aboutsummaryrefslogtreecommitdiffhomepage
path: root/src/core/lib/transport/metadata.c
blob: d358bea76ffbea5f1ffc5332008c340af1944f0b (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
/*
 *
 * 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.
 *
 */

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

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

#include <grpc/compression.h>
#include <grpc/grpc.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/ext/transport/chttp2/transport/bin_encoder.h"
#include "src/core/lib/iomgr/iomgr_internal.h"
#include "src/core/lib/profiling/timers.h"
#include "src/core/lib/support/murmur_hash.h"
#include "src/core/lib/support/string.h"
#include "src/core/lib/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);

#define SIZE_IN_DECODER_TABLE_NOT_SET -1
/* 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;

  gpr_atm size_in_decoder_table;

  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);
      if (grpc_iomgr_abort_on_leaks()) {
        abort();
      }
    }
    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);
      if (grpc_iomgr_abort_on_leaks()) {
        abort();
      }
    }
    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->size_in_decoder_table = SIZE_IN_DECODER_TABLE_NOT_SET;
  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));
}

static size_t get_base64_encoded_size(size_t raw_length) {
  static const uint8_t tail_xtra[3] = {0, 2, 3};
  return raw_length / 3 * 4 + tail_xtra[raw_length % 3];
}

size_t grpc_mdelem_get_size_in_hpack_table(grpc_mdelem *elem) {
  size_t overhead_and_key = 32 + GPR_SLICE_LENGTH(elem->key->slice);
  size_t value_len = GPR_SLICE_LENGTH(elem->value->slice);
  if (is_mdstr_static(elem->value)) {
    if (grpc_is_binary_header(
            (const char *)GPR_SLICE_START_PTR(elem->key->slice),
            GPR_SLICE_LENGTH(elem->key->slice))) {
      return overhead_and_key + get_base64_encoded_size(value_len);
    } else {
      return overhead_and_key + value_len;
    }
  } else {
    internal_string *is = (internal_string *)elem->value;
    gpr_atm current_size = gpr_atm_acq_load(&is->size_in_decoder_table);
    if (current_size == SIZE_IN_DECODER_TABLE_NOT_SET) {
      if (grpc_is_binary_header(
              (const char *)GPR_SLICE_START_PTR(elem->key->slice),
              GPR_SLICE_LENGTH(elem->key->slice))) {
        current_size = (gpr_atm)get_base64_encoded_size(value_len);
      } else {
        current_size = (gpr_atm)value_len;
      }
      gpr_atm_rel_store(&is->size_in_decoder_table, current_size);
    }
    return overhead_and_key + (size_t)current_size;
  }
}

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;
}