Surface the real cause when the diagnostic log file cannot be created

[Evangelink]

Why

A .NET SDK CI run crashed at MTP test host startup with an error that made the actual problem impossible to diagnose:

Unhandled exception. System.InvalidOperationException: Failed to create a unique log file after 3 seconds.
Lastly tried file name is 'Microsoft.DotNet.PackageInstall.Tests_net11.0_x64_260730124750326.diag'.
   at Microsoft.Testing.Platform.Logging.FileLogger.CreateFileStream(...)

Two createdump full crash dumps had just been written to the same Helix disk immediately before, so the real cause was almost certainly disk exhaustion, not a file name collision. FileLogger.CreateFileStream caught every IOException in a 3 second retry loop, swallowed it, and then reported a generic "cannot create a unique log file" with no inner exception. Retrying only ever recovers from a name collision, so disk-full, missing-folder, and path-too-long failures were spun on and then reported as something they were not.

Investigating that turned up two further defects in the same loop.

What changed

The deadline was evaluated before the first attempt. IClock.UtcNow is wall time, not monotonic. A stall of more than 3 seconds, or a forward clock jump (NTP / VM time sync), between two adjacent statements made the loop throw without ever calling Create, leaving fileName null and rendering Lastly tried file name is ''. A Helix agent that just flushed two full crash dumps is exactly the environment where that happens, so this may well be the shape of the reported failure. The loop is now bounded by attempt count, which is provably terminating and immune to the clock.

Every retry regenerated the identical name. The candidate came from a millisecond-resolution timestamp, so a lost race was retried unchanged until the clock ticked (about 15ms on Windows), burning thousands of doomed attempts over the 3 second window on a machine that is often already sick. Retry candidates now carry the process id and the attempt index, making them unique both within the process and against any other process racing on the same log folder, so a collision can only ever happen on the first attempt. The first candidate keeps its existing name shape, so the file name users see in the normal case is unchanged.

The real error is now reported. On exhaustion we throw IOException (the same type a single failed attempt throws, so a caller needs only one filter rather than IOException plus InvalidOperationException) carrying the last failure in the message and chained as InnerException. The message now reports the full path and the attempt count instead of a hardcoded "3 seconds" that was baked into 13 translations.

Notes for reviewers

The .xlf files were regenerated with /t:UpdateXlf, not hand-edited.

Two things I deliberately left alone, both worth a second opinion:

• A failure to create the diagnostic log file is still fatal to the test run. This file already has the opposite documented policy on the dispose path ("a logger failing to flush must never crash an otherwise successful test run", see Microsoft.Testing.Platform test host crash on shutdown: "Failed to flush logs before the timeout of '300' seconds" (FileLogger.Dispose) dotnet/sdk#55215), and here we trade "no diagnostics" for "no test results at all". Making it degrade gracefully is a user visible behavior change that deserves its own discussion, so this PR only makes the failure precisely catchable so that can be layered on later.
• Directory.CreateDirectory in TestApplication.CreateFileLoggerIfDiagnosticIsEnabled sits on the same fatal path with the same disk-full trigger and is not covered here.

Testing

Full repo build is clean across all target frameworks. Unit tests pass on net9.0 (1977) and net462 (1936), the latter covering the netstandard2.0 build of the platform.

The new tests were mutation checked rather than just observed passing: separately removing the name uniqueness, the process id discriminator, the message detail, and the inner exception chaining each fails between one and three tests.

[Copilot]

Pull request overview

Improves diagnostic log creation reliability and error reporting in Microsoft.Testing.Platform.

Changes:

• Replaces time-based retries with bounded, distinct filename attempts.
• Preserves and reports the underlying IOException.
• Adds unit coverage and regenerates localization resources.

Show a summary per file

File	Description
src/Platform/Microsoft.Testing.Platform/Logging/FileLogger.cs	Revises file creation retries and errors.
src/Platform/Microsoft.Testing.Platform/Resources/PlatformResources.resx	Updates the diagnostic failure message.
src/Platform/Microsoft.Testing.Platform/Resources/xlf/PlatformResources.cs.xlf	Regenerates Czech localization.
src/Platform/Microsoft.Testing.Platform/Resources/xlf/PlatformResources.de.xlf	Regenerates German localization.
src/Platform/Microsoft.Testing.Platform/Resources/xlf/PlatformResources.es.xlf	Regenerates Spanish localization.
src/Platform/Microsoft.Testing.Platform/Resources/xlf/PlatformResources.fr.xlf	Regenerates French localization.
src/Platform/Microsoft.Testing.Platform/Resources/xlf/PlatformResources.it.xlf	Regenerates Italian localization.
src/Platform/Microsoft.Testing.Platform/Resources/xlf/PlatformResources.ja.xlf	Regenerates Japanese localization.
src/Platform/Microsoft.Testing.Platform/Resources/xlf/PlatformResources.ko.xlf	Regenerates Korean localization.
src/Platform/Microsoft.Testing.Platform/Resources/xlf/PlatformResources.pl.xlf	Regenerates Polish localization.
src/Platform/Microsoft.Testing.Platform/Resources/xlf/PlatformResources.pt-BR.xlf	Regenerates Brazilian Portuguese localization.
src/Platform/Microsoft.Testing.Platform/Resources/xlf/PlatformResources.ru.xlf	Regenerates Russian localization.
src/Platform/Microsoft.Testing.Platform/Resources/xlf/PlatformResources.tr.xlf	Regenerates Turkish localization.
src/Platform/Microsoft.Testing.Platform/Resources/xlf/PlatformResources.zh-Hans.xlf	Regenerates Simplified Chinese localization.
src/Platform/Microsoft.Testing.Platform/Resources/xlf/PlatformResources.zh-Hant.xlf	Regenerates Traditional Chinese localization.
test/UnitTests/Microsoft.Testing.Platform.UnitTests/Logging/FileLoggerTests.cs	Tests retry naming and exception details.

Review details

• Files reviewed: 16/16 changed files
• Comments generated: 1
• Review effort level: Balanced

[Copilot]

Review details

• Files reviewed: 16/16 changed files
• Comments generated: 0 new
• Review effort level: Balanced

[github-actions]

🧪 Test quality grade — PR #10374

Grade	Test	Mutation	Notes	How to improve
A (90–100)	new FileLoggerTests. FileLogger_ NullFileSyncFlush_ ConcurrentLoggersInTheSameTickNeverShareACandidateName	3/3 killed	Intersection assertion kills the process-wide counter vs attempt-index mutation; shared-name identity pin is precise.	—
A (90–100)	new FileLoggerTests. FileLogger_ NullFileSyncFlush_ EveryAttemptUsesADistinctFileName	4/4 killed	Count + distinctness + exact path[0] + StartsWith for all retries protect the uniqueness invariant and loop bound.	—
A (90–100)	new FileLoggerTests. FileLogger_ NullFileSyncFlush_ WhenEveryAttemptFails_ ReportsAndChainsTheLastFailure	5/5 killed	ThrowsExactly + AreSame on InnerException + Contains for last path, attempt count, and original message text.	—
A (90–100)	mod FileLoggerTests. FileLogger_ NullFileSyncFlush_ FileStreamCreated	4/4 killed	HasCount(2) + exact path[0] + StartsWith with PID prefix + fileLoggerName identity; all retry-logic mutations killed.	—

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🤖 Automated content by GitHub Copilot. Generated by the Grade Tests on PR (on open / sync) workflow. · sonnet46 101.1 AIC · ⌖ 4.3 AIC · ⊞ 11.8K · [◷]( · ◷)