fix(session-replay): Stabilize video assembly#8041
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Extract the replay video-assembly fixes from the capture-backoff branch so they can land and ship independently: - Retain the last frame captured before a segment window so recovered and resumed segments start with the correct screen state instead of dropping it (this moves the recovered replay start timestamp to the retained frame's time). - Fill and bound fractional gaps between captured frames so video timing stays stable when captures are skipped under load. - Use half-open video windows to avoid duplicating boundary frames in consecutive segments. - Drop video segments that contain no frames instead of emitting empty videos. - Keep video timing stable when a cached frame image cannot be read. - Serialize oldestFrameDate reads on the processing queue.
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Codecov Report❌ Patch coverage is Additional details and impacted files@@ Coverage Diff @@
## main #8041 +/- ##
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+ Coverage 87.601% 87.633% +0.031%
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Files 559 559
Lines 32174 32289 +115
Branches 13158 13223 +65
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+ Hits 28185 28296 +111
- Misses 3941 3944 +3
- Partials 48 49 +1
... and 6 files with indirect coverage changes Continue to review full report in Codecov by Harness.
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romtsn
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NinjaLikesCheez,
itaybre,
noahsmartin,
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philprime
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Removed merge conflict markers and updated changelog for version 9.17.1.
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Performance metrics 🚀
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| Revision | Plain | With Sentry | Diff |
|---|---|---|---|
| 71e6611 | 1213.27 ms | 1242.25 ms | 28.98 ms |
| 4014441 | 1232.16 ms | 1266.41 ms | 34.25 ms |
| 1c5ecda | 1219.35 ms | 1253.76 ms | 34.41 ms |
| 655e96d | 1217.63 ms | 1258.34 ms | 40.72 ms |
| a48979e | 1244.58 ms | 1269.00 ms | 24.42 ms |
| ba5a1dd | 1234.15 ms | 1257.50 ms | 23.35 ms |
| 8bb6aeb | 1215.82 ms | 1244.84 ms | 29.02 ms |
| da35aa2 | 1224.21 ms | 1249.00 ms | 24.79 ms |
| c3064bc | 1227.20 ms | 1248.25 ms | 21.05 ms |
| 943b250 | 1214.69 ms | 1257.32 ms | 42.63 ms |
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| 71e6611 | 24.14 KiB | 1.15 MiB | 1.12 MiB |
| 4014441 | 24.14 KiB | 1.16 MiB | 1.14 MiB |
| 1c5ecda | 24.14 KiB | 1.15 MiB | 1.12 MiB |
| 655e96d | 24.14 KiB | 1.18 MiB | 1.16 MiB |
| a48979e | 24.14 KiB | 1.17 MiB | 1.15 MiB |
| ba5a1dd | 24.14 KiB | 1.18 MiB | 1.16 MiB |
| 8bb6aeb | 24.14 KiB | 1.17 MiB | 1.15 MiB |
| da35aa2 | 24.14 KiB | 1.15 MiB | 1.13 MiB |
| c3064bc | 24.14 KiB | 1.18 MiB | 1.15 MiB |
| 943b250 | 24.14 KiB | 1.17 MiB | 1.15 MiB |
Previous results on branch: fix/replay-video-assembly
Startup times
| Revision | Plain | With Sentry | Diff |
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| e8e4651 | 1217.60 ms | 1247.39 ms | 29.79 ms |
| 4832942 | 1230.43 ms | 1265.88 ms | 35.45 ms |
| f17e4d9 | 1224.52 ms | 1255.33 ms | 30.81 ms |
| f98aa1c | 1229.22 ms | 1259.48 ms | 30.26 ms |
| 104ce0c | 1215.65 ms | 1245.04 ms | 29.39 ms |
| 5756a29 | 1234.65 ms | 1258.87 ms | 24.22 ms |
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| e8e4651 | 24.14 KiB | 1.18 MiB | 1.16 MiB |
| 4832942 | 24.14 KiB | 1.18 MiB | 1.16 MiB |
| f17e4d9 | 24.14 KiB | 1.18 MiB | 1.16 MiB |
| f98aa1c | 24.14 KiB | 1.18 MiB | 1.16 MiB |
| 104ce0c | 24.14 KiB | 1.18 MiB | 1.16 MiB |
| 5756a29 | 24.14 KiB | 1.18 MiB | 1.16 MiB |
Inline videoFramesForRendering and replaceRetainedFrame at their only call sites and drop the one-line frame(_:movedTo:) wrapper. No behavior change.
This was referenced Jun 11, 2026
Inline appendLastFrameUntilVideoEnd, cancelWriting, presentationFrameCount, presentationFrameIndex, processCurrentFrame, processUnreadableFrame, and the appendLastFrame(date) overload into their single call sites. Keeps appendLastFrame(untilFrameIndex:), append(image:forFrame:), and handleAppendResult which have multiple callers.
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Use distinct timestamps so the gap-filling logic is actually exercised, and update assertions to expect the gap-fill frame.
philprime
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Jun 15, 2026
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Good progress, left a couple of comments to discuss
Document why bounded replay segments repeat the last captured frame until the requested video end.
Make append result handling return a boolean or throw, instead of taking the video completion callback.
Name the helper for what it does: repeat the last appended frame until the video reaches a target output index.
Replace duplicate replay frame and empty video deletion helpers with a single file removal helper.
Document why clean shutdown removes the retained frame file while crash recovery leaves it for the next launch.
Read loaded replay frames directly during recovery because recording has not started yet.
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Extracts the replay video-assembly fixes from #7851 into their own PR so they can be reviewed and land first; #7851 (the capture-scheduler/backoff rework) builds on top of them and will be retargeted onto this branch. Only
SentryOnDemandReplayandSentryVideoFrameProcessor(plus their tests) change; the public surface of both is untouched, and nothing here references the new capture pacing.These fixes were motivated by the backoff work, which makes irregular capture cadence the norm — but the conditions they handle already occur on
mainwhen captures are skipped under load, and two of them are flat-out pre-existing issues. Behavior changes, all in how segment videos are assembled from captured frames:testSessionReplayForCrashexpectation).oldestFrameDatereads are serialized on the processing queue to avoid racing frame mutations.Review the
SentryVideoFrameProcessordiff first (frame-index based appending is the core change), thenSentryOnDemandReplay(window selection and retained-frame bookkeeping). Verified with the four session-replay test suites (116 tests) against main's display-link capture path.