fix(ffmpeg-patches): libvmaf_sycl filter — free sycl_state on close + NULL-guard QSV chain

[lusoris]

Summary

Two verified defects from the adversarial round-4 audit in the dedicated libvmaf_sycl FFmpeg filter added by ffmpeg-patches/0005.

sev	fix
high	sycl_state leak — uninit_sycl called vmaf_close() but never vmaf_sycl_state_free(). vmaf_close() explicitly does not free the SYCL state (ownership isn't transferred), so the state + its USM allocations leaked on every filter close. Freed explicitly now; false comment removed.
med	QSV NULL deref — do_vmaf_sycl walked the QSV zero-copy chain (data[3] → mfxFrameSurface1* → Data.MemId → mfxHDLPair* → ->first) with no NULL guards; a software-fallback QSV surface (no VA backing) crashed it. All three links NULL-checked → AVERROR(EINVAL) + diagnostic.

Deferred: finding #21 (a redundant but idempotent check_pkg_config libvmaf_sycl configure probe) is intentionally left — removing the wrong one risks breaking SYCL detection for no correctness gain.

Reproducer / verification

# Full series replay against the pinned base (CLAUDE rule #14):
git -C /path/to/ffmpeg-8 reset --hard n8.1.1 && git -C ffmpeg-8 clean -fdq
while read p; do p="${p%%#*}"; [ -z "${p// }" ] && continue; \
  git -C ffmpeg-8 apply --3way "ffmpeg-patches/$p" || break; done < ffmpeg-patches/series.txt
# → all 16 patches apply CLEAN (verified)

The patch was regenerated surgically — only the two + blocks in the vf_libvmaf.c hunk + the hunk-header recount (335→353); the configure/Makefile/allfilters hunks are byte-unchanged (a full format-patch/am --3way regeneration was rejected because it fuzzed the configure probe >= 3.0.0→2.0.0).

Deep-dive deliverables (ADR-0108)

• Research digest — no digest needed: audit-derived bug-fix (audit in .workingdir2/BUGHUNT_2026-06-27.md).
• Decision matrix — no alternatives: free-after-close + NULL-guard are the only correct fixes; feat(meta): VMAFX Phase 4b distributed platform umbrella ADR #21 deferred with rationale.
• AGENTS.md invariant — the "keep these two + blocks; regenerate surgically not via format-patch" invariant is in this PR's docs/rebase-notes.md entry.
• Reproducer — series-replay command above.
• Changelog fragment — changelog.d/fixed/round4-ffmpeg-patches-audit.md.
• Rebase-notes entry — docs/rebase-notes.md.

state.md / golden / patch-stack rule

• docs/state.md — T-ROUND4-FFMPEG-PATCHES-2026-06-27 Recently-closed row.
• CLAUDE rule feat(meta): VMAFX Phase 4 language-modernization foundation #14 satisfied — series-replay against n8.1.1 is CLEAN (16/16 patches).
• Netflix golden unaffected (ffmpeg integration patch, not the metric path).
• No new user surface. no docs needed: bug fix in an existing ffmpeg filter patch, no user-visible delta.

🤖 Generated with Claude Code