Fix directory enumeration hang

[Michael Pysson (mpysson)]

Summary

Fix a directory enumeration failure that can leave Component Detection waiting indefinitely when the source tree changes during a scan.

Problem

Component Detection first discovers directories and then enumerates them concurrently. In a mutable build tree, a discovered directory can be deleted before recursive enumeration begins. The resulting DirectoryNotFoundException faults the directory enumeration ActionBlock.

Errors from the concatenated enumeration observables were also not forwarded to the outer observer. As a result, an unexpected enumeration failure could leave the shared replay observable without a terminal notification. Detectors waiting for enumeration to complete would then remain blocked indefinitely.

This behavior was observed in a process dump where:

• A previously discovered directory no longer existed.
• Recursive directory enumeration had faulted with DirectoryNotFoundException.
• The replay observable had neither completed nor failed.
• Detector tasks remained incomplete while worker threads were idle.

Solution

• Treat DirectoryNotFoundException during recursive enumeration as a recoverable race and continue scanning the remaining directories.
• Log unexpected enumeration failures and propagate them through OnError so the scan fails instead of hanging.

Other I/O and enumeration exceptions are not suppressed.

Tests

Added regression coverage that verifies:

• Enumeration completes when a directory is deleted after discovery but before recursive enumeration.
• Unexpected recursive enumeration failures are logged and propagated to subscribers rather than timing out.

[Copilot]

🟡 Changes recommended

The recursive enumeration observable can call IObserver.OnNext concurrently from multiple ActionBlock workers, which violates Rx observer thread-safety expectations and can cause race-condition failures.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR fixes a potential hang in the FastDirectoryWalkerFactory directory enumeration pipeline when the filesystem changes during a scan (e.g., a directory is deleted between discovery and recursive enumeration). It makes DirectoryNotFoundException during recursive enumeration recoverable and ensures unexpected enumeration failures are logged and propagated via OnError, allowing scans to fail fast instead of stalling subscribers indefinitely.

Changes:

• Catch and suppress DirectoryNotFoundException during recursive enumeration to handle directory-deletion races.
• Forward enumeration errors to observers (and log them) so the replayed observable terminates instead of hanging.
• Add regression tests covering the “directory disappears” race and unexpected enumeration failure propagation/logging.

File summaries

File	Description
src/Microsoft.ComponentDetection.Common/FastDirectoryWalkerFactory.cs	Adds recoverable handling for DirectoryNotFoundException during recursive enumeration and forwards unexpected errors via OnError with logging.
test/Microsoft.ComponentDetection.Common.Tests/FastDirectoryWalkerFactoryTests.cs	Adds regression tests verifying completion on disappearing directories and error propagation/logging for unexpected enumeration failures.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 3
• Review effort level: Lite

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[github-actions]

👋 Hi! It looks like you modified some files in the Detectors folder.
You may need to bump the detector versions if any of the following scenarios apply:

• The detector detects more or fewer components than before
• The detector generates different parent/child graph relationships than before
• The detector generates different devDependencies values than before

If none of the above scenarios apply, feel free to ignore this comment 🙂