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fegger ce52f64e52 feat(capture): implement Phase 3 PipeWire/portal desktop capture

Implement the xdg-desktop-portal ScreenCast backend via libportal: a
blocking portal handshake (interactive source picker), a PipeWire stream
on the portal's node enumerating BGRx/BGRA/RGBx/RGBA, and a latest-frame
slot handing frames to next_frame(). stop() is thread-safe; teardown
follows the order PipeWire requires. All proxy operations run under the
thread-loop lock to satisfy the protocol extension context checks
('impl_ext_end_proxy called from wrong context' otherwise).

The encoder now accepts padded strides for packed RGB inputs (real
PipeWire row pitches) and maps the new PixelFormat::Bgrx to
AV_PIX_FMT_BGRA.

Add tools/capture_smoke: a manual smoke tool (interactive, not in
meson test) that captures N frames, encodes them, and writes a
self-contained Annex-B elementary stream with prepended SPS/PPS.

Validated manually on Wayland/Hyprland: 2256x1504 H.264 elementary
stream, ffprobe clean. Phase 3 marked complete in docs/PHASES.md.

2026-09-07 10:37:14 +02:00

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Runbook — build and validation commands

Reusable validation steps for this repository. Run the relevant ones before considering a change complete.

Build and test

meson setup build          # once
meson compile -C build
meson test -C build --print-errorlogs

Memory and correctness checks (codec changes)

Run the codec round-trip test under valgrind after touching the encoder or decoder:

valgrind --leak-check=full --errors-for-leak-kinds=definite \
    build/tests/test_codec_roundtrip

Expected: no "definitely lost" bytes and no "Invalid read/write" errors. FFmpeg may keep some "still reachable" allocations at exit; that is normal.

This check caught two real defects in the Phase 2 decoder: a per-packet payload leak and an unpadded extradata buffer over-read. Keep using it.

Capture smoke test (Phase 3, manual)

Requires a running desktop session with xdg-desktop-portal and a backend that implements the ScreenCast portal (e.g. Hyprland, GNOME, KDE). The portal shows an interactive source picker, so this cannot run unattended.

meson compile -C build                       # builds tools/capture_smoke
./build/tools/capture_smoke 10 out.h264      # captures 10 frames
ffprobe -v error -show_entries stream=codec_name,width,height out.h264

Expected: the tool prints the negotiated resolution/format/stride and writes a non-empty file; ffprobe reports codec_name=h264 and the correct width/height. The encoder prepends its Annex-B SPS/PPS extradata so the file is a self-contained elementary stream.

If you see impl_ext_end_proxy called from wrong context warnings, a PipeWire proxy operation ran without the thread-loop lock — all pw calls that send messages (connect, stream, core) must happen under pw_thread_loop_lock.

Formatting

find include src tests -type f \( -name '*.cpp' -o -name '*.h' \) \
    -exec clang-format --dry-run --Werror {} +

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