Fix a crash generating thumbnails for self-hosted videos

[jkmassel]

Summary

• Fixes an intermittent SIGSEGV in the video-thumbnail async bridge. It surfaces as a hard crash of MediaImageServiceTests.testSmallThumbnailForRemoteSelfHostedVideo, and this bridge runs in production every time the app generates a thumbnail for a self-hosted video — so it is a shipping bug, not just a test artifact.
• The test is retained deliberately as the regression guard: it exercises the real AVFoundation path and is the only thing that caught this.

Root Cause

Self-hosted videos have no remoteThumbnailURL, so the app generates the thumbnail from the video itself via live AVFoundation: MediaImageService.generateThumbnailForVideo → MediaThumbnailExporter.exportThumbnail(forVideoURL:) → MediaVideoExporter.exportPreviewImageForVideo.

The callback-to-async bridge was unsafe in two compounding ways:

1. Unsafe continuation, resumed more than once. The bridge used withUnsafeThrowingContinuation. AVAssetImageGenerator.generateCGImagesAsynchronously can invoke its completion handler a second time (a .cancelled callback on teardown, or racing the cancellation handler), resuming the same continuation twice. A double-resume of an unsafe continuation is undefined behavior — a raw SIGSEGV — instead of the deterministic trap a checked continuation gives.
2. Progress mutated after the resume. In exportPreviewImageForVideo the image export is synchronous, so exporter.export(onCompletion:onError:) resumes the continuation inline. The next line — progress.addChild(imageProgress, …) — then mutated the Progress object graph on AVFoundation's queue after the awaiting task had resumed and could be racing forward on another thread.

Fix

1. Make the async bridges resume-once and checked

MediaExporter.export(), MediaThumbnailExporter.exportThumbnail(forFileURL:) and exportThumbnail(forVideoURL:) now use withCheckedThrowingContinuation routed through a small NSLock-backed ResumeOnce<T> that nils the continuation on first resume. A stray second callback degrades to a no-op. The checked variant stays permanently — the unsafe one buys nothing here and hides exactly this class of bug.

2. Stop mutating progress after the resume

exportPreviewImageForVideo finalizes progress.completedUnitCount = .done inside the inner export's onCompletion/onError, before the resuming callback fires. The post-resume addChild is removed. Because the image export is synchronous and returns an already-complete Progress, this is equivalent for progress reporting while removing the cross-thread mutation.

What We Explored

• Reproducing the SEGV locally. Ran the pre-fix binary 200× with per-iteration relaunch on an iOS 26 simulator: 200/200 passed, 0 crashes. The crash does not reproduce in an isolated local loop — it needs CI-level executor contention (the test runs alongside hundreds of others). The test never cancels, so the operative mechanism is the double-resume (1), which is not a TSan-detectable data race — so TSan is clean on this path and adds nothing here. Part 1 makes the crash impossible regardless of the exact micro-mechanism.

Test Plan

• rake lint clean on all changed files.
• WordPressUnitTests builds; testSmallThumbnailForRemoteSelfHostedVideo green across 121 executions of the real AVFoundation path (0 crashes / 0 failures).
• Pre-fix SEGV confirmed not locally reproducible (200/200 clean) — documented above.
• Full WordPressUnitTests suite on CI (running the pipeline repeatedly to confirm the flake is gone).

Related

• The sibling testSmallThumbnailForRemoteVideo ~3% flake is a separate MemoryCache.shared / ImageDownloader.shared test-isolation issue, out of scope here.

[dangermattic]

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