- Author: alishananda
- Approver: ctiller
- Status: In Review
- Implemented in: C++
- Last updated: 2021-08-27
- Discussion at: https://groups.google.com/g/grpc-io/c/UNBHGr6feeY
There is a race between ServerContext::IsCancelled and Read in the sync API where a call is cancelled and Read returns false but IsCancelled also returns false. We need to ensure we propagate the cancellation to IsCancelled so it always returns true if Read returns false and the call was cancelled.
As part of this fix, we have to move grpc_call_failed_before_recv_message into the C-core surface API so the implementation of MaybeMarkCancelledOnRead can be inlined and found by proto_library.
A user reported that after cancelling a call, IsCancelled would sometimes return false when Read would also return false.
We saw this same issue with the callback API, where the server couldn't tell whether OnReadDone returned false because the stream was cancelled or actually closed cleanly. This PR added a core subsurface grpc_call_failed_before_recv_message function in src/core/lib/surface/call.h and MaybeMarkCancelledOnRead function depending on this value in src/cpp/server/server_context.cc. Together, these distinguish a cancellation from a cleanly closed stream by marking places in the transport where OnReadDone will return an empty message because of a stream failure and using grpc_call_failed_before_recv_message to propagate this information out of the grpc_call struct.
N/A
The proposal is to move grpc_call_failed_before_recv_message from subsurface to the C-core surface API.
It seems that the build structure of the callback API is different than that of the sync API. When using MaybeMarkCancelledOnRead with the Sync API, the proto library returns a linker error that it cannot find the function because the proto library does not depend on the file implementing the function. We thus have to inline MaybeMarkCancelledOnRead in server_context.h, which will mean adding the grpc_call_failed_before_recv_message function to our public API.
We will move grpc_call_failed_before_recv_message out of call.h and into the C-core surface API.
The PR with this implementation is grpc/grpc#27056.