Everything compiles...

src/core/lib/channel/http_server_filter.c

@@ -197,7 +197,7 @@ static grpc_error *server_filter_incoming_metadata(grpc_exec_ctx *exec_ctx,
   if (b->idx.named.host != NULL) {
     add_error(error_name, &error,
               grpc_metadata_batch_substitute(
-                  b, b->idx.named.host,
+                  exec_ctx, b, b->idx.named.host,
                   grpc_mdelem_from_slices(
                       exec_ctx, GRPC_MDSTR_AUTHORITY,
                       grpc_slice_ref(GRPC_MDVALUE(b->idx.named.host->md)))));

src/core/lib/surface/call.c

@@ -940,8 +940,9 @@ static void publish_app_metadata(grpc_call *call, grpc_metadata_batch *b,
   grpc_metadata_array *dest;
   grpc_metadata *mdusr;
   dest = call->buffered_metadata[is_trailing];
-  if (dest->count + b->count > dest->capacity) {
-    dest->capacity = GPR_MAX(dest->capacity + b->count, dest->capacity * 3 / 2);
+  if (dest->count + b->list.count > dest->capacity) {
+    dest->capacity =
+        GPR_MAX(dest->capacity + b->list.count, dest->capacity * 3 / 2);
     dest->metadata =
         gpr_realloc(dest->metadata, sizeof(grpc_metadata) * dest->capacity);
   }

src/core/lib/transport/metadata_batch.c

@@ -52,13 +52,16 @@ static void assert_valid_list(grpc_mdelem_list *list) {
   GPR_ASSERT(list->tail->next == NULL);
   GPR_ASSERT((list->head == list->tail) == (list->head->next == NULL));
 
+  size_t verified_count = 0;
   for (l = list->head; l; l = l->next) {
     GPR_ASSERT(!GRPC_MDISNULL(l->md));
     GPR_ASSERT((l->prev == NULL) == (l == list->head));
     GPR_ASSERT((l->next == NULL) == (l == list->tail));
     if (l->next) GPR_ASSERT(l->next->prev == l);
     if (l->prev) GPR_ASSERT(l->prev->next == l);
+    verified_count++;
   }
+  GPR_ASSERT(list->count == verified_count);
 #endif /* NDEBUG */
 }
 
@@ -154,6 +157,7 @@ static void link_head(grpc_mdelem_list *list, grpc_linked_mdelem *storage) {
     list->tail = storage;
   }
   list->head = storage;
+  list->count++;
   assert_valid_list(list);
 }
 
@@ -219,6 +223,7 @@ static void unlink_storage(grpc_mdelem_list *list,
   } else {
     list->tail = storage->prev;
   }
+  list->count--;
   assert_valid_list(list);
 }
 
@@ -230,6 +235,33 @@ void grpc_metadata_batch_remove(grpc_metadata_batch *batch,
   assert_valid_callouts(batch);
 }
 
+void grpc_metadata_batch_set_value(grpc_exec_ctx *exec_ctx,
+                                   grpc_linked_mdelem *storage,
+                                   grpc_slice value) {
+  grpc_mdelem old = storage->md;
+  grpc_mdelem new =
+      grpc_mdelem_from_slices(exec_ctx, grpc_slice_ref(GRPC_MDKEY(old)), value);
+  storage->md = new;
+  GRPC_MDELEM_UNREF(exec_ctx, old);
+}
+
+grpc_error *grpc_metadata_batch_substitute(grpc_exec_ctx *exec_ctx,
+                                           grpc_metadata_batch *batch,
+                                           grpc_linked_mdelem *storage,
+                                           grpc_mdelem new) {
+  grpc_error *error = GRPC_ERROR_NONE;
+  grpc_mdelem old = storage->md;
+  if (!grpc_slice_eq(GRPC_MDKEY(new), GRPC_MDKEY(old))) {
+    maybe_unlink_callout(batch, storage);
+    storage->md = new;
+    error = maybe_link_callout(batch, storage);
+  } else {
+    storage->md = new;
+  }
+  GRPC_MDELEM_UNREF(exec_ctx, old);
+  return error;
+}
+
 void grpc_metadata_batch_clear(grpc_exec_ctx *exec_ctx,
                                grpc_metadata_batch *batch) {
   grpc_metadata_batch_destroy(exec_ctx, batch);

src/core/lib/transport/metadata_batch.h

@@ -55,13 +55,12 @@ typedef struct grpc_linked_mdelem {
 } grpc_linked_mdelem;
 
 typedef struct grpc_mdelem_list {
+  size_t count;
   grpc_linked_mdelem *head;
   grpc_linked_mdelem *tail;
 } grpc_mdelem_list;
 
 typedef struct grpc_metadata_batch {
-  /* number of elements in the batch */
-  size_t count;
   /** Metadata elements in this batch */
   grpc_mdelem_list list;
   grpc_metadata_batch_callouts idx;
@@ -86,7 +85,8 @@ void grpc_metadata_batch_remove(grpc_metadata_batch *batch,
                                 grpc_linked_mdelem *storage);
 
 /** Substitute a new mdelem for an old value */
-grpc_error *grpc_metadata_batch_substitute(grpc_metadata_batch *batch,
+grpc_error *grpc_metadata_batch_substitute(grpc_exec_ctx *exec_ctx,
+                                           grpc_metadata_batch *batch,
                                            grpc_linked_mdelem *storage,
                                            grpc_mdelem new_value);
 

src/cpp/common/channel_filter.cc

@@ -49,7 +49,8 @@ grpc_linked_mdelem *MetadataBatch::AddMetadata(grpc_exec_ctx *exec_ctx,
   memset(storage, 0, sizeof(grpc_linked_mdelem));
   storage->md = grpc_mdelem_from_slices(exec_ctx, SliceFromCopiedString(key),
                                         SliceFromCopiedString(value));
-  grpc_metadata_batch_link_head(batch_, storage);
+  GRPC_LOG_IF_ERROR("MetadataBatch::AddMetadata",
+                    grpc_metadata_batch_link_head(batch_, storage));
   return storage;
 }