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[release/9.0] Fix DeflateStream.FlushAsync if no data has been written #121798

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[release/9.0] Fix DeflateStream.FlushAsync if no data has been written #121798

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29 changes: 16 additions & 13 deletions src/libraries/System.IO.Compression/src/System/IO/Compression/DeflateZLib/DeflateStream.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -220,21 +220,24 @@ async Task Core(CancellationToken cancellationToken)
{
Debug.Assert(_deflater != null && _buffer != null);

// Compress any bytes left:
await WriteDeflaterOutputAsync(cancellationToken).ConfigureAwait(false);

// Pull out any bytes left inside deflater:
bool flushSuccessful;
do
if (_wroteBytes)
{
int compressedBytes;
flushSuccessful = _deflater.Flush(_buffer, out compressedBytes);
if (flushSuccessful)
// Compress any bytes left:
await WriteDeflaterOutputAsync(cancellationToken).ConfigureAwait(false);

// Pull out any bytes left inside deflater:
bool flushSuccessful;
do
{
await _stream.WriteAsync(new ReadOnlyMemory<byte>(_buffer, 0, compressedBytes), cancellationToken).ConfigureAwait(false);
}
Debug.Assert(flushSuccessful == (compressedBytes > 0));
} while (flushSuccessful);
int compressedBytes;
flushSuccessful = _deflater.Flush(_buffer, out compressedBytes);
if (flushSuccessful)
{
await _stream.WriteAsync(new ReadOnlyMemory<byte>(_buffer, 0, compressedBytes), cancellationToken).ConfigureAwait(false);
}
Debug.Assert(flushSuccessful == (compressedBytes > 0));
} while (flushSuccessful);
}
Comment on lines +223 to +240
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Copilot AI Nov 19, 2025

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[nitpick] The entire deflater flush block is indented one level deeper, but the comment on line 225 still says 'Compress any bytes left' which may be slightly misleading when _wroteBytes is false and no compression occurs. Consider updating the comment to clarify the conditional nature, such as '// Compress any bytes left if data was written:'

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// Always flush on the underlying stream
await _stream.FlushAsync(cancellationToken).ConfigureAwait(false);
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