[release/10.0] Don't flush data in DeflateStream.FlushAsync if no data was written #121796
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rzikm
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rzikm
November 19, 2025 16:08
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Pull Request Overview
This PR fixes a regression in DeflateStream and GZipStream where calling FlushAsync without writing any data could produce a non-empty partial payload that doesn't decompress correctly. The fix ensures consistency between the sync and async flush implementations.
Key Changes
- Added
if (_wroteBytes)check to the asyncFlushAsyncmethod to match the existing syncFlushbehavior - Prevents compression operations when no data has been written to the stream
alinpahontu2912
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Nov 19, 2025
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Fixes #121710.
Customer Impact
Certain valid code patterns using DeflateStream and GZipStream may produce invalid bytes when compressing empty input. The invalid output then cannot be correctly decompressed by 3rd party tools.
Workarounds are either not using async path or ensuring FlushAsync is not called by user code if nothing was written to the stream.
Regression
Behavior differs w.r.t. .NET 8.
The bug has been fixed for .NET 11 by #118570
Testing
Tested scenario reported by customer. Additional validation is provided by CI
Risk
Low, fix is small and the root cause is well understood.