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FileNameSanitizer: hostile-name edge cases survive mutation (73% score)

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#720 0 comments 0 reactions 0 assignees View on GitHub

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@DeepDiver1975 is already working on this.

Since Oct 21, 2026.

  • #722 by @DeepDiver1975 — open

Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
25/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Stale
Tech stack
csharp
Domain
security, testing

Research direction

Read Files/FileNameSanitizer.cs, the listed mutation survivors, and the relevant tests under tests/Lagebuch.Domain.Tests; first run make mutate PROJECT=Domain to inspect the report. Add a test for each listed hostile input and verify each test fails against its mutant. The issue also asks to raise the floor in tests/Lagebuch.Domain.Tests/stryker-config.json; done means the new tests cover the cases and the Domain mutation score floor is raised. An open linked pull request (#722) already covers this work.

Written by the indexing model from the issue text.

Description

The weekly mutation run (2026-10-21, run 39560645989) scores Files/FileNameSanitizer.cs at 73 %. That is low for code at the trust boundary: it cleans the file names a sync peer sends and the names read back from an .fwincident. Line numbers below are as of 058273e.

These survivors are hostile-input paths with no test:

Line Survivor What goes untested
61 the IsNullOrWhiteSpace early return null removed TrySanitize(null). Without the guard, fileName.Trim() throws, so no test passes null.
80 TrySanitize(...) ?? fallback (no coverage) Sanitize returning its fallback when nothing usable is left, e.g. "..." or "‮"
100 <= MaxFileNameBytes → < a name of exactly 255 UTF-8 bytes being kept as is
107, 110 >= MaxFileNameBytes → >; the pathological branch (no coverage) an "extension" of 255 bytes or more. Without that branch, new byte[MaxFileNameBytes - extensionBytes] gets a negative size and throws.
119 flush: false → true a stem cut right at a surrogate pair. With flush: true, half an emoji is encoded as a replacement character.

Line 68 (IndexOf(...) < 0 → <= 0) is equivalent on Linux, because the first invalid character there is \0, which IsHidden already drops. Leave it.

Reproduce
make mutate PROJECT=Domain
# report: StrykerOutput/Domain/reports/mutation-report.html, file Files/FileNameSanitizer.cs
Suggested approach

Follow AGENTS.md: write one test per row that replays the hostile name and fails against its mutant first:

  • null;
  • a name made only of dots or bidi overrides;
  • a 255-byte name;
  • "a." + new string('x', 300);
  • a long stem of emoji cut at the limit.

Fixing these raises Domain's score. Raise its floor in tests/LageBuch.Domain.Tests/stryker-config.json in the same pull request.

Dominant language
C#
Stars
1
Forks
3
Avg merge
4h 34m
Merged PRs (30d)
182

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  4. Open a pull request that references the issue number.

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