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, 2 insertions, 2 deletions

diff --git a/src/core/surface/server.c b/src/core/surface/server.c
index 42cffccb4c..fb5e0d4b9e 100644
--- a/src/core/surface/server.c
+++ b/src/core/surface/server.c
@@ -596,8 +596,8 @@ static void server_mutate_op(grpc_call_element *elem,
 
   if (op->recv_initial_metadata != NULL) {
     calld->recv_initial_metadata = op->recv_initial_metadata;
-    calld->on_done_recv_initial_metadata = op->on_complete;
-    op->on_complete = &calld->server_on_recv_initial_metadata;
+    calld->on_done_recv_initial_metadata = op->recv_initial_metadata_ready;
+    op->recv_initial_metadata_ready = &calld->server_on_recv_initial_metadata;
   }
 }