Force receiving a SETTINGS frame as the first frame

src/core/ext/transport/chttp2/transport/chttp2_transport.c

@@ -264,6 +264,7 @@ static void init_transport(grpc_exec_ctx *exec_ctx, grpc_chttp2_transport *t,
   t->parsing.is_client = is_client;
   t->parsing.deframe_state =
       is_client ? GRPC_DTS_FH_0 : GRPC_DTS_CLIENT_PREFIX_0;
+  t->parsing.is_first_frame = true;
   t->writing.is_client = is_client;
   t->optional_drop_message = gpr_empty_slice();
   grpc_connectivity_state_init(
@@ -1805,7 +1806,7 @@ static void post_reading_action_locked(grpc_exec_ctx *exec_ctx,
     UNREF_TRANSPORT(exec_ctx, t, "reading_action");
   }
 
-  GRPC_ERROR_UNREF(error);
+  GRPC_LOG_IF_ERROR("close_transport", error);
 }
 
 /*******************************************************************************

src/core/ext/transport/chttp2/transport/internal.h

@@ -265,6 +265,7 @@ struct grpc_chttp2_transport_parsing {
   uint8_t incoming_frame_type;
   uint8_t incoming_frame_flags;
   uint8_t header_eof;
+  bool is_first_frame;
   uint32_t expect_continuation_stream_id;
   uint32_t incoming_frame_size;
   uint32_t incoming_stream_id;

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

@@ -468,6 +468,17 @@ grpc_error *grpc_chttp2_perform_read(
 
 static grpc_error *init_frame_parser(
     grpc_exec_ctx *exec_ctx, grpc_chttp2_transport_parsing *transport_parsing) {
+  if (transport_parsing->is_first_frame &&
+      transport_parsing->incoming_frame_type != GRPC_CHTTP2_FRAME_SETTINGS) {
+    char *msg;
+    gpr_asprintf(
+        &msg, "Expected SETTINGS frame as the first frame, got frame type %d",
+        transport_parsing->incoming_frame_type);
+    grpc_error *err = GRPC_ERROR_CREATE(msg);
+    gpr_free(msg);
+    return err;
+  }
+  transport_parsing->is_first_frame = false;
   if (transport_parsing->expect_continuation_stream_id != 0) {
     if (transport_parsing->incoming_frame_type !=
         GRPC_CHTTP2_FRAME_CONTINUATION) {