[matt-edmondson]

Fixes #171

What was wrong

To allow concatenated members, CompressedStreamEnd.Gzip.EndsWith accepted any input whose last 4 bytes, read as ISIZE, were smaller than the output so far (|| trailerLength < length), and it never checked a CRC for that case. In truncated input those bytes are payload. Deflate copies stored blocks verbatim, so a cut just after a zero field or a small little-endian integer was reported as success with partial output on the span, stream and async paths. The repro in the issue reproduces on main.

Change

A trailer whose size is less than the whole output is now accepted only if its CRC-32 matches the output it claims to cover:

• Span path (IsComplete): the whole output is available, so the CRC of output[^trailerLength..] is computed and compared directly. This goes through a new EndsWith(inputTail, output) overload, which ZLib leaves at its default.
• Stream and async paths: Append records where each read's output began, along with the CRC-32 of the output before it. GZipStream returns from a read at the end of each member, so a later member starts at one of these points. The suffix CRC is derived as crc(all) ^ shift(crc(before), suffixLength), using zlib's crc32_combine arithmetic (Multiply / XToThe), so no output has to be kept.

A last member that is empty (trailer size 0) is no longer accepted. Accepting a zero size is what made eight zero bytes at a cut look like a valid trailer, and an empty trailing member is not something the providers produce. ZLib and Brotli are unchanged.

Tests (TruncatedCompressionTests)

• Gzip_Rejects_A_Stored_Block_Payload_Cut_After_A_Zeroed_Field: the issue's repro, checked on all four entry points
• Gzip_Rejects_A_Stored_Block_Payload_Cut_At_Every_Offset: random bytes with small fields followed by zero runs, cut at every offset, on all four entry points
• Gzip_Rejects_Concatenated_Members_Whose_Last_Member_Is_Cut: every cut inside the last member
• Gzip_Accepts_Concatenated_Members_On_Every_Path: three members, the first larger than one 80 KiB read, so the CRC combine runs on the stream paths

The three rejection tests fail on main and pass with the fix. The existing Gzip_Accepts_Concatenated_Members still passes. Full suite: 1349 passed, 1 skipped. Essentials.CompressionProviders.Gzip builds for all its targets with -warnaserror.

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Generated by Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
3 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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