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swayfx-mosaic/tooling/probes/step20c-buffer-identity/README.md

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STEP20C buffer identity fixture — EXPERIMENTAL

The protocol records the wlroots 0.20.2 and SceneFX 0.5 lifetime audit before this private instrumentation. Reconstruct only in a disposable lab from the published presentation fixture. Its prepare.sh starts from published Mosaic commit 006adf69d7364181917cfc266863794b6d13a65b, using pinned SwayFX fd71a6bdc061bd633b488ae7b83e8a6981d22f86, private Mosaic cd550382b311efdbfe3ed892f4e1d281d852e754, private SceneFX 37ccd723bef49e6891156ffafce8f549f01446cc, and the historical patches. The new buffer-identity.patch, SHA-256 d62e8c3a166e60a6eb394de58f51da1e37c3e7ac49671a361bd106ac64e2b427, applies after presentation-events.patch to private sway/desktop/output.c only. It changes no renderer, clipping decision, public ABI or installed library.

Rebuild and replay

Requires the presentation fixture's dependencies plus Pillow for RGB normalization. BI_OUTPUT_DIR must be a new private directory. The runner creates its own runtime, Wayland socket and explicit IPC socket. It never uses the daily session. The buffer patch caps snapshots at 80 × 1,920,000 bytes, holds at most one extra HEADLESS-1 buffer lock, and releases it on matching present, any subsequent repaint, rejected commit, timeout or output teardown. The timeout is two seconds. A second read is limited to three buffers that actually reach present while locked: visible, hidden and visible after return. Four other selected buffers in the final run were released by an instrumentation scope defect: a HEADLESS-2 repaint called global bi_release_hold("superseded") while it held a HEADLESS-1 buffer. Their later native present events do not establish retention or byte stability through presentation. The historical patch and traces preserve this negative result.

The historical five-source manifest includes the README before this scientific rectification, SHA-256 add56c3de06671fd14bbb504456f1339eaaa7d0209f11444a9cb940f3435aa59. That version is retained in the private lab as buffer-fixture-README-before-rectification.md. A fresh prepare.sh creates a manifest for the current corrected README; do not replace the historical manifest or patch hash.

prep=$(sh tooling/probes/step20c-buffer-identity/prepare.sh)
printf '%s\n' "$prep"
LAB=$(printf '%s\n' "$prep" | sed -n 's/^LAB=//p')
(cd "$LAB" && sha256sum -c presentation-source-sha256.txt && \
    sha256sum -c buffer-source-sha256.txt)
mkdir "$LAB/buffers-final"
BI_OUTPUT_DIR="$LAB/buffers-final" PYTHONDONTWRITEBYTECODE=1 \
  python tooling/probes/step20c-buffer-identity/run.py \
    "$LAB" private_mosaic buffer_cycles --cycles 3
RUN=$(find "$LAB/runs" -maxdepth 1 -type d -name 'private_mosaic-buffer_cycles-*' | sort | tail -1)
PYTHONDONTWRITEBYTECODE=1 python tooling/probes/step20c-buffer-identity/analyze.py \
  "$LAB" "$RUN" "$LAB/buffers-final"

For a timing control, repeat the same buffer_cycles invocation without BI_OUTPUT_DIR, sequentially, using the same private binary. The probe still logs stages but performs no buffer readback. For the initial logical/applied camera lag, run a separate one-pair case with BI_INCLUDE_TRANSIENT=1 and a new output directory. Pointer visible, b_cross and hidden_click scenarios can be run separately with the negative-origin fixture against this same private lab. Standard splith controls use the ordinary presentation runner, once each for control and private_standard.

The analyzer writes LAB/results/{commits,presentations,buffer-lifetime, buffer-fingerprints,scene-states,correlation,pixel-analysis,matrix,summary}.json. The raw files are XRGB8888 readback copies with stride 3200. It preserves raw SHA-256 and a top-down RGB SHA-256. The actual output-state source is a GBM DMA-BUF; the CPU data-pointer attempt returned false in the measured renderer, so the successful read is through SceneFX FX texture APIs. The pixel comparison uses the declared XRGB8888 read format and actual source/destination defaults, not an assumed source GBM format. The private patch's texture read calls SceneFX's internal glFinish(); the result is timing-sensitive and cannot be treated as a neutral observation.

Measured laboratory

/tmp/swayfx-mosaic-nested-grab-QwCf4FLm/ retains the final three-pair run, no-readback control, transient-camera run, three separate pointer runs, two splith controls, private build and copied buffers. The final three-pair run private_mosaic-buffer_cycles-1791774958990683685 had 80 native commits, 78 native presents and two initial sequence-2 commits with no present observed. All 32 selected true output buffers were read, confirmed in the synchronous native commit callback and paired to a native headless present. Twenty-eight extra locks were released on that event; four were released early by HEADLESS-2 repaint before their later present. The same owner ID 8 and one A/B map persisted through all three pairs. The three second reads (visible, hidden, visible) matched their precommit raw bytes. The transient run separately read the buffer for native HEADLESS-1 sequence 10 at logical 1760/applied -1120.

The report records the limited PARTIALLY_FEASIBLE result and measured readback timing impact. Generated builds, logs, PNGs, sockets and copied buffers remain outside Git. STEP20C is NEXT; multi-output Mosaic is NOT YET SUPPORTED.