What

Summary

Phase 2A.2 of #1074. Extends mock_h2_server_peer with an opt-in reply_mode parameter so tests can exercise the http2_client response-success path against a hermetic loopback peer. The default mode (drain_only) is unchanged.

Change Type

• Feature (test infrastructure addition)
• Bugfix
• Refactor
• Documentation
• Test

Affected Components

• tests/support/mock_h2_server_peer.{h,cpp} -- new reply_mode::echo_one path
• tests/unit/http2_client_branch_test.cpp -- new demo TEST_F
• tests/support/README.md -- usage example + Phase 2 progress update

Why

Problem Solved

http2_client::handle_headers_frame and handle_data_frame success branches were only reachable via the DISABLED_ConnectToHttpbin integration test that requires an external network. The Phase 2A mock_h2_server_peer only performed the SETTINGS exchange and then drained client frames -- http2_client::get() always timed out, so the post-SETTINGS success paths were unreachable from CI.

Related Issues

• Part of infra(test): protocol-aware loopback peers + error injection (Phase 2 of #1060) #1074 (Phase 2 of infra: hermetic transport fixture for HTTP/2 / QUIC / WebSocket / gRPC unit tests #1060: protocol-aware loopback peers + error injection)
• Unblocks coverage expansion for http2_client.cpp in test(http2): expand http2_client.cpp coverage to 80% line / 70% branch #1062

Where

Files Changed

API Changes

• mock_h2_server_peer constructor gains a defaulted reply_mode mode = reply_mode::drain_only parameter (backward compatible -- all existing callers compile unchanged)
• New public getters: request_received(), response_sent(), last_request_stream_id()

How

Implementation Details

1. New reply_mode enum with two values: drain_only (Phase 2A behavior, default) and echo_one (Phase 2A.2).
2. After the SETTINGS exchange, run() checks the mode. In echo_one:
   • Loop reading client frames until a HEADERS frame opens a stream and END_STREAM is observed (either on HEADERS or on a subsequent DATA frame on the same stream_id).
   • Send response HEADERS frame with :status: 200 (HPACK indexed header field 0x88, RFC 7541 Appendix A static-table index 8) on the same stream_id.
   • Send response DATA frame with body "ok" and END_STREAM set.
   • Set request_received_ and response_sent_ atomic flags.
3. The drain loop after the mode-specific block continues to absorb GOAWAY/PING frames emitted during disconnect().

Testing Done

• Local build verification -- toolchain unavailable in dev environment; relying on CI
• Static review of frame_type / frame_flags / frame_header members against src/internal/protocols/http2/frame.h
• Backward compatibility verified by inspection: every existing mock_h2_server_peer peer(io()) call retains the Phase 2A drain_only behavior via the default argument

Test Plan for Reviewers

1. CI must pass on Ubuntu / macOS / Windows (build, sanitizers, ctest)
2. New test Http2ClientHermeticTransportTest.GetSucceedsWhenPeerRepliesWithHeadersAndData should pass
3. All existing Phase 2A tests in http2_client_branch_test.cpp and grpc_client_branch_test.cpp should continue to pass unchanged

Breaking Changes

None -- reply_mode parameter has a default value matching prior behavior.