third_party/grpc/src/core/transport/metadata.c

/*
 *
 * Copyright 2015-2016, 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.
 *
 */

#include "src/core/transport/metadata.h"

#include <assert.h>
#include <stddef.h>
#include <string.h>

#include <grpc/compression.h>
#include <grpc/support/alloc.h>
#include <grpc/support/atm.h>
#include <grpc/support/log.h>
#include <grpc/support/string_util.h>
#include <grpc/support/time.h>
#include "src/core/profiling/timers.h"
#include "src/core/support/murmur_hash.h"
#include "src/core/support/string.h"
#include "src/core/transport/chttp2/bin_encoder.h"
#include "src/core/transport/static_metadata.h"

/* There are two kinds of mdelem and mdstr instances.
 * Static instances are declared in static_metadata.{h,c} and
 * are initialized by grpc_mdctx_global_init().
 * Dynamic instances are stored in hash tables on grpc_mdctx, and are backed
 * by internal_string and internal_element structures.
 * Internal helper functions here-in (is_mdstr_static, is_mdelem_static) are
 * used to determine which kind of element a pointer refers to.
 */

#define INITIAL_STRTAB_CAPACITY 4
#define INITIAL_MDTAB_CAPACITY 4

#ifdef GRPC_METADATA_REFCOUNT_DEBUG
#define DEBUG_ARGS , const char *file, int line
#define FWD_DEBUG_ARGS , file, line
#define REF_MD_LOCKED(shard, s) ref_md_locked((shard), (s), __FILE__, __LINE__)
#else
#define DEBUG_ARGS
#define FWD_DEBUG_ARGS
#define REF_MD_LOCKED(shard, s) ref_md_locked((shard), (s))
#endif

#define TABLE_IDX(hash, log2_shards, capacity) \
  (((hash) >> (log2_shards)) % (capacity))
#define SHARD_IDX(hash, log2_shards) ((hash) & ((1 << (log2_shards)) - 1))

typedef void (*destroy_user_data_func)(void *user_data);

/* Shadow structure for grpc_mdstr for non-static values */
typedef struct internal_string {
  /* must be byte compatible with grpc_mdstr */
  gpr_slice slice;
  uint32_t hash;

  /* private only data */
  gpr_atm refcnt;

  uint8_t has_base64_and_huffman_encoded;
  gpr_slice_refcount refcount;

  gpr_slice base64_and_huffman;

  struct internal_string *bucket_next;
} internal_string;

/* Shadow structure for grpc_mdelem for non-static elements */
typedef struct internal_metadata {
  /* must be byte compatible with grpc_mdelem */
  internal_string *key;
  internal_string *value;

  /* private only data */
  gpr_atm refcnt;

  gpr_mu mu_user_data;
  gpr_atm destroy_user_data;
  gpr_atm user_data;

  struct internal_metadata *bucket_next;
} internal_metadata;

typedef struct strtab_shard {
  gpr_mu mu;
  internal_string **strs;
  size_t count;
  size_t capacity;
} strtab_shard;

typedef struct mdtab_shard {
  gpr_mu mu;
  internal_metadata **elems;
  size_t count;
  size_t capacity;
  size_t free;
} mdtab_shard;

#define LOG2_STRTAB_SHARD_COUNT 5
#define LOG2_MDTAB_SHARD_COUNT 4
#define STRTAB_SHARD_COUNT ((size_t)(1 << LOG2_STRTAB_SHARD_COUNT))
#define MDTAB_SHARD_COUNT ((size_t)(1 << LOG2_MDTAB_SHARD_COUNT))

/* hash seed: decided at initialization time */
static uint32_t g_hash_seed;
static int g_forced_hash_seed = 0;

/* linearly probed hash tables for static element lookup */
static grpc_mdstr *g_static_strtab[GRPC_STATIC_MDSTR_COUNT * 2];
static grpc_mdelem *g_static_mdtab[GRPC_STATIC_MDELEM_COUNT * 2];
static size_t g_static_strtab_maxprobe;
static size_t g_static_mdtab_maxprobe;

static strtab_shard g_strtab_shard[STRTAB_SHARD_COUNT];
static mdtab_shard g_mdtab_shard[MDTAB_SHARD_COUNT];

static void gc_mdtab(mdtab_shard *shard);

void grpc_test_only_set_metadata_hash_seed(uint32_t seed) {
  g_hash_seed = seed;
  g_forced_hash_seed = 1;
}

void grpc_mdctx_global_init(void) {
  size_t i, j;
  if (!g_forced_hash_seed) {
    g_hash_seed = (uint32_t)gpr_now(GPR_CLOCK_REALTIME).tv_nsec;
  }
  g_static_strtab_maxprobe = 0;
  g_static_mdtab_maxprobe = 0;
  /* build static tables */
  memset(g_static_mdtab, 0, sizeof(g_static_mdtab));
  memset(g_static_strtab, 0, sizeof(g_static_strtab));
  for (i = 0; i < GRPC_STATIC_MDSTR_COUNT; i++) {
    grpc_mdstr *elem = &grpc_static_mdstr_table[i];
    const char *str = grpc_static_metadata_strings[i];
    uint32_t hash = gpr_murmur_hash3(str, strlen(str), g_hash_seed);
    *(gpr_slice *)&elem->slice = gpr_slice_from_static_string(str);
    *(uint32_t *)&elem->hash = hash;
    for (j = 0;; j++) {
      size_t idx = (hash + j) % GPR_ARRAY_SIZE(g_static_strtab);
      if (g_static_strtab[idx] == NULL) {
        g_static_strtab[idx] = &grpc_static_mdstr_table[i];
        break;
      }
    }
    if (j > g_static_strtab_maxprobe) {
      g_static_strtab_maxprobe = j;
    }
  }
  for (i = 0; i < GRPC_STATIC_MDELEM_COUNT; i++) {
    grpc_mdelem *elem = &grpc_static_mdelem_table[i];
    grpc_mdstr *key =
        &grpc_static_mdstr_table[grpc_static_metadata_elem_indices[2 * i + 0]];
    grpc_mdstr *value =
        &grpc_static_mdstr_table[grpc_static_metadata_elem_indices[2 * i + 1]];
    uint32_t hash = GRPC_MDSTR_KV_HASH(key->hash, value->hash);
    *(grpc_mdstr **)&elem->key = key;
    *(grpc_mdstr **)&elem->value = value;
    for (j = 0;; j++) {
      size_t idx = (hash + j) % GPR_ARRAY_SIZE(g_static_mdtab);
      if (g_static_mdtab[idx] == NULL) {
        g_static_mdtab[idx] = elem;
        break;
      }
    }
    if (j > g_static_mdtab_maxprobe) {
      g_static_mdtab_maxprobe = j;
    }
  }
  /* initialize shards */
  for (i = 0; i < STRTAB_SHARD_COUNT; i++) {
    strtab_shard *shard = &g_strtab_shard[i];
    gpr_mu_init(&shard->mu);
    shard->count = 0;
    shard->capacity = INITIAL_STRTAB_CAPACITY;
    shard->strs = gpr_malloc(sizeof(*shard->strs) * shard->capacity);
    memset(shard->strs, 0, sizeof(*shard->strs) * shard->capacity);
  }
  for (i = 0; i < MDTAB_SHARD_COUNT; i++) {
    mdtab_shard *shard = &g_mdtab_shard[i];
    gpr_mu_init(&shard->mu);
    shard->count = 0;
    shard->free = 0;
    shard->capacity = INITIAL_MDTAB_CAPACITY;
    shard->elems = gpr_malloc(sizeof(*shard->elems) * shard->capacity);
    memset(shard->elems, 0, sizeof(*shard->elems) * shard->capacity);
  }
}

void grpc_mdctx_global_shutdown(void) {
  size_t i;
  for (i = 0; i < MDTAB_SHARD_COUNT; i++) {
    mdtab_shard *shard = &g_mdtab_shard[i];
    gpr_mu_destroy(&shard->mu);
    gc_mdtab(shard);
    /* TODO(ctiller): GPR_ASSERT(shard->count == 0); */
    if (shard->count != 0) {
      gpr_log(GPR_DEBUG, "WARNING: %d metadata elements were leaked",
              shard->count);
    }
    gpr_free(shard->elems);
  }
  for (i = 0; i < STRTAB_SHARD_COUNT; i++) {
    strtab_shard *shard = &g_strtab_shard[i];
    gpr_mu_destroy(&shard->mu);
    /* TODO(ctiller): GPR_ASSERT(shard->count == 0); */
    if (shard->count != 0) {
      gpr_log(GPR_DEBUG, "WARNING: %d metadata strings were leaked",
              shard->count);
    }
    gpr_free(shard->strs);
  }
}

static int is_mdstr_static(grpc_mdstr *s) {
  return s >= &grpc_static_mdstr_table[0] &&
         s < &grpc_static_mdstr_table[GRPC_STATIC_MDSTR_COUNT];
}

static int is_mdelem_static(grpc_mdelem *e) {
  return e >= &grpc_static_mdelem_table[0] &&
         e < &grpc_static_mdelem_table[GRPC_STATIC_MDELEM_COUNT];
}

static void ref_md_locked(mdtab_shard *shard,
                          internal_metadata *md DEBUG_ARGS) {
#ifdef GRPC_METADATA_REFCOUNT_DEBUG
  gpr_log(file, line, GPR_LOG_SEVERITY_DEBUG,
          "ELM   REF:%p:%d->%d: '%s' = '%s'", md,
          gpr_atm_no_barrier_load(&md->refcnt),
          gpr_atm_no_barrier_load(&md->refcnt) + 1,
          grpc_mdstr_as_c_string((grpc_mdstr *)md->key),
          grpc_mdstr_as_c_string((grpc_mdstr *)md->value));
#endif
  if (0 == gpr_atm_no_barrier_fetch_add(&md->refcnt, 2)) {
    shard->free--;
  } else {
    GPR_ASSERT(1 != gpr_atm_no_barrier_fetch_add(&md->refcnt, -1));
  }
}

static void grow_strtab(strtab_shard *shard) {
  size_t capacity = shard->capacity * 2;
  size_t i;
  internal_string **strtab;
  internal_string *s, *next;

  GPR_TIMER_BEGIN("grow_strtab", 0);

  strtab = gpr_malloc(sizeof(internal_string *) * capacity);
  memset(strtab, 0, sizeof(internal_string *) * capacity);

  for (i = 0; i < shard->capacity; i++) {
    for (s = shard->strs[i]; s; s = next) {
      size_t idx = TABLE_IDX(s->hash, LOG2_STRTAB_SHARD_COUNT, capacity);
      next = s->bucket_next;
      s->bucket_next = strtab[idx];
      strtab[idx] = s;
    }
  }

  gpr_free(shard->strs);
  shard->strs = strtab;
  shard->capacity = capacity;

  GPR_TIMER_END("grow_strtab", 0);
}

static void internal_destroy_string(strtab_shard *shard, internal_string *is) {
  internal_string **prev_next;
  internal_string *cur;
  GPR_TIMER_BEGIN("internal_destroy_string", 0);
  if (is->has_base64_and_huffman_encoded) {
    gpr_slice_unref(is->base64_and_huffman);
  }
  for (prev_next = &shard->strs[TABLE_IDX(is->hash, LOG2_STRTAB_SHARD_COUNT,
                                          shard->capacity)],
      cur = *prev_next;
       cur != is; prev_next = &cur->bucket_next, cur = cur->bucket_next)
    ;
  *prev_next = cur->bucket_next;
  shard->count--;
  gpr_free(is);
  GPR_TIMER_END("internal_destroy_string", 0);
}

static void slice_ref(void *p) {
  internal_string *is =
      (internal_string *)((char *)p - offsetof(internal_string, refcount));
  GRPC_MDSTR_REF((grpc_mdstr *)(is));
}

static void slice_unref(void *p) {
  internal_string *is =
      (internal_string *)((char *)p - offsetof(internal_string, refcount));
  GRPC_MDSTR_UNREF((grpc_mdstr *)(is));
}

grpc_mdstr *grpc_mdstr_from_string(const char *str) {
  return grpc_mdstr_from_buffer((const uint8_t *)str, strlen(str));
}

grpc_mdstr *grpc_mdstr_from_slice(gpr_slice slice) {
  grpc_mdstr *result = grpc_mdstr_from_buffer(GPR_SLICE_START_PTR(slice),
                                              GPR_SLICE_LENGTH(slice));
  gpr_slice_unref(slice);
  return result;
}

grpc_mdstr *grpc_mdstr_from_buffer(const uint8_t *buf, size_t length) {
  uint32_t hash = gpr_murmur_hash3(buf, length, g_hash_seed);
  internal_string *s;
  strtab_shard *shard =
      &g_strtab_shard[SHARD_IDX(hash, LOG2_STRTAB_SHARD_COUNT)];
  size_t i;
  size_t idx;

  GPR_TIMER_BEGIN("grpc_mdstr_from_buffer", 0);

  /* search for a static string */
  for (i = 0; i <= g_static_strtab_maxprobe; i++) {
    grpc_mdstr *ss;
    idx = (hash + i) % GPR_ARRAY_SIZE(g_static_strtab);
    ss = g_static_strtab[idx];
    if (ss == NULL) break;
    if (ss->hash == hash && GPR_SLICE_LENGTH(ss->slice) == length &&
        0 == memcmp(buf, GPR_SLICE_START_PTR(ss->slice), length)) {
      GPR_TIMER_END("grpc_mdstr_from_buffer", 0);
      return ss;
    }
  }

  gpr_mu_lock(&shard->mu);

  /* search for an existing string */
  idx = TABLE_IDX(hash, LOG2_STRTAB_SHARD_COUNT, shard->capacity);
  for (s = shard->strs[idx]; s; s = s->bucket_next) {
    if (s->hash == hash && GPR_SLICE_LENGTH(s->slice) == length &&
        0 == memcmp(buf, GPR_SLICE_START_PTR(s->slice), length)) {
      GRPC_MDSTR_REF((grpc_mdstr *)s);
      gpr_mu_unlock(&shard->mu);
      GPR_TIMER_END("grpc_mdstr_from_buffer", 0);
      return (grpc_mdstr *)s;
    }
  }

  /* not found: create a new string */
  if (length + 1 < GPR_SLICE_INLINED_SIZE) {
    /* string data goes directly into the slice */
    s = gpr_malloc(sizeof(internal_string));
    gpr_atm_rel_store(&s->refcnt, 2);
    s->slice.refcount = NULL;
    memcpy(s->slice.data.inlined.bytes, buf, length);
    s->slice.data.inlined.bytes[length] = 0;
    s->slice.data.inlined.length = (uint8_t)length;
  } else {
    /* string data goes after the internal_string header, and we +1 for null
       terminator */
    s = gpr_malloc(sizeof(internal_string) + length + 1);
    gpr_atm_rel_store(&s->refcnt, 2);
    s->refcount.ref = slice_ref;
    s->refcount.unref = slice_unref;
    s->slice.refcount = &s->refcount;
    s->slice.data.refcounted.bytes = (uint8_t *)(s + 1);
    s->slice.data.refcounted.length = length;
    memcpy(s->slice.data.refcounted.bytes, buf, length);
    /* add a null terminator for cheap c string conversion when desired */
    s->slice.data.refcounted.bytes[length] = 0;
  }
  s->has_base64_and_huffman_encoded = 0;
  s->hash = hash;
  s->bucket_next = shard->strs[idx];
  shard->strs[idx] = s;

  shard->count++;

  if (shard->count > shard->capacity * 2) {
    grow_strtab(shard);
  }

  gpr_mu_unlock(&shard->mu);
  GPR_TIMER_END("grpc_mdstr_from_buffer", 0);

  return (grpc_mdstr *)s;
}

static void gc_mdtab(mdtab_shard *shard) {
  size_t i;
  internal_metadata **prev_next;
  internal_metadata *md, *next;

  GPR_TIMER_BEGIN("gc_mdtab", 0);
  for (i = 0; i < shard->capacity; i++) {
    prev_next = &shard->elems[i];
    for (md = shard->elems[i]; md; md = next) {
      void *user_data = (void *)gpr_atm_no_barrier_load(&md->user_data);
      next = md->bucket_next;
      if (gpr_atm_acq_load(&md->refcnt) == 0) {
        GRPC_MDSTR_UNREF((grpc_mdstr *)md->key);
        GRPC_MDSTR_UNREF((grpc_mdstr *)md->value);
        if (md->user_data) {
          ((destroy_user_data_func)gpr_atm_no_barrier_load(
              &md->destroy_user_data))(user_data);
        }
        gpr_free(md);
        *prev_next = next;
        shard->free--;
        shard->count--;
      } else {
        prev_next = &md->bucket_next;
      }
    }
  }
  GPR_TIMER_END("gc_mdtab", 0);
}

static void grow_mdtab(mdtab_shard *shard) {
  size_t capacity = shard->capacity * 2;
  size_t i;
  internal_metadata **mdtab;
  internal_metadata *md, *next;
  uint32_t hash;

  GPR_TIMER_BEGIN("grow_mdtab", 0);

  mdtab = gpr_malloc(sizeof(internal_metadata *) * capacity);
  memset(mdtab, 0, sizeof(internal_metadata *) * capacity);

  for (i = 0; i < shard->capacity; i++) {
    for (md = shard->elems[i]; md; md = next) {
      size_t idx;
      hash = GRPC_MDSTR_KV_HASH(md->key->hash, md->value->hash);
      next = md->bucket_next;
      idx = TABLE_IDX(hash, LOG2_MDTAB_SHARD_COUNT, capacity);
      md->bucket_next = mdtab[idx];
      mdtab[idx] = md;
    }
  }

  gpr_free(shard->elems);
  shard->elems = mdtab;
  shard->capacity = capacity;

  GPR_TIMER_END("grow_mdtab", 0);
}

static void rehash_mdtab(mdtab_shard *shard) {
  if (shard->free > shard->capacity / 4) {
    gc_mdtab(shard);
  } else {
    grow_mdtab(shard);
  }
}

grpc_mdelem *grpc_mdelem_from_metadata_strings(grpc_mdstr *mkey,
                                               grpc_mdstr *mvalue) {
  internal_string *key = (internal_string *)mkey;
  internal_string *value = (internal_string *)mvalue;
  uint32_t hash = GRPC_MDSTR_KV_HASH(mkey->hash, mvalue->hash);
  internal_metadata *md;
  mdtab_shard *shard = &g_mdtab_shard[SHARD_IDX(hash, LOG2_MDTAB_SHARD_COUNT)];
  size_t i;
  size_t idx;

  GPR_TIMER_BEGIN("grpc_mdelem_from_metadata_strings", 0);

  if (is_mdstr_static(mkey) && is_mdstr_static(mvalue)) {
    for (i = 0; i <= g_static_mdtab_maxprobe; i++) {
      grpc_mdelem *smd;
      idx = (hash + i) % GPR_ARRAY_SIZE(g_static_mdtab);
      smd = g_static_mdtab[idx];
      if (smd == NULL) break;
      if (smd->key == mkey && smd->value == mvalue) {
        GPR_TIMER_END("grpc_mdelem_from_metadata_strings", 0);
        return smd;
      }
    }
  }

  gpr_mu_lock(&shard->mu);

  idx = TABLE_IDX(hash, LOG2_MDTAB_SHARD_COUNT, shard->capacity);
  /* search for an existing pair */
  for (md = shard->elems[idx]; md; md = md->bucket_next) {
    if (md->key == key && md->value == value) {
      REF_MD_LOCKED(shard, md);
      GRPC_MDSTR_UNREF((grpc_mdstr *)key);
      GRPC_MDSTR_UNREF((grpc_mdstr *)value);
      gpr_mu_unlock(&shard->mu);
      GPR_TIMER_END("grpc_mdelem_from_metadata_strings", 0);
      return (grpc_mdelem *)md;
    }
  }

  /* not found: create a new pair */
  md = gpr_malloc(sizeof(internal_metadata));
  gpr_atm_rel_store(&md->refcnt, 2);
  md->key = key;
  md->value = value;
  md->user_data = 0;
  md->destroy_user_data = 0;
  md->bucket_next = shard->elems[idx];
  shard->elems[idx] = md;
  gpr_mu_init(&md->mu_user_data);
#ifdef GRPC_METADATA_REFCOUNT_DEBUG
  gpr_log(GPR_DEBUG, "ELM   NEW:%p:%d: '%s' = '%s'", md,
          gpr_atm_no_barrier_load(&md->refcnt),
          grpc_mdstr_as_c_string((grpc_mdstr *)md->key),
          grpc_mdstr_as_c_string((grpc_mdstr *)md->value));
#endif
  shard->count++;

  if (shard->count > shard->capacity * 2) {
    rehash_mdtab(shard);
  }

  gpr_mu_unlock(&shard->mu);

  GPR_TIMER_END("grpc_mdelem_from_metadata_strings", 0);

  return (grpc_mdelem *)md;
}

grpc_mdelem *grpc_mdelem_from_strings(const char *key, const char *value) {
  return grpc_mdelem_from_metadata_strings(grpc_mdstr_from_string(key),
                                           grpc_mdstr_from_string(value));
}

grpc_mdelem *grpc_mdelem_from_slices(gpr_slice key, gpr_slice value) {
  return grpc_mdelem_from_metadata_strings(grpc_mdstr_from_slice(key),
                                           grpc_mdstr_from_slice(value));
}

grpc_mdelem *grpc_mdelem_from_string_and_buffer(const char *key,
                                                const uint8_t *value,
                                                size_t value_length) {
  return grpc_mdelem_from_metadata_strings(
      grpc_mdstr_from_string(key), grpc_mdstr_from_buffer(value, value_length));
}

grpc_mdelem *grpc_mdelem_ref(grpc_mdelem *gmd DEBUG_ARGS) {
  internal_metadata *md = (internal_metadata *)gmd;
  if (is_mdelem_static(gmd)) return gmd;
#ifdef GRPC_METADATA_REFCOUNT_DEBUG
  gpr_log(file, line, GPR_LOG_SEVERITY_DEBUG,
          "ELM   REF:%p:%d->%d: '%s' = '%s'", md,
          gpr_atm_no_barrier_load(&md->refcnt),
          gpr_atm_no_barrier_load(&md->refcnt) + 1,
          grpc_mdstr_as_c_string((grpc_mdstr *)md->key),
          grpc_mdstr_as_c_string((grpc_mdstr *)md->value));
#endif
  /* we can assume the ref count is >= 1 as the application is calling
     this function - meaning that no adjustment to mdtab_free is necessary,
     simplifying the logic here to be just an atomic increment */
  /* use C assert to have this removed in opt builds */
  assert(gpr_atm_no_barrier_load(&md->refcnt) >= 2);
  gpr_atm_no_barrier_fetch_add(&md->refcnt, 1);
  return gmd;
}

void grpc_mdelem_unref(grpc_mdelem *gmd DEBUG_ARGS) {
  internal_metadata *md = (internal_metadata *)gmd;
  if (!md) return;
  if (is_mdelem_static(gmd)) return;
#ifdef GRPC_METADATA_REFCOUNT_DEBUG
  gpr_log(file, line, GPR_LOG_SEVERITY_DEBUG,
          "ELM UNREF:%p:%d->%d: '%s' = '%s'", md,
          gpr_atm_no_barrier_load(&md->refcnt),
          gpr_atm_no_barrier_load(&md->refcnt) - 1,
          grpc_mdstr_as_c_string((grpc_mdstr *)md->key),
          grpc_mdstr_as_c_string((grpc_mdstr *)md->value));
#endif
  if (2 == gpr_atm_full_fetch_add(&md->refcnt, -1)) {
    uint32_t hash = GRPC_MDSTR_KV_HASH(md->key->hash, md->value->hash);
    mdtab_shard *shard =
        &g_mdtab_shard[SHARD_IDX(hash, LOG2_MDTAB_SHARD_COUNT)];
    GPR_TIMER_BEGIN("grpc_mdelem_unref.to_zero", 0);
    gpr_mu_lock(&shard->mu);
    if (1 == gpr_atm_no_barrier_load(&md->refcnt)) {
      shard->free++;
      gpr_atm_no_barrier_store(&md->refcnt, 0);
    }
    gpr_mu_unlock(&shard->mu);
    GPR_TIMER_END("grpc_mdelem_unref.to_zero", 0);
  }
}

const char *grpc_mdstr_as_c_string(grpc_mdstr *s) {
  return (const char *)GPR_SLICE_START_PTR(s->slice);
}

grpc_mdstr *grpc_mdstr_ref(grpc_mdstr *gs DEBUG_ARGS) {
  internal_string *s = (internal_string *)gs;
  if (is_mdstr_static(gs)) return gs;
  GPR_ASSERT(gpr_atm_full_fetch_add(&s->refcnt, 1) != 0);
  return gs;
}

void grpc_mdstr_unref(grpc_mdstr *gs DEBUG_ARGS) {
  internal_string *s = (internal_string *)gs;
  if (is_mdstr_static(gs)) return;
  if (2 == gpr_atm_full_fetch_add(&s->refcnt, -1)) {
    strtab_shard *shard =
        &g_strtab_shard[SHARD_IDX(s->hash, LOG2_STRTAB_SHARD_COUNT)];
    gpr_mu_lock(&shard->mu);
    if (1 == gpr_atm_no_barrier_load(&s->refcnt)) {
      internal_destroy_string(shard, s);
    }
    gpr_mu_unlock(&shard->mu);
  }
}

void *grpc_mdelem_get_user_data(grpc_mdelem *md, void (*destroy_func)(void *)) {
  internal_metadata *im = (internal_metadata *)md;
  void *result;
  if (is_mdelem_static(md)) {
    return (void *)grpc_static_mdelem_user_data[md - grpc_static_mdelem_table];
  }
  if (gpr_atm_acq_load(&im->destroy_user_data) == (gpr_atm)destroy_func) {
    return (void *)gpr_atm_no_barrier_load(&im->user_data);
  } else {
    return NULL;
  }
  return result;
}

void grpc_mdelem_set_user_data(grpc_mdelem *md, void (*destroy_func)(void *),
                               void *user_data) {
  internal_metadata *im = (internal_metadata *)md;
  GPR_ASSERT(!is_mdelem_static(md));
  GPR_ASSERT((user_data == NULL) == (destroy_func == NULL));
  gpr_mu_lock(&im->mu_user_data);
  if (gpr_atm_no_barrier_load(&im->destroy_user_data)) {
    /* user data can only be set once */
    gpr_mu_unlock(&im->mu_user_data);
    if (destroy_func != NULL) {
      destroy_func(user_data);
    }
    return;
  }
  gpr_atm_no_barrier_store(&im->user_data, (gpr_atm)user_data);
  gpr_atm_rel_store(&im->destroy_user_data, (gpr_atm)destroy_func);
  gpr_mu_unlock(&im->mu_user_data);
}

gpr_slice grpc_mdstr_as_base64_encoded_and_huffman_compressed(grpc_mdstr *gs) {
  internal_string *s = (internal_string *)gs;
  gpr_slice slice;
  strtab_shard *shard =
      &g_strtab_shard[SHARD_IDX(s->hash, LOG2_STRTAB_SHARD_COUNT)];
  gpr_mu_lock(&shard->mu);
  if (!s->has_base64_and_huffman_encoded) {
    s->base64_and_huffman =
        grpc_chttp2_base64_encode_and_huffman_compress(s->slice);
    s->has_base64_and_huffman_encoded = 1;
  }
  slice = s->base64_and_huffman;
  gpr_mu_unlock(&shard->mu);
  return slice;
}