Fix race condition in transport API

Specifically:

Receiving trailing and initial metadata had to be published in
lock-step.
=> If we wanted trailing metadata, we might not get initial metadata processed
   until messages arrived.
=> Compression code had no idea what codec to use.

To fix it, publish initial metadata as soon as it's ready (this is a
transport API change).

Requires changes to grpc_call to ensure ordering in processing initial
metadata and messages (one may be delayed).

Exposed at least some bugs in C++ where we never read initial metadata.

I expect at least one more similar bug.

1 files changed, 4 insertions, 1 deletions

diff --git a/src/core/transport/transport.h b/src/core/transport/transport.h
index f5cac77adc..8902c5d2f6 100644
--- a/src/core/transport/transport.h
+++ b/src/core/transport/transport.h
@@ -92,6 +92,8 @@ typedef struct grpc_transport_stream_op {
 
   /** Receive initial metadata from the stream, into provided metadata batch. */
   grpc_metadata_batch *recv_initial_metadata;
+  /** Should be enqueued when initial metadata is ready to be processed. */
+  grpc_closure *recv_initial_metadata_ready;
 
   /** Receive message data from the stream, into provided byte stream. */
   grpc_byte_stream **recv_message;
@@ -103,7 +105,8 @@ typedef struct grpc_transport_stream_op {
   grpc_metadata_batch *recv_trailing_metadata;
 
   /** Should be enqueued when all requested operations (excluding recv_message
-     which has its own closure) in a given batch have been completed. */
+      and recv_initial_metadata which have their own closures) in a given batch
+      have been completed. */
   grpc_closure *on_complete;
 
   /** If != GRPC_STATUS_OK, cancel this stream */