src/core/lib/transport/metadata.h

/*
 *
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 * in the documentation and/or other materials provided with the
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#ifndef GRPC_CORE_LIB_TRANSPORT_METADATA_H
#define GRPC_CORE_LIB_TRANSPORT_METADATA_H

#include <grpc/slice.h>
#include <grpc/support/useful.h>

#include "src/core/lib/iomgr/exec_ctx.h"

#ifdef __cplusplus
extern "C" {
#endif

/* 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_mdelem grpc_mdelem;

/* if changing this, make identical changes in internal_metadata in
   metadata.c */
struct grpc_mdelem {
  const grpc_slice key;
  const grpc_slice value;
  /* there is a private part to this in metadata.c */
};

/* Unrefs the slices. */
grpc_mdelem *grpc_mdelem_from_slices(grpc_exec_ctx *exec_ctx, grpc_slice key,
                                     grpc_slice value);

size_t grpc_mdelem_get_size_in_hpack_table(grpc_mdelem *elem);

/* 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 */
//#define GRPC_METADATA_REFCOUNT_DEBUG
#ifdef GRPC_METADATA_REFCOUNT_DEBUG
#define GRPC_MDELEM_REF(s) grpc_mdelem_ref((s), __FILE__, __LINE__)
#define GRPC_MDELEM_UNREF(exec_ctx, s) \
  grpc_mdelem_unref((exec_ctx), (s), __FILE__, __LINE__)
grpc_mdelem *grpc_mdelem_ref(grpc_mdelem *md, const char *file, int line);
void grpc_mdelem_unref(grpc_exec_ctx *exec_ctx, grpc_mdelem *md,
                       const char *file, int line);
#else
#define GRPC_MDELEM_REF(s) grpc_mdelem_ref((s))
#define GRPC_MDELEM_UNREF(exec_ctx, s) grpc_mdelem_unref((exec_ctx), (s))
grpc_mdelem *grpc_mdelem_ref(grpc_mdelem *md);
void grpc_mdelem_unref(grpc_exec_ctx *exec_ctx, grpc_mdelem *md);
#endif

#define GRPC_MDSTR_LENGTH(s) (GRPC_SLICE_LENGTH(s->slice))

/* We add 32 bytes of padding as per RFC-7540 section 6.5.2. */
#define GRPC_MDELEM_LENGTH(e) \
  (GRPC_MDSTR_LENGTH((e)->key) + GRPC_MDSTR_LENGTH((e)->value) + 32)

#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(grpc_exec_ctx *exec_ctx);

#ifdef __cplusplus
}
#endif

#endif /* GRPC_CORE_LIB_TRANSPORT_METADATA_H */