[billythekidz]

io.open(path, "w+") / "w+b" threw when the file did not exist. LuaFileOpenMode.WriteUpdate was mapped to FileMode.Truncate, which requires an existing file. Lua's w+ creates the file if needed and truncates it otherwise, so it is now mapped to FileMode.Create (still FileAccess.ReadWrite).

Verification

• New test OpenWritePlus_CreatesMissingFileAndTruncatesExisting (create, write, seek, read back, reopen and check it is truncated) fails on main and passes with the change.
• dotnet test tests/Lua.Tests/Lua.Tests.csproj -c Release --filter "TestCategory!=ExpectedFailure": 611 passed, 0 failed (610 on main + 1 new).
• dotnet csharpier check . is clean.
• Run on macOS (arm64), .NET 8 runtime only.

[Copilot]

🟢 Approval recommended

The narrow mapping change implements the required semantics, and the regression test covers creation and truncation with no identified blocking issues.

No open issues 🎉

What changed in this PR

Fixes io.open write-update mode so it creates missing files and truncates existing files while preserving read/write access.

Changes:

• Maps LuaFileOpenMode.WriteUpdate to FileMode.Create instead of FileMode.Truncate.
• Adds a regression test covering creation, writing, seeking, reading, and truncation on reopening.

File	Description
tests/​Lua.Tests/​IOTests.cs	Adds regression coverage for w+ file behavior.
src/​Lua/​IO/​FileSystem.cs	Corrects the write-update file mode mapping.

_{Review effort: Balanced}

[akeit0]

Thank you for the w+ creation fix and the regression test! Your original commit has been merged through #368 with its authorship preserved.

The broader seek/write checks also uncovered an adjacent existing issue: w+ was classified as append mode, so writes after a seek went to the end of the file. We fixed that in a follow-up and expanded coverage for both w+ and w+b, while checking that a+ and a+b still append after a seek. The combined changes passed local tests and Linux CI.