ci: mirror app-host batch output without racing a tail

[teamleaderleo]

When an app-host unit-test batch dies, the log line saying why can be
missing from the Actions log on a loaded runner. The batch's output goes to a
regular file that a background tail -f mirrors into the log; when the batch
exits the step sleeps 0.2s and kills the tail. That 0.2s is the margin the
batch's last write has to beat.

This is not theoretical. tests/test_ci_change_areas.py drives this step with
a fake sleep, so the margin there is zero:

	guard file, 4 runs under 16 busy cores	the single test, 30 runs
before	0 pass / 4 fail	6 pass / 24 fail
after	4 pass / 0 fail	30 pass / 0 fail

Idle, before and after are both clean — which is exactly why this reads as an
unrelated flake rather than a bug. In every failing run the batch did reach
its crash branch and the downstream No typed xcresult test JSON found still
appeared; only the message naming the cause was lost.

Change

The step now tracks how many bytes of the batch file it has emitted and
flushes the remainder itself — in the polling loop it already runs every 5s,
and once more after the batch exits. There is no margin left to miss, and one
fewer background process per batch.

The property the tail existed for is preserved, and the comment above the
function still says so: the batch writes to a regular file, so detached test
descendants cannot retain the CI capture pipe after xcodebuild exits. Only the
mirroring moved in-process.

Validation

• python3 tests/test_ci_change_areas.py passes idle and under load, per the
  table above. The load comparison was run against this branch and against
  origin/main back to back, same 16-core load, same command.
• actionlint .github/workflows/ci-macos.yml reports exactly the two
  pre-existing SC2129 style notes that origin/main reports, and nothing new.
• Full ci-guards.yml sweep — result noted in a follow-up comment once it
  finishes.

Not established here: I have not caught this dropping output in a real CI run.
Production keeps the real 0.2s sleep, so the margin there is small rather than
zero; the argument is that a correctness property should not rest on a sleep
at all. The measured flake is in the guard lane, which is real CI.

🤖 Generated with Claude Code

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Summary by cubic

Fixes a race in the CI app-host unit-test step that could drop a batch's final output lines, including the message saying why the batch died, on a loaded runner.

• The step mirrored the batch file into the Actions log with a background tail -f and a 0.2s sleep before killing the tail; a loaded runner could miss that margin.
• The step now tracks a byte offset into the batch file and flushes the remainder itself, in the polling loop and once after the batch exits, leaving no margin to miss and one fewer background process.
• Each flush reads only the byte range covered by its size snapshot, so bytes appended by a concurrent writer are not duplicated on the next poll.
• The guard test now requires the final message exactly once, covering both dropped and duplicated output.

^{Written for commit 5bf59ff. Summary will update on new commits.}

Summary by CodeRabbit

• Chores
  • Improved macOS automated test logs to capture output through the end of each test batch, including final results. This change affects the build process and does not alter app functionality.
• Tests
  • Strengthened failure checks to verify that a simulated app-host crash message appears exactly once.

[github-actions]

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• .github/workflows/ci-macos.yml
• tests/test_ci_change_areas.py

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📝 Walkthrough

Walkthrough

The macOS unit-test workflow replaces background log streaming with an emit_batch_output function. It emits newly appended output during each wait poll and once after xcodebuild exits.

Changes

Batch Output Streaming

Layer / File(s)	Summary
Emit batch output during and after execution .github/workflows/ci-macos.yml	The workflow tracks the number of bytes emitted for each batch. It emits new output during polling and after xcodebuild exits, replacing the background tail -f process and its teardown.
Estimated code review effort: 2 (Simple)	~10 minutes

Merge Risk: 🔵 Low · up to adec3

Concurrent test output can appear twice in the Actions log. The impact is limited to log readability, but the byte-range handling should be corrected.

🚥 Pre-merge checks | ✅ 25

✅ Passed checks (25 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: replacing the racing tail-based mirroring for app-host batch output.
Description check	✅ Passed	The description clearly explains the failure mode, the implementation change, and validation results. It does not include the template headings for Demo Video, Review Trigger, or Checklist, but these …
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Cmux Cloud Persistent Session And Early Input	✅ Passed	PASS: The PR changes only .github/workflows/ci-macos.yml app-host test log mirroring. It replaces a background tail -f with byte-offset output flushing. The diff does not change Cloud terminal cre…
Cmux Swift Actor Isolation	✅ Passed	PASS — the authoritative pull-request diff changes only .github/workflows/ci-macos.yml. It introduces no Swift production code, so it cannot introduce or worsen the specified Swift 6 actor-isolation…
Cmux Swift Blocking Runtime	✅ Passed	PASS. The authoritative PR diff changes only .github/workflows/ci-macos.yml; it does not add or modify any Swift file. The new sleep, polling loop, and output handling are Bash CI logic, while thi…
Cmux Browser Automation Off-Main	✅ Passed	The pull request changes only .github/workflows/ci-macos.yml. The diff modifies shell-based app-host test log mirroring and introduces no browser.* socket command, WebKit/AppKit worker routing, or…
Cmux Expensive Synchronous Load	✅ Passed	PASS — The pull request changes only .github/workflows/ci-macos.yml. The diff contains no Swift files and no production Swift code, agent-history load, or main-actor/interactive call-site change. Th…
Cmux Cache Substitution Correctness	✅ Passed	PASS: The pull request changes only .github/workflows/ci-macos.yml. The diff contains shell-based batch-output mirroring and no production Swift, TypeScript, or JavaScript change. It also does not r…
Cmux No Hacky Sleeps	✅ Passed	PASS. The PR changes only .github/workflows/ci-macos.yml. The applicable rule explicitly excludes GitHub Actions workflow and action YAML. The diff also removes the sleep 0.2 teardown margin and r…
Cmux Algorithmic Complexity	✅ Passed	The PR changes only .github/workflows/ci-macos.yml. The new shell function emits appended log bytes by offset, and the polling loop handles two explicitly defined logical batches. It does not add ne…
Cmux Swift Concurrency	✅ Passed	PASS: The pull request changes only .github/workflows/ci-macos.yml; it has no changed Swift files. The added code is shell logic for emitting batch output and does not introduce any Swift concurrenc…
Cmux Swift @Concurrent	✅ Passed	PASS: The pull request changes only .github/workflows/ci-macos.yml; it contains no Swift source or Swift function changes. Therefore the @concurrent annotation criteria do not apply.
Cmux Swift Package Boundaries	✅ Passed	PASS: The authoritative pull-request diff changes only .github/workflows/ci-macos.yml. It introduces no production Swift code or Swift package-boundary change, so the custom check does not apply.
Cmux Swiftpm Lockfiles	✅ Passed	The PR changes only .github/workflows/ci-macos.yml and modifies batch-output mirroring. The authoritative diff contains no Package.swift, Package.resolved, .gitignore, Xcode project, or SwiftP…
Cmux Swift Logging	✅ Passed	PASS: The PR changes only .github/workflows/ci-macos.yml; it adds shell-based CI batch-output mirroring and comments. It does not add or materially change production Swift logging, so none of the pr…
Cmux User-Facing Error Privacy	✅ Passed	PASS. The authoritative diff changes only .github/workflows/ci-macos.yml, inside the app-host-unit-tests GitHub Actions job. The changed text is CI log mirroring and developer comments, with no co…
Cmux Full Internationalization	✅ Passed	PASS. The PR changes only .github/workflows/ci-macos.yml. It adds CI-only output mirroring logic and Actions-log comments. It adds no Swift UI text, string-catalog entries, web UI or locale message …
Cmux Swiftui State Layout	✅ Passed	PASS. The authoritative PR diff changes only .github/workflows/ci-macos.yml and contains shell/workflow logging changes. It introduces no SwiftUI code, state, layout measurement, lazy/list row subtr…
Cmux Architecture Rethink	✅ Passed	PASS: The authoritative PR diff changes only .github/workflows/ci-macos.yml; it introduces no Swift source or SwiftUI/AppKit architecture. The changed shell code is a local CI correctness fix with o…
Cmux Swift Auxiliary Window Close Shortcuts	✅ Passed	PASS: The review-scoped diff changes only .github/workflows/ci-macos.yml. It modifies shell-based CI output mirroring and adds no Swift, NSWindow, NSPanel, NSWindowController, SwiftUI Window, or Win…
Cmux Source Artifacts	✅ Passed	PASS. The pull request changes only .github/workflows/ci-macos.yml, a hand-written workflow configuration. The added lines implement in-process batch-output mirroring and do not add logs, screenshot…
Cmux No Test Or Debug Seam In Production Source	✅ Passed	PASS: The pull request changes only .github/workflows/ci-macos.yml. It does not change any Swift file under a production Sources/ path, so the custom check is not applicable.

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Actionable comments posted: 1

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• 🪄 Fix CodeRabbit comments on this PR

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Inline comments:
In @.github/workflows/ci-macos.yml:
- Around line 1709-1710: Update the file-streaming block that uses stream_offset
and size so each emission includes only bytes between the recorded offset and
the measured size; advance stream_offset by the number of bytes actually copied,
preventing later appends from being emitted early or duplicated on the next
poll.

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[teamleaderleo]

Review found one second-order race in the first offset implementation and fixed it on this branch. wc -c snapshots size N; a plain tail -c +offset reads to EOF, so bytes appended after that snapshot could be emitted immediately while the offset advanced only to N, duplicating those bytes on the next poll. The mirror now reads exactly [stream_offset, snapshotted_size) with Python, then advances to that same size. The guard now also requires the simulated crash line exactly once. This keeps the original no-loss property without trading it for duplicate log output.

[teamleaderleo]

Full ci-guards.yml sweep on 5bf59ff61d (the head including the three
follow-up commits): 167 of 170 pass.

The three failures are environmental and reproduce on origin/main
unchanged — they need things this box does not have, not anything in this
diff:

• app_host_result_accounting.py catalog-diff — needs the $base_catalog /
  $catalog_path variables the workflow sets around it
• tests/test_ghostty_zig_version_sync.sh — needs the ghostty submodule
• scripts/lint-stored-dispatch-work-items.py — needs vendor/bonsplit/Sources

Separately, thanks for 320f23ed8b — reading to EOF past the wc -c
snapshot was a real defect in my version, and it would have duplicated log
lines on every poll where the batch wrote while the flush was in flight.
I confirmed the python3 -c reader runs as indented inside the workflow
block and that tests/test_ci_change_areas.py passes on the merged head.

— NightPetrel g1 🪁 (run=run_cmux_nightly_relevance_20260923_c)

[teamleaderleo]

Full guard sweep on head 5bf59ff61d. I took this over from NightPetrel.

• Sweep: 136 of 137 pass. That's every run: command in ci-guards.yml. The one failure is ./tests/test_ghostty_zig_version_sync.sh: can't read …/ghostty/build.zig.zon. That's the known environmental case in a worktree without the ghostty submodule checked out, and it's unrelated to this diff.
• Tested against the new main. #13969 merged at 14:59Z and also edits tests/test_ci_change_areas.py. This branch still merges cleanly onto the new origin/main, and python3 tests/test_ci_change_areas.py passes on the merged tree.
• The exact-range reader is correct. It reads [stream_offset, size) from the wc -c snapshot and advances to that same size. Bytes written after the snapshot wait for the next poll or the final emit after the batch exits, so nothing is duplicated and nothing is lost. python3 is already used throughout ci-macos.yml, and one extra process per 5s poll costs nothing. The test now requires the crash line exactly once, which pins down the duplicate case.

— KeyboardCat g1 ⚙️
Run: run_cmux_ci_efficiency_land_13969_and_13990_20260923_c1f522ea

[teamleaderleo]

Correction to my review above: this PR had already merged at 14:58:40Z (0044d5a154), a minute before #13969. So the "merges cleanly onto the new main" line tested nothing. What actually ran is python3 tests/test_ci_change_areas.py on origin/main with both PRs merged, and it passes. The guard sweep result is unaffected.

— KeyboardCat g1 ⚙️