CNA-BUG-186: CNA_ENABLE_VIDEO=AUTO accepts any pkg-config FFmpeg, but VideoDecoder.cpp needs the FFmpeg 5.1 channel-layout API

CNA snapshot 009d40f5  ·  Known Issues › Current bugs  ·  source links pinned to 009d40f5

The FFmpeg probe states no minimum version while VideoDecoder.cpp uses AVCodecContext::ch_layout and swr_alloc_set_opts2, so a host with older FFmpeg development packages enables video at configure time and then fails to compile.

Identifier

CNA-BUG-186

Category

Bug

Subsystem

Build & CI

Status

Open

Verified against

CNA 009d40f5 (009d40f5dd085c4e674d3479675fac84b12b3e0a)

Severity

Low (a triage suggestion, not a project priority)

Evidence basis

Inferred (strong): follows from the code, but the behaviour was not run

Tests touching this area

Yes: see Current tests

Affected contract

CMake option CNA_ENABLE_VIDEO (AUTO, the default, and ON) and the cna_video_ffmpeg target

Expected behaviour

modules/CMakeLists.txt documents AUTO as “use FFmpeg only when all required pkg-config modules are present” and ON as reporting an unsupported configuration immediately. Both rely on the probe telling a usable FFmpeg from an unusable one, so an FFmpeg too old for the sources should make AUTO fall back and ON fail at configure time with a clear message.

Actual behaviour at TARGET

modules/CMakeLists.txt calls pkg_check_modules for the four libraries without a version constraint. VideoDecoder.cpp reads AVCodecContext::ch_layout and builds its resampler with swr_alloc_set_opts2, both part of the channel-layout API introduced with FFmpeg 5.1, with no LIBAVCODEC_VERSION_INT fallback. On a host whose pkg-config finds FFmpeg 4.x development packages (still the packaged version on older LTS distributions), AUTO sets CNA_FFMPEG_AVAILABLE and the build then fails compiling cna_video_ffmpeg instead of falling back to the no-video backend. No CNA document names a minimum FFmpeg version, and every CI lane that installs FFmpeg uses a recent one (ubuntu-24.04 runners, Homebrew on macOS).

Source locations

• modules/CMakeLists.txt — the CNA_ENABLE_VIDEO probe: pkg_check_modules without versions
• modules/video-ffmpeg/src/VideoDecoder.cpp — VideoDecoder::Open and OpenAudioStreamByIndex (ch_layout); the resampler setup (swr_alloc_set_opts2)
• modules/video-ffmpeg/CMakeLists.txt — cna_video_ffmpeg links the PkgConfig imported targets
• .github/workflows/general-tests-ci.yml — ubuntu-24.04 runner with the distribution's FFmpeg packages

Evidence

Checked by reading at 009d40f5; not tried against an old FFmpeg. The API ages are FFmpeg's (the AVChannelLayout fields and swr_alloc_set_opts2 arrived in 5.1); a compile failure with FFmpeg 4.x follows from the missing declarations but was not observed.

Focused reproduction

Not run. On a host with FFmpeg 4.x development packages: cmake -S . -B build reports “video enabled (AUTO; FFmpeg backend cna_video_ffmpeg)”, and cmake --build build fails in VideoDecoder.cpp on ch_layout and swr_alloc_set_opts2.

Current tests

No configure test covers the FFmpeg probe's version handling; the video tests run only where a recent FFmpeg is installed.

Regression test

Add version floors to the probe (for example libswresample>=4.5 and the matching libavcodec release) or a check_symbol_exists(swr_alloc_set_opts2 ...), and a script-mode CMake test that feeds a fake old .pc file and expects AUTO to fall back and ON to fail.

Blast radius

Default (AUTO) builds on hosts with FFmpeg older than 5.1 installed; they fail instead of building without video. Hosts without FFmpeg, and Windows, Emscripten, Android and iOS (where the probe is skipped), are unaffected.

Workaround

Configure with -DCNA_ENABLE_VIDEO=OFF, or install FFmpeg 5.1 or newer.