Stray extension-less snapshot file crashes every run of its test #1850

MatchingFileFinder.SubStringEquals sliced a fixed number of characters off each enumerated file name without checking the name was long enough:

var slice = value.Slice(start, match.Length);

The Win32 search pattern used to enumerate candidates is {prefix}*.received.* / {prefix}*.verified.*, and a trailing .* also matches a name with no extension. So a stray file named exactly {prefix}.received or {prefix}.verified is returned by the enumeration, but is one character shorter than the {prefix}.received. pattern it is compared against, and the slice throws.

Symptom

With a file such as MyTests.MyTest.verified (extension dropped by an editor, a git operation, or a manual rename) sitting beside the snapshots, every subsequent run of MyTests.MyTest throws:

System.ArgumentOutOfRangeException: Index and length must refer to a location within the string.

from MatchingFileFinder.FindVerified / DeleteReceived, during InnerVerifier construction — before the test body executes. The failure gives no hint that an unrelated stray file is the cause.

Fix

Length-check before slicing, so a name shorter than the pattern is simply not a match.

Fixed in a43e52e, covered by MatchingFileFinderTests.StrayFileWithNoExtensionIsIgnored.