What

Round 2 of protocols/http2/http2_client.cpp test coverage expansion using the Phase 2 substrate (frame_injector + mock_h2_server_peer) merged in PR #1105 (Part of #1074). Round 1 sub-issues #991 and #1062 closed but the file still stands at 18.8% line / 9.9% branch in the 2026-05-06 coverage measurement (run 25430202846).

This sub-issue narrows scope to error-branch coverage specifically, leveraging the fault-injection infrastructure that did not exist when Round 1 was opened.

Why

http2_client.cpp is the single largest contributor to the #953 coverage gap:

• Line gap: 576 LOC * (60% - 18.8%) = ~237 additional hit lines
• Branch gap: 979 branches * (40% - 9.9%) = ~295 additional hit branches

Without raising it, the epic cannot meet its acceptance criteria. Phase 2 substrate (#1074) was specifically designed to enable deterministic error-path testing of the HTTP/2 client by giving tests control over server-side frame timing and integrity.

Where

How

Acceptance criteria

• http2_client.cpp line coverage >= 60.0% (current 18.8%, +41.2pp)
• http2_client.cpp branch coverage >= 40.0% (current 9.9%, +30.1pp)
• At least 6 new TEST_F cases driving previously-unreachable error branches via frame_injector:
  • connect timeout via injection_mode::drop on first server SETTINGS
  • malformed SETTINGS-ACK via injection_mode::malform (flip type byte)
  • truncated server SETTINGS payload via injection_mode::truncate
  • slow-write byte-by-byte via injection_mode::slow_write
  • partial-response disconnect (HEADERS sent, DATA dropped)
  • WINDOW_UPDATE handling under back-pressure
• All new tests pass under Debug and Release builds
• No src/ changes; test-only PR

Test plan for reviewers

cmake --build build --target network_http2_client_branch_test -j 4
./build/bin/network_http2_client_branch_test
# Expect: all TEST_F PASS

Out of scope

• HTTP/2 frame parsing (protocols/http2/frame.cpp already at 98.4%)
• HPACK (protocols/http2/hpack.cpp already at 91.3%)
• Performance / load testing

Related

• Parent epic: Part of Expand unit test coverage from 40% to 80% #953
• Substrate provider: Part of infra(test): protocol-aware loopback peers + error injection (Phase 2 of #1060) #1074 (Phase 2E PR test(support): add frame_injector for protocol-aware fault injection (Phase 2E of #1074) #1105)
• Round 1 closed without hitting target: test: expand coverage for src/protocols/http2/http2_client.cpp #991, test(http2): expand http2_client.cpp coverage to 80% line / 70% branch #1062