[torlando-tech]

Follow-up to #240 (merged at 5ebff91, before this fix landed).

Greptile round 2 flagged a P2: logFailureOnce read/wrote loggedFailure without a lock, and the sampler calls DiskIO.sample() outside the trace lock, so it can race the stop marker's sample — worst case, two diagnosis lines in ext-diag.log instead of one. Fixed with a dedicated NSLock (check-and-set inside, log call outside).

Diff is 6 lines; full context and the 5/5 review it came from are on #240.

[torlando-tech]

@greptile review

[greptile-apps]

Confidence Score: 5/5

[Low impact] The PR appears safe to merge.

Summary

Protects DiskIO’s once-only failure message when the sampler and stop marker run at the same time.

• Disk probe failures now produce one diagnosis line per process.

_{Reviews (1) · Last reviewed commit: "DiskIO: lock the once-only failure log a..." · Reviewed by Greptile}