fix: age-filter in scanForLeftovers to guard against concurrent builds

[gnodet]

Problem

BackgroundCleaner.scanForLeftovers() scans the fast staging directory at session start and queues any leftover directories for background deletion. It did not protect against the case where two concurrent builds share the same staging directory: one build's leftover scan could delete directories that are actively being used by the other build.

Fixes #353.

Fix

Introduce a LEFTOVER_AGE_THRESHOLD_MS constant (30 seconds). scanForLeftovers() now reads the last-modified time of each staged directory and skips any directory younger than the threshold.

• A directory created by an ongoing concurrent build will always be younger than 30 s at the time of the scan (the BackgroundCleaner is initialised very early in the session, so there is no practical way a live build's staged directories are 30 s old at that point).
• A directory left behind by a killed build will be older than 30 s in virtually all cases.
• If getLastModifiedTime() fails (unlikely), the directory is also skipped — conservative, safe default.

An age filter was chosen over the alternative PID-marker approach because:

• It requires no additional file I/O on the hot path (only during the leftover scan at session start).
• It is robust on all platforms (no /proc or OS-specific PID checks).
• The 30-second threshold is conservative enough to make false negatives (live directory treated as leftover) virtually impossible.

Also fixed

A pre-existing missing closing } in CleanerTest.batchRetryLogsWarningAfterRetryWhenStillFailing that was causing a compile error on the spotless/checkstyle phase.

Tests

Two new tests in BackgroundCleanerTest:

• scanForLeftoversSkipsRecentDirectories — a directory with lastModified = now is not deleted.
• scanForLeftoversDeletesOldDirectories — a directory with lastModified = now − threshold − 60 s is deleted.

The existing scanForLeftoversDeletesOrphanedDirectories test is updated to set an old timestamp on the leftover directory (it would have been skipped by the new age filter with the default lastModified = now from createDirectory).

[gnodet-bot]

Solid fix. The age-filter approach is well-chosen — simpler than PID markers, no hot-path I/O overhead, and the 30-second threshold is conservative enough to make false positives (deleting a concurrent build's directory) effectively impossible under normal conditions.

The three critical paths are correctly handled:

• Recent directories (lastModified > threshold) → skipped
• Old directories (lastModified <= threshold) → queued for deletion
• getLastModifiedTime() failure → skipped (safe default)

Test coverage is adequate: both the positive and negative age-filter cases are exercised, and the existing scanForLeftoversDeletesOrphanedDirectories test is properly updated to set an old timestamp.

The CleanerTest fix (missing closing }) is a legitimate pre-existing compile error fix.

This review was generated by an AI agent, Hermès on behalf of @gnodet.