exec_ctx, s->recv_trailing_md_op->on_complete,
+                           GRPC_ERROR_NONE);
+        s->recv_trailing_md_op = NULL;
+        needs_close = true;
+      }
+    }
+    maybe_schedule_op_closure_locked(exec_ctx, other, GRPC_ERROR_NONE);
+    complete_if_batch_end_locked(
+        exec_ctx, s, GRPC_ERROR_NONE, s->send_trailing_md_op,
+        "op_state_machine scheduling send-trailing-metadata-on-complete");
+    s->send_trailing_md_op = NULL;
+  }
+  if (s->recv_initial_md_op) {
+    if (s->initial_md_recvd) {
       new_err =
           GRPC_ERROR_CREATE_FROM_STATIC_STRING("Already recvd initial md");
       INPROC_LOG(
           GPR_DEBUG,
-          "read_state_machine %p scheduling on_complete errors for already "
+          "op_state_machine %p scheduling on_complete errors for already "
           "recvd initial md %p",
           s, new_err);
       fail_helper_locked(exec_ctx, s, GRPC_ERROR_REF(new_err));
       goto done;
     }
 
-    s->initial_md_recvd = true;
-    new_err = fill_in_metadata(
-        exec_ctx, s, &s->to_read_initial_md, s->to_read_initial_md_flags,
-        s->recv_initial_md_op->payload->recv_initial_metadata
-            .recv_initial_metadata,
-        s->recv_initial_md_op->payload->recv_initial_metadata.recv_flags, NULL);
-    s->recv_initial_md_op->payload->recv_initial_metadata.recv_initial_metadata
-        ->deadline = s->deadline;
-    grpc_metadata_batch_clear(exec_ctx, &s->to_read_initial_md);
-    s->to_read_initial_md_filled = false;
-    INPROC_LOG(GPR_DEBUG,
-               "read_state_machine %p scheduling initial-metadata-ready %p", s,
-               new_err);
-    GRPC_CLOSURE_SCHED(exec_ctx,
-                       s->recv_initial_md_op->payload->recv_initial_metadata
-                           .recv_initial_metadata_ready,
-                       GRPC_ERROR_REF(new_err));
-    if ((s->recv_initial_md_op != s->recv_message_op) &&
-        (s->recv_initial_md_op != s->recv_trailing_md_op)) {
-      INPROC_LOG(
-          GPR_DEBUG,
-          "read_state_machine %p scheduling initial-metadata-on-complete %p", s,
-          new_err);
-      GRPC_CLOSURE_SCHED(exec_ctx, s->recv_initial_md_op->on_complete,
-                         GRPC_ERROR_REF(new_err));
-    }
-    s->recv_initial_md_op = NULL;
-
-    if (new_err != GRPC_ERROR_NONE) {
+    if (s->to_read_initial_md_filled) {
+      s->initial_md_recvd = true;
+      new_err = fill_in_metadata(
+          exec_ctx, s, &s->to_read_initial_md, s->to_read_initial_md_flags,
+          s->recv_initial_md_op->payload->recv_initial_metadata
+              .recv_initial_metadata,
+          s->recv_initial_md_op->payload->recv_initial_metadata.recv_flags,
+          NULL);
+      s->recv_initial_md_op->payload->recv_initial_metadata
+          .recv_initial_metadata->deadline = s->deadline;
+      grpc_metadata_batch_clear(exec_ctx, &s->to_read_initial_md);
+      s->to_read_initial_md_filled = false;
       INPROC_LOG(GPR_DEBUG,
-                 "read_state_machine %p scheduling on_complete errors2 %p", s,
+                 "op_state_machine %p scheduling initial-metadata-ready %p", s,
                  new_err);
-      fail_helper_locked(exec_ctx, s, GRPC_ERROR_REF(new_err));
-      goto done;
+      GRPC_CLOSURE_SCHED(exec_ctx,
+                         s->recv_initial_md_op->payload->recv_initial_metadata
+                             .recv_initial_metadata_ready,
+                         GRPC_ERROR_REF(new_err));
+      complete_if_batch_end_locked(
+          exec_ctx, s, new_err, s->recv_initial_md_op,
+          "op_state_machine scheduling recv-initial-metadata-on-complete");
+      s->recv_initial_md_op = NULL;
+
+      if (new_err != GRPC_ERROR_NONE) {
+        INPROC_LOG(GPR_DEBUG,
+                   "op_state_machine %p scheduling on_complete errors2 %p", s,
+                   new_err);
+        fail_helper_locked(exec_ctx, s, GRPC_ERROR_REF(new_err));
+        goto done;
+      }
     }
   }
-  if (s->to_read_initial_md_filled) {
-    new_err = GRPC_ERROR_CREATE_FROM_STATIC_STRING("Unexpected recv frame");
-    fail_helper_locked(exec_ctx, s, GRPC_ERROR_REF(new_err));
-    goto done;
-  }
-  if (!slice_buffer_list_empty(&s->to_read_message) && s->recv_message_op) {
-    inproc_slice_byte_stream_init(
-        &s->recv_message_stream,
-        slice_buffer_list_pophead(&s->to_read_message));
-    *s->recv_message_op->payload->recv_message.recv_message =
-        &s->recv_message_stream.base;
-    INPROC_LOG(GPR_DEBUG, "read_state_machine %p scheduling message-ready", s);
-    GRPC_CLOSURE_SCHED(
-        exec_ctx, s->recv_message_op->payload->recv_message.recv_message_ready,
-        GRPC_ERROR_NONE);
-    if (s->recv_message_op != s->recv_trailing_md_op) {
-      INPROC_LOG(GPR_DEBUG,
-                 "read_state_machine %p scheduling message-on-complete %p", s,
-                 new_err);
-      GRPC_CLOSURE_SCHED(exec_ctx, s->recv_message_op->on_complete,
-                         GRPC_ERROR_REF(new_err));
+  if (s->recv_message_op) {
+    if (other && other->send_message_op) {
+      message_transfer_locked(exec_ctx, other, s);
+      maybe_schedule_op_closure_locked(exec_ctx, other, GRPC_ERROR_NONE);
     }
-    s->recv_message_op = NULL;
+  }
+  if (s->recv_trailing_md_op && s->t->is_client && other &&
+      other->send_message_op) {
+    maybe_schedule_op_closure_locked(exec_ctx, other, GRPC_ERROR_NONE);
   }
   if (s->to_read_trailing_md_filled) {
     if (s->trailing_md_recvd) {
@@ -722,7 +707,7 @@ static void read_state_machine(grpc_exec_ctx *exec_ctx, void *arg,
           GRPC_ERROR_CREATE_FROM_STATIC_STRING("Already recvd trailing md");
       INPROC_LOG(
           GPR_DEBUG,
-          "read_state_machine %p scheduling on_complete errors for already "
+          "op_state_machine %p scheduling on_complete errors for already "
           "recvd trailing md %p",
           s, new_err);
       fail_helper_locked(exec_ctx, s, GRPC_ERROR_REF(new_err));
@@ -731,21 +716,24 @@ static void read_state_machine(grpc_exec_ctx *exec_ctx, void *arg,
     if (s->recv_message_op != NULL) {
       // This message needs to be wrapped up because it will never be
       // satisfied
-      INPROC_LOG(GPR_DEBUG, "read_state_machine %p scheduling message-ready",
-                 s);
+      INPROC_LOG(GPR_DEBUG, "op_state_machine %p scheduling message-ready", s);
       GRPC_CLOSURE_SCHED(
           exec_ctx,
           s->recv_message_op->payload->recv_message.recv_message_ready,
           GRPC_ERROR_NONE);
-      if (s->recv_message_op != s->recv_trailing_md_op) {
-        INPROC_LOG(GPR_DEBUG,
-                   "read_state_machine %p scheduling message-on-complete %p", s,
-                   new_err);
-        GRPC_CLOSURE_SCHED(exec_ctx, s->recv_message_op->on_complete,
-                           GRPC_ERROR_REF(new_err));
-      }
+      complete_if_batch_end_locked(
+          exec_ctx, s, new_err, s->recv_message_op,
+          "op_state_machine scheduling recv-message-on-complete");
       s->recv_message_op = NULL;
     }
+    if ((s->trailing_md_sent || s->t->is_client) && s->send_message_op) {
+      // Nothing further will try to receive from this stream, so finish off
+      // any outstanding send_message op
+      complete_if_batch_end_locked(
+          exec_ctx, s, new_err, s->send_message_op,
+          "op_state_machine scheduling send-message-on-complete");
+      s->send_message_op = NULL;
+    }
     if (s->recv_trailing_md_op != NULL) {
       // We wanted trailing metadata and we got it
       s->trailing_md_recvd = true;
@@ -763,61 +751,65 @@ static void read_state_machine(grpc_exec_ctx *exec_ctx, void *arg,
       //    (If the server hasn't already sent its trailing md, it doesn't have
       //     a final status, so don't mark this op complete)
       if (s->t->is_client || s->trailing_md_sent) {
-        INPROC_LOG(
-            GPR_DEBUG,
-            "read_state_machine %p scheduling trailing-md-on-complete %p", s,
-            new_err);
+        INPROC_LOG(GPR_DEBUG,
+                   "op_state_machine %p scheduling trailing-md-on-complete %p",
+                   s, new_err);
         GRPC_CLOSURE_SCHED(exec_ctx, s->recv_trailing_md_op->on_complete,
                            GRPC_ERROR_REF(new_err));
         s->recv_trailing_md_op = NULL;
         needs_close = true;
       } else {
         INPROC_LOG(GPR_DEBUG,
-                   "read_state_machine %p server needs to delay handling "
+                   "op_state_machine %p server needs to delay handling "
                    "trailing-md-on-complete %p",
                    s, new_err);
       }
     } else {
       INPROC_LOG(
           GPR_DEBUG,
-          "read_state_machine %p has trailing md but not yet waiting for it",
-          s);
+          "op_state_machine %p has trailing md but not yet waiting for it", s);
     }
   }
   if (s->trailing_md_recvd && s->recv_message_op) {
     // No further message will come on this stream, so finish off the
     // recv_message_op
-    INPROC_LOG(GPR_DEBUG, "read_state_machine %p scheduling message-ready", s);
+    INPROC_LOG(GPR_DEBUG, "op_state_machine %p scheduling message-ready", s);
     GRPC_CLOSURE_SCHED(
         exec_ctx, s->recv_message_op->payload->recv_message.recv_message_ready,
         GRPC_ERROR_NONE);
-    if (s->recv_message_op != s->recv_trailing_md_op) {
-      INPROC_LOG(GPR_DEBUG,
-                 "read_state_machine %p scheduling message-on-complete %p", s,
-                 new_err);
-      GRPC_CLOSURE_SCHED(exec_ctx, s->recv_message_op->on_complete,
-                         GRPC_ERROR_REF(new_err));
-    }
+    complete_if_batch_end_locked(
+        exec_ctx, s, new_err, s->recv_message_op,
+        "op_state_machine scheduling recv-message-on-complete");
     s->recv_message_op = NULL;
   }
-  if (s->recv_message_op || s->recv_trailing_md_op) {
+  if (s->trailing_md_recvd && (s->trailing_md_sent || s->t->is_client) &&
+      s->send_message_op) {
+    // Nothing further will try to receive from this stream, so finish off
+    // any outstanding send_message op
+    complete_if_batch_end_locked(
+        exec_ctx, s, new_err, s->send_message_op,
+        "op_state_machine scheduling send-message-on-complete");
+    s->send_message_op = NULL;
+  }
+  if (s->send_message_op || s->send_trailing_md_op || s->recv_initial_md_op ||
+      s->recv_message_op || s->recv_trailing_md_op) {
     // Didn't get the item we wanted so we still need to get
     // rescheduled
-    INPROC_LOG(GPR_DEBUG, "read_state_machine %p still needs closure %p %p", s,
-               s->recv_message_op, s->recv_trailing_md_op);
-    s->reads_needed = true;
+    INPROC_LOG(
+        GPR_DEBUG, "op_state_machine %p still needs closure %p %p %p %p %p", s,
+        s->send_message_op, s->send_trailing_md_op, s->recv_initial_md_op,
+        s->recv_message_op, s->recv_trailing_md_op);
+    s->ops_needed = true;
   }
 done:
   if (needs_close) {
-    close_other_side_locked(exec_ctx, s, "read_state_machine");
+    close_other_side_locked(exec_ctx, s, "op_state_machine");
     close_stream_locked(exec_ctx, s);
   }
   gpr_mu_unlock(mu);
   GRPC_ERROR_UNREF(new_err);
 }
 
-static grpc_closure do_nothing_closure;
-
 static bool cancel_stream_locked(grpc_exec_ctx *exec_ctx, inproc_stream *s,
                                  grpc_error *error) {
   bool ret = false;  // was the cancel accepted
@@ -826,14 +818,7 @@ static bool cancel_stream_locked(grpc_exec_ctx *exec_ctx, inproc_stream *s,
   if (s->cancel_self_error == GRPC_ERROR_NONE) {
     ret = true;
     s->cancel_self_error = GRPC_ERROR_REF(error);
-    if (s->reads_needed) {
-      if (!s->read_closure_scheduled) {
-        GRPC_CLOSURE_SCHED(exec_ctx, &s->read_closure,
-                           GRPC_ERROR_REF(s->cancel_self_error));
-        s->read_closure_scheduled = true;
-      }
-      s->reads_needed = false;
-    }
+    maybe_schedule_op_closure_locked(exec_ctx, s, s->cancel_self_error);
     // Send trailing md to the other side indicating cancellation, even if we
     // already have
     s->trailing_md_sent = true;
@@ -853,14 +838,8 @@ static bool cancel_stream_locked(grpc_exec_ctx *exec_ctx, inproc_stream *s,
       if (other->cancel_other_error == GRPC_ERROR_NONE) {
         other->cancel_other_error = GRPC_ERROR_REF(s->cancel_self_error);
       }
-      if (other->reads_needed) {
-        if (!other->read_closure_scheduled) {
-          GRPC_CLOSURE_SCHED(exec_ctx, &other->read_closure,
-                             GRPC_ERROR_REF(other->cancel_other_error));
-          other->read_closure_scheduled = true;
-        }
-        other->reads_needed = false;
-      }
+      maybe_schedule_op_closure_locked(exec_ctx, other,
+                                       other->cancel_other_error);
     } else if (s->write_buffer_cancel_error == GRPC_ERROR_NONE) {
       s->write_buffer_cancel_error = GRPC_ERROR_REF(s->cancel_self_error);
     }
@@ -869,11 +848,9 @@ static bool cancel_stream_locked(grpc_exec_ctx *exec_ctx, inproc_stream *s,
     // couldn't complete that because we hadn't yet sent out trailing
     // md, now's the chance
     if (!s->t->is_client && s->trailing_md_recvd && s->recv_trailing_md_op) {
-      INPROC_LOG(GPR_DEBUG,
-                 "cancel_stream %p scheduling trailing-md-on-complete %p", s,
-                 s->cancel_self_error);
-      GRPC_CLOSURE_SCHED(exec_ctx, s->recv_trailing_md_op->on_complete,
-                         GRPC_ERROR_REF(s->cancel_self_error));
+      complete_if_batch_end_locked(
+          exec_ctx, s, s->cancel_self_error, s->recv_trailing_md_op,
+          "cancel_stream scheduling trailing-md-on-complete");
       s->recv_trailing_md_op = NULL;
     }
   }
@@ -918,7 +895,8 @@ static void perform_stream_op(grpc_exec_ctx *exec_ctx, grpc_transport *gt,
     // already self-canceled so still give it an error
     error = GRPC_ERROR_REF(s->cancel_self_error);
   } else {
-    INPROC_LOG(GPR_DEBUG, "perform_stream_op %p%s%s%s%s%s%s", s,
+    INPROC_LOG(GPR_DEBUG, "perform_stream_op %p %s%s%s%s%s%s%s", s,
+               s->t->is_client ? "client" : "server",
                op->send_initial_metadata ? " send_initial_metadata" : "",
                op->send_message ? " send_message" : "",
                op->send_trailing_metadata ? " send_trailing_metadata" : "",
@@ -929,10 +907,9 @@ static void perform_stream_op(grpc_exec_ctx *exec_ctx, grpc_transport *gt,
 
   bool needs_close = false;
 
+  inproc_stream *other = s->other_side;
   if (error == GRPC_ERROR_NONE &&
-      (op->send_initial_metadata || op->send_message ||
-       op->send_trailing_metadata)) {
-    inproc_stream *other = s->other_side;
+      (op->send_initial_metadata || op->send_trailing_metadata)) {
     if (s->t->is_closed) {
       error = GRPC_ERROR_CREATE_FROM_STATIC_STRING("Endpoint already shutdown");
     }
@@ -963,72 +940,21 @@ static void perform_stream_op(grpc_exec_ctx *exec_ctx, grpc_transport *gt,
           s->initial_md_sent = true;
         }
       }
-    }
-    if (error == GRPC_ERROR_NONE && op->send_message) {
-      size_t remaining = op->payload->send_message.send_message->length;
-      grpc_slice_buffer *dest = slice_buffer_list_append(
-          (other == NULL) ? &s->write_buffer_message : &other->to_read_message);
-      do {
-        grpc_slice message_slice;
-        grpc_closure unused;
-        GPR_ASSERT(grpc_byte_stream_next(exec_ctx,
-                                         op->payload->send_message.send_message,
-                                         SIZE_MAX, &unused));
-        error = grpc_byte_stream_pull(
-            exec_ctx, op->payload->send_message.send_message, &message_slice);
-        if (error != GRPC_ERROR_NONE) {
-          cancel_stream_locked(exec_ctx, s, GRPC_ERROR_REF(error));
-          break;
-        }
-        GPR_ASSERT(error == GRPC_ERROR_NONE);
-        remaining -= GRPC_SLICE_LENGTH(message_slice);
-        grpc_slice_buffer_add(dest, message_slice);
-      } while (remaining != 0);
-      grpc_byte_stream_destroy(exec_ctx,
-                               op->payload->send_message.send_message);
-    }
-    if (error == GRPC_ERROR_NONE && op->send_trailing_metadata) {
-      grpc_metadata_batch *dest = (other == NULL) ? &s->write_buffer_trailing_md
-                                                  : &other->to_read_trailing_md;
-      bool *destfilled = (other == NULL) ? &s->write_buffer_trailing_md_filled
-                                         : &other->to_read_trailing_md_filled;
-      if (*destfilled || s->trailing_md_sent) {
-        // The buffer is already in use; that's an error!
-        INPROC_LOG(GPR_DEBUG, "Extra trailing metadata %p", s);
-        error = GRPC_ERROR_CREATE_FROM_STATIC_STRING("Extra trailing metadata");
-      } else {
-        if (!other->closed) {
-          fill_in_metadata(
-              exec_ctx, s,
-              op->payload->send_trailing_metadata.send_trailing_metadata, 0,
-              dest, NULL, destfilled);
-        }
-        s->trailing_md_sent = true;
-        if (!s->t->is_client && s->trailing_md_recvd &&
-            s->recv_trailing_md_op) {
-          INPROC_LOG(GPR_DEBUG,
-                     "perform_stream_op %p scheduling trailing-md-on-complete",
-                     s);
-          GRPC_CLOSURE_SCHED(exec_ctx, s->recv_trailing_md_op->on_complete,
-                             GRPC_ERROR_NONE);
-          s->recv_trailing_md_op = NULL;
-          needs_close = true;
-        }
-      }
-    }
-    if (other != NULL && other->reads_needed) {
-      if (!other->read_closure_scheduled) {
-        GRPC_CLOSURE_SCHED(exec_ctx, &other->read_closure, error);
-        other->read_closure_scheduled = true;
-      }
-      other->reads_needed = false;
+      maybe_schedule_op_closure_locked(exec_ctx, other, error);
     }
   }
+
   if (error == GRPC_ERROR_NONE &&
-      (op->recv_initial_metadata || op->recv_message ||
+      (op->send_message || op->send_trailing_metadata ||
+       op->recv_initial_metadata || op->recv_message ||
        op->recv_trailing_metadata)) {
-    // If there are any reads, mark it so that the read closure will react to
-    // them
+    // Mark ops that need to be processed by the closure
+    if (op->send_message) {
+      s->send_message_op = op;
+    }
+    if (op->send_trailing_metadata) {
+      s->send_trailing_md_op = op;
+    }
     if (op->recv_initial_metadata) {
       s->recv_initial_md_op = op;
     }
@@ -1040,25 +966,28 @@ static void perform_stream_op(grpc_exec_ctx *exec_ctx, grpc_transport *gt,
     }
 
     // We want to initiate the closure if:
-    // 1. There is initial metadata and something ready to take that
-    // 2. There is a message and something ready to take it
-    // 3. There is trailing metadata, even if nothing specifically wants
-    //    that because that can shut down the message as well
-    if ((s->to_read_initial_md_filled && op->recv_initial_metadata) ||
-        ((!slice_buffer_list_empty(&s->to_read_message) ||
-          s->trailing_md_recvd) &&
-         op->recv_message) ||
-        (s->to_read_trailing_md_filled)) {
-      if (!s->read_closure_scheduled) {
-        GRPC_CLOSURE_SCHED(exec_ctx, &s->read_closure, GRPC_ERROR_NONE);
-        s->read_closure_scheduled = true;
+    // 1. We want to send a message and the other side wants to receive or end
+    // 2. We want to send trailing metadata and there isn't an unmatched send
+    // 3. We want initial metadata and the other side has sent it
+    // 4. We want to receive a message and there is a message ready
+    // 5. There is trailing metadata, even if nothing specifically wants
+    //    that because that can shut down the receive message as well
+    if ((op->send_message && other && ((other->recv_message_op != NULL) ||
+                                       (other->recv_trailing_md_op != NULL))) ||
+        (op->send_trailing_metadata && !op->send_message) ||
+        (op->recv_initial_metadata && s->to_read_initial_md_filled) ||
+        (op->recv_message && (other && other->send_message_op != NULL)) ||
+        (s->to_read_trailing_md_filled || s->trailing_md_recvd)) {
+      if (!s->op_closure_scheduled) {
+        GRPC_CLOSURE_SCHED(exec_ctx, &s->op_closure, GRPC_ERROR_NONE);
+        s->op_closure_scheduled = true;
       }
     } else {
-      s->reads_needed = true;
+      s->ops_needed = true;
     }
   } else {
     if (error != GRPC_ERROR_NONE) {
-      // Schedule op's read closures that we didn't push to read state machine
+      // Schedule op's closures that we didn't push to op state machine
       if (op->recv_initial_metadata) {
         INPROC_LOG(
             GPR_DEBUG,
diff --git a/src/core/lib/debug/stats_data.cc b/src/core/lib/debug/stats_data.cc
index 5bd7884e28..5d737c56cb 100644
--- a/src/core/lib/debug/stats_data.cc
+++ b/src/core/lib/debug/stats_data.cc
@@ -104,6 +104,10 @@ const char *grpc_stats_counter_name[GRPC_STATS_COUNTER_COUNT] = {
     "combiner_locks_scheduled_items",
     "combiner_locks_scheduled_final_items",
     "combiner_locks_offloaded",
+    "call_combiner_locks_initiated",
+    "call_combiner_locks_scheduled_items",
+    "call_combiner_set_notify_on_cancel",
+    "call_combiner_cancelled",
     "executor_scheduled_short_items",
     "executor_scheduled_long_items",
     "executor_scheduled_to_self",
@@ -112,6 +116,9 @@ const char *grpc_stats_counter_name[GRPC_STATS_COUNTER_COUNT] = {
     "executor_push_retries",
     "server_requested_calls",
     "server_slowpath_requests_queued",
+    "cq_ev_queue_trylock_failures",
+    "cq_ev_queue_trylock_successes",
+    "cq_ev_queue_transient_pop_failures",
 };
 const char *grpc_stats_counter_doc[GRPC_STATS_COUNTER_COUNT] = {
     "Number of client side calls created by this process",
@@ -210,6 +217,11 @@ const char *grpc_stats_counter_doc[GRPC_STATS_COUNTER_COUNT] = {
     "Number of items scheduled against combiner locks",
     "Number of final items scheduled against combiner locks",
     "Number of combiner locks offloaded to different threads",
+    "Number of call combiner lock entries by process (first items queued to a "
+    "call combiner)",
+    "Number of items scheduled against call combiner locks",
+    "Number of times a cancellation callback was set on a call combiner",
+    "Number of times a call combiner was cancelled",
     "Number of finite runtime closures scheduled against the executor (gRPC "
     "thread pool)",
     "Number of potentially infinite runtime closures scheduled against the "
@@ -222,6 +234,12 @@ const char *grpc_stats_counter_doc[GRPC_STATS_COUNTER_COUNT] = {
     "How many calls were requested (not necessarily received) by the server",
     "How many times was the server slow path taken (indicates too few "
     "outstanding requests)",
+    "Number of lock (trylock) acquisition failures on completion queue event "
+    "queue. High value here indicates high contention on completion queues",
+    "Number of lock (trylock) acquisition successes on completion queue event "
+    "queue.",
+    "Number of times NULL was popped out of completion queue's event queue "
+    "even though the event queue was not empty",
 };
 const char *grpc_stats_histogram_name[GRPC_STATS_HISTOGRAM_COUNT] = {
     "call_initial_size",
diff --git a/src/core/lib/debug/stats_data.h b/src/core/lib/debug/stats_data.h
index d8e4e7d264..031942df5c 100644
--- a/src/core/lib/debug/stats_data.h
+++ b/src/core/lib/debug/stats_data.h
@@ -110,6 +110,10 @@ typedef enum {
   GRPC_STATS_COUNTER_COMBINER_LOCKS_SCHEDULED_ITEMS,
   GRPC_STATS_COUNTER_COMBINER_LOCKS_SCHEDULED_FINAL_ITEMS,
   GRPC_STATS_COUNTER_COMBINER_LOCKS_OFFLOADED,
+  GRPC_STATS_COUNTER_CALL_COMBINER_LOCKS_INITIATED,
+  GRPC_STATS_COUNTER_CALL_COMBINER_LOCKS_SCHEDULED_ITEMS,
+  GRPC_STATS_COUNTER_CALL_COMBINER_SET_NOTIFY_ON_CANCEL,
+  GRPC_STATS_COUNTER_CALL_COMBINER_CANCELLED,
   GRPC_STATS_COUNTER_EXECUTOR_SCHEDULED_SHORT_ITEMS,
   GRPC_STATS_COUNTER_EXECUTOR_SCHEDULED_LONG_ITEMS,
   GRPC_STATS_COUNTER_EXECUTOR_SCHEDULED_TO_SELF,
@@ -118,6 +122,9 @@ typedef enum {
   GRPC_STATS_COUNTER_EXECUTOR_PUSH_RETRIES,
   GRPC_STATS_COUNTER_SERVER_REQUESTED_CALLS,
   GRPC_STATS_COUNTER_SERVER_SLOWPATH_REQUESTS_QUEUED,
+  GRPC_STATS_COUNTER_CQ_EV_QUEUE_TRYLOCK_FAILURES,
+  GRPC_STATS_COUNTER_CQ_EV_QUEUE_TRYLOCK_SUCCESSES,
+  GRPC_STATS_COUNTER_CQ_EV_QUEUE_TRANSIENT_POP_FAILURES,
   GRPC_STATS_COUNTER_COUNT
 } grpc_stats_counters;
 extern const char *grpc_stats_counter_name[GRPC_STATS_COUNTER_COUNT];
@@ -404,6 +411,17 @@ typedef enum {
 #define GRPC_STATS_INC_COMBINER_LOCKS_OFFLOADED(exec_ctx) \
   GRPC_STATS_INC_COUNTER((exec_ctx),                      \
                          GRPC_STATS_COUNTER_COMBINER_LOCKS_OFFLOADED)
+#define GRPC_STATS_INC_CALL_COMBINER_LOCKS_INITIATED(exec_ctx) \
+  GRPC_STATS_INC_COUNTER((exec_ctx),                           \
+                         GRPC_STATS_COUNTER_CALL_COMBINER_LOCKS_INITIATED)
+#define GRPC_STATS_INC_CALL_COMBINER_LOCKS_SCHEDULED_ITEMS(exec_ctx) \
+  GRPC_STATS_INC_COUNTER(                                            \
+      (exec_ctx), GRPC_STATS_COUNTER_CALL_COMBINER_LOCKS_SCHEDULED_ITEMS)
+#define GRPC_STATS_INC_CALL_COMBINER_SET_NOTIFY_ON_CANCEL(exec_ctx) \
+  GRPC_STATS_INC_COUNTER(                                           \
+      (exec_ctx), GRPC_STATS_COUNTER_CALL_COMBINER_SET_NOTIFY_ON_CANCEL)
+#define GRPC_STATS_INC_CALL_COMBINER_CANCELLED(exec_ctx) \
+  GRPC_STATS_INC_COUNTER((exec_ctx), GRPC_STATS_COUNTER_CALL_COMBINER_CANCELLED)
 #define GRPC_STATS_INC_EXECUTOR_SCHEDULED_SHORT_ITEMS(exec_ctx) \
   GRPC_STATS_INC_COUNTER((exec_ctx),                            \
                          GRPC_STATS_COUNTER_EXECUTOR_SCHEDULED_SHORT_ITEMS)
@@ -425,6 +443,15 @@ typedef enum {
 #define GRPC_STATS_INC_SERVER_SLOWPATH_REQUESTS_QUEUED(exec_ctx) \
   GRPC_STATS_INC_COUNTER((exec_ctx),                             \
                          GRPC_STATS_COUNTER_SERVER_SLOWPATH_REQUESTS_QUEUED)
+#define GRPC_STATS_INC_CQ_EV_QUEUE_TRYLOCK_FAILURES(exec_ctx) \
+  GRPC_STATS_INC_COUNTER((exec_ctx),                          \
+                         GRPC_STATS_COUNTER_CQ_EV_QUEUE_TRYLOCK_FAILURES)
+#define GRPC_STATS_INC_CQ_EV_QUEUE_TRYLOCK_SUCCESSES(exec_ctx) \
+  GRPC_STATS_INC_COUNTER((exec_ctx),                           \
+                         GRPC_STATS_COUNTER_CQ_EV_QUEUE_TRYLOCK_SUCCESSES)
+#define GRPC_STATS_INC_CQ_EV_QUEUE_TRANSIENT_POP_FAILURES(exec_ctx) \
+  GRPC_STATS_INC_COUNTER(                                           \
+      (exec_ctx), GRPC_STATS_COUNTER_CQ_EV_QUEUE_TRANSIENT_POP_FAILURES)
 #define GRPC_STATS_INC_CALL_INITIAL_SIZE(exec_ctx, value) \
   grpc_stats_inc_call_initial_size((exec_ctx), (int)(value))
 void grpc_stats_inc_call_initial_size(grpc_exec_ctx *exec_ctx, int x);
diff --git a/src/core/lib/debug/stats_data.yaml b/src/core/lib/debug/stats_data.yaml
index 5c0ab2262e..af4553028e 100644
--- a/src/core/lib/debug/stats_data.yaml
+++ b/src/core/lib/debug/stats_data.yaml
@@ -245,6 +245,16 @@
   doc: Number of final items scheduled against combiner locks
 - counter: combiner_locks_offloaded
   doc: Number of combiner locks offloaded to different threads
+# call combiner locks
+- counter: call_combiner_locks_initiated
+  doc: Number of call combiner lock entries by process
+       (first items queued to a call combiner)
+- counter: call_combiner_locks_scheduled_items
+  doc: Number of items scheduled against call combiner locks
+- counter: call_combiner_set_notify_on_cancel
+  doc: Number of times a cancellation callback was set on a call combiner
+- counter: call_combiner_cancelled
+  doc: Number of times a call combiner was cancelled
 # executor
 - counter: executor_scheduled_short_items
   doc: Number of finite runtime closures scheduled against the executor
@@ -272,4 +282,13 @@
 - counter: server_slowpath_requests_queued
   doc: How many times was the server slow path taken (indicates too few
        outstanding requests)
-
+# cq
+- counter: cq_ev_queue_trylock_failures
+  doc: Number of lock (trylock) acquisition failures on completion queue event
+       queue. High value here indicates high contention on completion queues
+- counter: cq_ev_queue_trylock_successes
+  doc: Number of lock (trylock) acquisition successes on completion queue event
+       queue.
+- counter: cq_ev_queue_transient_pop_failures
+  doc: Number of times NULL was popped out of completion queue's event queue
+       even though the event queue was not empty
diff --git a/src/core/lib/debug/stats_data_bq_schema.sql b/src/core/lib/debug/stats_data_bq_schema.sql
index 54869977b0..04b6d471f6 100644
--- a/src/core/lib/debug/stats_data_bq_schema.sql
+++ b/src/core/lib/debug/stats_data_bq_schema.sql
@@ -79,6 +79,10 @@ combiner_locks_initiated_per_iteration:FLOAT,
 combiner_locks_scheduled_items_per_iteration:FLOAT,
 combiner_locks_scheduled_final_items_per_iteration:FLOAT,
 combiner_locks_offloaded_per_iteration:FLOAT,
+call_combiner_locks_initiated_per_iteration:FLOAT,
+call_combiner_locks_scheduled_items_per_iteration:FLOAT,
+call_combiner_set_notify_on_cancel_per_iteration:FLOAT,
+call_combiner_cancelled_per_iteration:FLOAT,
 executor_scheduled_short_items_per_iteration:FLOAT,
 executor_scheduled_long_items_per_iteration:FLOAT,
 executor_scheduled_to_self_per_iteration:FLOAT,
@@ -86,4 +90,7 @@ executor_wakeup_initiated_per_iteration:FLOAT,
 executor_queue_drained_per_iteration:FLOAT,
 executor_push_retries_per_iteration:FLOAT,
 server_requested_calls_per_iteration:FLOAT,
-server_slowpath_requests_queued_per_iteration:FLOAT
+server_slowpath_requests_queued_per_iteration:FLOAT,
+cq_ev_queue_trylock_failures_per_iteration:FLOAT,
+cq_ev_queue_trylock_successes_per_iteration:FLOAT,
+cq_ev_queue_transient_pop_failures_per_iteration:FLOAT
diff --git a/src/core/lib/iomgr/call_combiner.cc b/src/core/lib/iomgr/call_combiner.cc
index bab3df021a..0ba32a1e46 100644
--- a/src/core/lib/iomgr/call_combiner.cc
+++ b/src/core/lib/iomgr/call_combiner.cc
@@ -21,6 +21,7 @@
 #include <inttypes.h>
 
 #include <grpc/support/log.h>
+#include "src/core/lib/debug/stats.h"
 
 grpc_tracer_flag grpc_call_combiner_trace =
     GRPC_TRACER_INITIALIZER(false, "call_combiner");
@@ -73,7 +74,9 @@ void grpc_call_combiner_start(grpc_exec_ctx* exec_ctx,
     gpr_log(GPR_DEBUG, "  size: %" PRIdPTR " -> %" PRIdPTR, prev_size,
             prev_size + 1);
   }
+  GRPC_STATS_INC_CALL_COMBINER_LOCKS_SCHEDULED_ITEMS(exec_ctx);
   if (prev_size == 0) {
+    GRPC_STATS_INC_CALL_COMBINER_LOCKS_INITIATED(exec_ctx);
     if (GRPC_TRACER_ON(grpc_call_combiner_trace)) {
       gpr_log(GPR_DEBUG, "  EXECUTING IMMEDIATELY");
     }
@@ -135,6 +138,7 @@ void grpc_call_combiner_stop(grpc_exec_ctx* exec_ctx,
 void grpc_call_combiner_set_notify_on_cancel(grpc_exec_ctx* exec_ctx,
                                              grpc_call_combiner* call_combiner,
                                              grpc_closure* closure) {
+  GRPC_STATS_INC_CALL_COMBINER_SET_NOTIFY_ON_CANCEL(exec_ctx);
   while (true) {
     // Decode original state.
     gpr_atm original_state = gpr_atm_acq_load(&call_combiner->cancel_state);
@@ -179,6 +183,7 @@ void grpc_call_combiner_set_notify_on_cancel(grpc_exec_ctx* exec_ctx,
 void grpc_call_combiner_cancel(grpc_exec_ctx* exec_ctx,
                                grpc_call_combiner* call_combiner,
                                grpc_error* error) {
+  GRPC_STATS_INC_CALL_COMBINER_CANCELLED(exec_ctx);
   while (true) {
     gpr_atm original_state = gpr_atm_acq_load(&call_combiner->cancel_state);
     grpc_error* original_error = decode_cancel_state_error(original_state);
diff --git a/src/core/lib/iomgr/combiner.cc b/src/core/lib/iomgr/combiner.cc
index 0e707ef839..53f4b7eaa7 100644
--- a/src/core/lib/iomgr/combiner.cc
+++ b/src/core/lib/iomgr/combiner.cc
@@ -165,6 +165,7 @@ static void combiner_exec(grpc_exec_ctx *exec_ctx, grpc_closure *cl,
                               lock, cl, last));
   if (last == 1) {
     GRPC_STATS_INC_COMBINER_LOCKS_INITIATED(exec_ctx);
+    GPR_TIMER_MARK("combiner.initiated", 0);
     gpr_atm_no_barrier_store(&lock->initiating_exec_ctx_or_null,
                              (gpr_atm)exec_ctx);
     // first element on this list: add it to the list of combiner locks
diff --git a/src/core/lib/iomgr/ev_posix.cc b/src/core/lib/iomgr/ev_posix.cc
index e4033fab1d..3a1dd8d30b 100644
--- a/src/core/lib/iomgr/ev_posix.cc
+++ b/src/core/lib/iomgr/ev_posix.cc
@@ -61,12 +61,43 @@ typedef struct {
   event_engine_factory_fn factory;
 } event_engine_factory;
 
+namespace {
+
+extern "C" {
+
+grpc_poll_function_type real_poll_function;
+
+int dummy_poll(struct pollfd fds[], nfds_t nfds, int timeout) {
+  if (timeout == 0) {
+    return real_poll_function(fds, nfds, 0);
+  } else {
+    gpr_log(GPR_ERROR, "Attempted a blocking poll when declared non-polling.");
+    GPR_ASSERT(false);
+    return -1;
+  }
+}
+}  // extern "C"
+
+const grpc_event_engine_vtable *init_non_polling(bool explicit_request) {
+  if (!explicit_request) {
+    return nullptr;
+  }
+  // return the simplest engine as a dummy but also override the poller
+  auto ret = grpc_init_poll_posix(explicit_request);
+  real_poll_function = grpc_poll_function;
+  grpc_poll_function = dummy_poll;
+
+  return ret;
+}
+}  // namespace
+
 static const event_engine_factory g_factories[] = {
     {"epoll1", grpc_init_epoll1_linux},
     {"epollsig", grpc_init_epollsig_linux},
     {"poll", grpc_init_poll_posix},
     {"poll-cv", grpc_init_poll_cv_posix},
     {"epollex", grpc_init_epollex_linux},
+    {"none", init_non_polling},
 };
 
 static void add(const char *beg, const char *end, char ***ss, size_t *ns) {
diff --git a/src/core/lib/iomgr/port.h b/src/core/lib/iomgr/port.h
index 42033d0ba4..1cc6d98491 100644
--- a/src/core/lib/iomgr/port.h
+++ b/src/core/lib/iomgr/port.h
@@ -109,6 +109,16 @@
 #define GRPC_POSIX_SOCKETUTILS 1
 #define GRPC_POSIX_WAKEUP_FD 1
 #define GRPC_TIMER_USE_GENERIC 1
+#elif defined(GPR_OPENBSD)
+#define GRPC_HAVE_IFADDRS 1
+#define GRPC_HAVE_IPV6_RECVPKTINFO 1
+#define GRPC_HAVE_UNIX_SOCKET 1
+#define GRPC_POSIX_NO_SPECIAL_WAKEUP_FD 1
+#define GRPC_POSIX_SOCKET 1
+#define GRPC_POSIX_SOCKETADDR 1
+#define GRPC_POSIX_SOCKETUTILS 1
+#define GRPC_POSIX_WAKEUP_FD 1
+#define GRPC_TIMER_USE_GENERIC 1
 #elif defined(GPR_NACL)
 #define GRPC_HAVE_ARPA_NAMESER 1
 #define GRPC_POSIX_NO_SPECIAL_WAKEUP_FD 1
diff --git a/src/core/lib/profiling/basic_timers.cc b/src/core/lib/profiling/basic_timers.cc
index ab9d60481c..0ae7d7f600 100644
--- a/src/core/lib/profiling/basic_timers.cc
+++ b/src/core/lib/profiling/basic_timers.cc
@@ -209,9 +209,9 @@ static void init_output() {
 
 static void rotate_log() {
   /* Using malloc here, as this code could end up being called by gpr_malloc */
-  gpr_timer_log *new = malloc(sizeof(*new));
+  gpr_timer_log *log = static_cast<gpr_timer_log *>(malloc(sizeof(*log)));
   gpr_once_init(&g_once_init, init_output);
-  new->num_entries = 0;
+  log->num_entries = 0;
   pthread_mutex_lock(&g_mu);
   if (g_thread_log != NULL) {
     timer_log_remove(&g_in_progress_logs, g_thread_log);
@@ -221,9 +221,9 @@ static void rotate_log() {
   } else {
     g_thread_id = g_next_thread_id++;
   }
-  timer_log_push_back(&g_in_progress_logs, new);
+  timer_log_push_back(&g_in_progress_logs, log);
   pthread_mutex_unlock(&g_mu);
-  g_thread_log = new;
+  g_thread_log = log;
 }
 
 static void gpr_timers_log_add(const char *tagstr, marker_type type,
diff --git a/src/core/lib/support/time_posix.cc b/src/core/lib/support/time_posix.cc
index 3267ea6b54..3f8a9094fd 100644
--- a/src/core/lib/support/time_posix.cc
+++ b/src/core/lib/support/time_posix.cc
@@ -42,7 +42,7 @@ static struct timespec timespec_from_gpr(gpr_timespec gts) {
   return rv;
 }
 
-#if _POSIX_TIMERS > 0
+#if _POSIX_TIMERS > 0 || defined(__OpenBSD__)
 static gpr_timespec gpr_from_timespec(struct timespec ts,
                                       gpr_clock_type clock_type) {
   /*
diff --git a/src/core/lib/surface/completion_queue.cc b/src/core/lib/surface/completion_queue.cc
index 36b4b835f8..21664f03c8 100644
--- a/src/core/lib/surface/completion_queue.cc
+++ b/src/core/lib/surface/completion_queue.cc
@@ -362,11 +362,24 @@ static bool cq_event_queue_push(grpc_cq_event_queue *q, grpc_cq_completion *c) {
 
 static grpc_cq_completion *cq_event_queue_pop(grpc_cq_event_queue *q) {
   grpc_cq_completion *c = NULL;
+  grpc_exec_ctx exec_ctx = GRPC_EXEC_CTX_INIT;
+
   if (gpr_spinlock_trylock(&q->queue_lock)) {
-    c = (grpc_cq_completion *)gpr_mpscq_pop(&q->queue);
+    GRPC_STATS_INC_CQ_EV_QUEUE_TRYLOCK_SUCCESSES(&exec_ctx);
+
+    bool is_empty = false;
+    c = (grpc_cq_completion *)gpr_mpscq_pop_and_check_end(&q->queue, &is_empty);
     gpr_spinlock_unlock(&q->queue_lock);
+
+    if (c == NULL && !is_empty) {
+      GRPC_STATS_INC_CQ_EV_QUEUE_TRANSIENT_POP_FAILURES(&exec_ctx);
+    }
+  } else {
+    GRPC_STATS_INC_CQ_EV_QUEUE_TRYLOCK_FAILURES(&exec_ctx);
   }
 
+  grpc_exec_ctx_finish(&exec_ctx);
+
   if (c) {
     gpr_atm_no_barrier_fetch_add(&q->num_queue_items, -1);
   }