Fix character classes to updated spec

src/core/transport/metadata.c

@@ -695,11 +695,9 @@ static int conforms_to(grpc_mdstr *s, const gpr_uint8 *legal_bits) {
 
 int grpc_mdstr_is_legal_header(grpc_mdstr *s) {
   static const gpr_uint8 legal_header_bits[256 / 8] = {
-      0x00, 0x00, 0x00, 0x00, 0x00, 0x20, 0xff, 0x03, 0xfe, 0xff, 0xff,
+      0x00, 0x00, 0x00, 0x00, 0x00, 0x20, 0xff, 0x03, 0x00, 0x00, 0x00,
-      0x07, 0xfe, 0xff, 0xff, 0x07, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+      0x80, 0xfe, 0xff, 0xff, 0x07, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
       0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00};
-
-  /* TODO(ctiller): consider caching this, or computing it on construction */
   return conforms_to(s, legal_header_bits);
 }
 

tools/codegen/core/gen_legal_metadata_characters.c

@@ -60,9 +60,9 @@ int main(void) {
 
   clear();
   for (i = 'a'; i <= 'z'; i++) legal(i);
-  for (i = 'A'; i <= 'Z'; i++) legal(i);
   for (i = '0'; i <= '9'; i++) legal(i);
   legal('-');
+  legal('_');
   dump();
 
   clear();