CH2 interned metadata key parsing fastpath.

on_initial_header() and on_trailing_header() are called by the hpack parser. The
hpack parser interns the key elements for metadata that it parses. Thus, the
called functions don't need to do a slowpath slice value check when performing
metadata key comparisons.

src/core/ext/transport/chttp2/transport/parsing.cc

@@ -394,6 +394,32 @@ error_handler:
 
 static void free_timeout(void* p) { gpr_free(p); }
 
+static bool md_key_cmp(grpc_mdelem md, const grpc_slice& reference) {
+  GPR_DEBUG_ASSERT(grpc_slice_is_interned(GRPC_MDKEY(md)));
+  return GRPC_MDKEY(md).refcount == reference.refcount;
+}
+
+static bool md_cmp(grpc_mdelem md, grpc_mdelem ref_md,
+                   const grpc_slice& ref_key) {
+  if (GPR_LIKELY(GRPC_MDELEM_IS_INTERNED(md))) {
+    return md.payload == ref_md.payload;
+  }
+  if (md_key_cmp(md, ref_key)) {
+    return grpc_slice_eq_static_interned(GRPC_MDVALUE(md),
+                                         GRPC_MDVALUE(ref_md));
+  }
+  return false;
+}
+
+static bool is_nonzero_status(grpc_mdelem md) {
+  // If md.payload == GRPC_MDELEM_GRPC_STATUS_1 or GRPC_MDELEM_GRPC_STATUS_2,
+  // then we have seen an error. In fact, if it is a GRPC_STATUS and it's
+  // not equal to GRPC_MDELEM_GRPC_STATUS_0, then we have seen an error.
+  // TODO(ctiller): check for a status like " 0"
+  return md_key_cmp(md, GRPC_MDSTR_GRPC_STATUS) &&
+         !md_cmp(md, GRPC_MDELEM_GRPC_STATUS_0, GRPC_MDSTR_GRPC_STATUS);
+}
+
 static void on_initial_header(void* tp, grpc_mdelem md) {
   GPR_TIMER_SCOPE("on_initial_header", 0);
 
@@ -411,15 +437,9 @@ static void on_initial_header(void* tp, grpc_mdelem md) {
     gpr_free(value);
   }
 
-  // If md.payload == GRPC_MDELEM_GRPC_STATUS_1 or GRPC_MDELEM_GRPC_STATUS_2,
-  // then we have seen an error. In fact, if it is a GRPC_STATUS and it's
-  // not equal to GRPC_MDELEM_GRPC_STATUS_0, then we have seen an error.
-  if (grpc_slice_eq_static_interned(GRPC_MDKEY(md), GRPC_MDSTR_GRPC_STATUS) &&
-      !grpc_mdelem_static_value_eq(md, GRPC_MDELEM_GRPC_STATUS_0)) {
-    /* TODO(ctiller): check for a status like " 0" */
+  if (is_nonzero_status(md)) {  // not GRPC_MDELEM_GRPC_STATUS_0?
     s->seen_error = true;
-  } else if (grpc_slice_eq_static_interned(GRPC_MDKEY(md),
-                                           GRPC_MDSTR_GRPC_TIMEOUT)) {
+  } else if (md_key_cmp(md, GRPC_MDSTR_GRPC_TIMEOUT)) {
     grpc_millis* cached_timeout =
         static_cast<grpc_millis*>(grpc_mdelem_get_user_data(md, free_timeout));
     grpc_millis timeout;
@@ -496,12 +516,7 @@ static void on_trailing_header(void* tp, grpc_mdelem md) {
     gpr_free(value);
   }
 
-  // If md.payload == GRPC_MDELEM_GRPC_STATUS_1 or GRPC_MDELEM_GRPC_STATUS_2,
-  // then we have seen an error. In fact, if it is a GRPC_STATUS and it's
-  // not equal to GRPC_MDELEM_GRPC_STATUS_0, then we have seen an error.
-  if (grpc_slice_eq_static_interned(GRPC_MDKEY(md), GRPC_MDSTR_GRPC_STATUS) &&
-      !grpc_mdelem_static_value_eq(md, GRPC_MDELEM_GRPC_STATUS_0)) {
-    /* TODO(ctiller): check for a status like " 0" */
+  if (is_nonzero_status(md)) {  // not GRPC_MDELEM_GRPC_STATUS_0?
     s->seen_error = true;
   }