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+/*
+ *
+ * Copyright 2015, Google Inc.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google Inc. nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ */
+
+#ifndef GRPC_CORE_LIB_TRANSPORT_METADATA_H
+#define GRPC_CORE_LIB_TRANSPORT_METADATA_H
+
+#include <grpc/support/slice.h>
+#include <grpc/support/useful.h>
+
+/* This file provides a mechanism for tracking metadata through the grpc stack.
+   It's not intended for consumption outside of the library.
+
+   Metadata is tracked in the context of a grpc_mdctx. For the time being there
+   is one of these per-channel, avoiding cross channel interference with memory
+   use and lock contention.
+
+   The context tracks unique strings (grpc_mdstr) and pairs of strings
+   (grpc_mdelem). Any of these objects can be checked for equality by comparing
+   their pointers. These objects are reference counted.
+
+   grpc_mdelem can additionally store a (non-NULL) user data pointer. This
+   pointer is intended to be used to cache semantic meaning of a metadata
+   element. For example, an OAuth token may cache the credentials it represents
+   and the time at which it expires in the mdelem user data.
+
+   Combining this metadata cache and the hpack compression table allows us to
+   simply lookup complete preparsed objects quickly, incurring a few atomic
+   ops per metadata element on the fast path.
+
+   grpc_mdelem instances MAY live longer than their refcount implies, and are
+   garbage collected periodically, meaning cached data can easily outlive a
+   single request.
+
+   STATIC METADATA: in static_metadata.h we declare a set of static metadata.
+   These mdelems and mdstrs are available via pre-declared code generated macros
+   and are available to code anywhere between grpc_init() and grpc_shutdown().
+   They are not refcounted, but can be passed to _ref and _unref functions
+   declared here - in which case those functions are effectively no-ops. */
+
+/* Forward declarations */
+typedef struct grpc_mdstr grpc_mdstr;
+typedef struct grpc_mdelem grpc_mdelem;
+
+/* if changing this, make identical changes in internal_string in metadata.c */
+struct grpc_mdstr {
+  const gpr_slice slice;
+  const uint32_t hash;
+  /* there is a private part to this in metadata.c */
+};
+
+/* if changing this, make identical changes in internal_metadata in
+   metadata.c */
+struct grpc_mdelem {
+  grpc_mdstr *const key;
+  grpc_mdstr *const value;
+  /* there is a private part to this in metadata.c */
+};
+
+void grpc_test_only_set_metadata_hash_seed(uint32_t seed);
+
+/* Constructors for grpc_mdstr instances; take a variety of data types that
+   clients may have handy */
+grpc_mdstr *grpc_mdstr_from_string(const char *str);
+/* Unrefs the slice. */
+grpc_mdstr *grpc_mdstr_from_slice(gpr_slice slice);
+grpc_mdstr *grpc_mdstr_from_buffer(const uint8_t *str, size_t length);
+
+/* Returns a borrowed slice from the mdstr with its contents base64 encoded
+   and huffman compressed */
+gpr_slice grpc_mdstr_as_base64_encoded_and_huffman_compressed(grpc_mdstr *str);
+
+/* Constructors for grpc_mdelem instances; take a variety of data types that
+   clients may have handy */
+grpc_mdelem *grpc_mdelem_from_metadata_strings(grpc_mdstr *key,
+                                               grpc_mdstr *value);
+grpc_mdelem *grpc_mdelem_from_strings(const char *key, const char *value);
+/* Unrefs the slices. */
+grpc_mdelem *grpc_mdelem_from_slices(gpr_slice key, gpr_slice value);
+grpc_mdelem *grpc_mdelem_from_string_and_buffer(const char *key,
+                                                const uint8_t *value,
+                                                size_t value_length);
+
+/* Mutator and accessor for grpc_mdelem user data. The destructor function
+   is used as a type tag and is checked during user_data fetch. */
+void *grpc_mdelem_get_user_data(grpc_mdelem *md,
+                                void (*if_destroy_func)(void *));
+void grpc_mdelem_set_user_data(grpc_mdelem *md, void (*destroy_func)(void *),
+                               void *user_data);
+
+/* Reference counting */
+#ifdef GRPC_METADATA_REFCOUNT_DEBUG
+#define GRPC_MDSTR_REF(s) grpc_mdstr_ref((s), __FILE__, __LINE__)
+#define GRPC_MDSTR_UNREF(s) grpc_mdstr_unref((s), __FILE__, __LINE__)
+#define GRPC_MDELEM_REF(s) grpc_mdelem_ref((s), __FILE__, __LINE__)
+#define GRPC_MDELEM_UNREF(s) grpc_mdelem_unref((s), __FILE__, __LINE__)
+grpc_mdstr *grpc_mdstr_ref(grpc_mdstr *s, const char *file, int line);
+void grpc_mdstr_unref(grpc_mdstr *s, const char *file, int line);
+grpc_mdelem *grpc_mdelem_ref(grpc_mdelem *md, const char *file, int line);
+void grpc_mdelem_unref(grpc_mdelem *md, const char *file, int line);
+#else
+#define GRPC_MDSTR_REF(s) grpc_mdstr_ref((s))
+#define GRPC_MDSTR_UNREF(s) grpc_mdstr_unref((s))
+#define GRPC_MDELEM_REF(s) grpc_mdelem_ref((s))
+#define GRPC_MDELEM_UNREF(s) grpc_mdelem_unref((s))
+grpc_mdstr *grpc_mdstr_ref(grpc_mdstr *s);
+void grpc_mdstr_unref(grpc_mdstr *s);
+grpc_mdelem *grpc_mdelem_ref(grpc_mdelem *md);
+void grpc_mdelem_unref(grpc_mdelem *md);
+#endif
+
+/* Recover a char* from a grpc_mdstr. The returned string is null terminated.
+   Does not promise that the returned string has no embedded nulls however. */
+const char *grpc_mdstr_as_c_string(grpc_mdstr *s);
+
+#define GRPC_MDSTR_LENGTH(s) (GPR_SLICE_LENGTH(s->slice))
+
+int grpc_mdstr_is_legal_header(grpc_mdstr *s);
+int grpc_mdstr_is_legal_nonbin_header(grpc_mdstr *s);
+int grpc_mdstr_is_bin_suffixed(grpc_mdstr *s);
+
+#define GRPC_MDSTR_KV_HASH(k_hash, v_hash) (GPR_ROTL((k_hash), 2) ^ (v_hash))
+
+void grpc_mdctx_global_init(void);
+void grpc_mdctx_global_shutdown(void);
+
+#endif /* GRPC_CORE_LIB_TRANSPORT_METADATA_H */