fix(timeline): isolate preview renders from concurrent exports#38
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Intermediate segment/text/fold filenames were deterministic (tl-seg-<index>-<clip>), so a preview frame render and a concurrent export wrote and ffmpeg -y-overwrote the SAME files — and each unlinked the other's in run-plan's finally — yielding a transient garbled frame or 500. The timeline doc (source of truth) was never touched, so it self-healed on retry, but it is a real race on the unauthenticated localhost dev server. Add an optional tag to CompileContext, mixed into the intermediate names. Export stays deterministic (no tag); the clip ui sets a per-request tag on each frame render (next commit), so previews get their own paths and cannot collide. Default (untagged) paths are unchanged — existing plans/tests are untouched.
- The frame route passes a per-request tag to buildFrameAtPlan, so a preview's intermediate segments never collide with a concurrent export's. - Frame and export routes translate a non-CompileError (a transient FFmpeg failure) into a clean 503 instead of a raw 500. - Viewer fetches the frame as a blob behind an AbortController, so a superseded scrub is cancelled instead of left rendering, and a 503 reads as a message rather than a broken img. The previous frame stays on screen until the next decodes — no blank flash.
This was referenced Jun 19, 2026
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Fix concurrent frame/export render collisions
Closes the last v0.3.0 pre-release-audit follow-up (non-blocking). On the localhost
clip ui, scrubbing the viewer during an export could produce a transient garbled frame or a raw 500.Root cause
compile.tsnamed intermediate segments deterministically —tl-seg-<index>-<clip>.mp4— with no random component (unlikenewOutputPath).run-plan.tsrunsffmpeg -ythen unlinks intermediates in afinally, and nothing serializes the UI render path. So a preview frame render and a concurrent export wrote/-y-overwrote the same segment files and unlinked each other's → a transient 500 or garbled frame. Self-healing on retry, and the timeline document (source of truth) is never touched — hence low severity — but a real race on the unauthenticated dev server.Fix
CompileContextgains an optionaltagmixed into intermediate (segment/text/fold) filenames. Export stays deterministic (no tag); the frame route sets a per-requesttag = frame-<rand>, so previews get their own paths and can't collide — with an export or with each other. Default (untagged) paths are unchanged, so existing plans and tests are untouched.CompileError(a transient FFmpeg failure) into a clean 503 instead of a raw 500.CompileError(a real doc problem) stays 422.Viewerfetches the frame as a blob behind anAbortController, so a superseded scrub is cancelled instead of left rendering server-side, and a 503 reads as a message rather than a broken<img>. The previous frame stays on screen until the next decodes — no blank flash.Verification
pnpm type-check✓ · 543 tests ✓ (new: tagged context → collision-free segment path, untagged unchanged) ·pnpm lintexit 0 ·pnpm build✓ (Vite compiles the newViewer).🤖 Generated with Claude Code