[bfs2021]

Problem

Fixes #242. DiskUtils.TryGetReparseDataBuffer rents a 16 KiB buffer from ArrayPool<byte>.Shared, calls DeviceIoControl(FSCTL_GET_REPARSE_POINT), then reads a SymbolicLinkReparseBuffer out of the whole array. The driver-reported byte count is discarded, and pooled arrays are not zeroed, so MemoryMarshal.TryRead always succeeds. When the driver returns fewer bytes than the struct size, the remaining fields are leftovers from whatever previously used the pooled array. The method also interprets the symlink-specific layout for every reparse tag, although fields such as Flags only exist for IO_REPARSE_TAG_SYMLINK.

Root cause

• lpBytesReturned was passed as out _ and ignored.
• The span covered the entire rented array rather than the bytes the driver actually wrote.
• The tag-specific union was unconditionally reinterpreted as SymbolicLinkReparseBuffer.
• The rented buffer was returned to the pool only on the success path.

Fix

• Capture lpBytesReturned, validate it against the rented array size, and parse only buffer[0..bytesReturned]. A response shorter than the common header now returns null.
• Clear the fixed header region of the rented array before the call, so stale pooled data cannot leak into the result.
• Replace SymbolicLinkReparseBuffer with ReparseDataBufferHeader (ReparseTag, ReparseDataLength, Reserved), the part common to all reparse tags. Tag-specific data is never interpreted here.
• Return the rented array from a finally block on every path.
• IsJunction is unchanged and still only compares ReparseTag.

Testing

• New tests in TruePath.Tests/DiskUtilsTests.cs: empty and too-short buffers return null; header fields are parsed correctly; tag-specific bytes after the header are ignored; a real non-reparse directory returns null. No elevated privileges needed.
• dotnet build TruePath.sln and dotnet build TruePath.sln -p:CheckXmlDoc=true --no-incremental: clean.
• dotnet test TruePath.Tests: 477 passed, 1 pre-existing failure (AbsolutePathTests.ReadKind_IsSymlink, which needs symlink-creation privilege; verified identical without this change).
• dotnet test ... -- --filter-class TruePath.Tests.DiskUtilsTests: 10/10 passed.