Fix StreamPipeReader.CopyToAsync not advancing past buffered data#124989
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BrennanConroy merged 2 commits intodotnet:mainfrom Mar 2, 2026
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Both CopyToAsync overloads (PipeWriter and Stream destinations) iterate buffered segments, but on successful drain the loop exits with segment set to null. The finally block only called AdvanceTo when segment was non-null, so buffered data remained in _readHead/_readIndex and would be returned again on subsequent reads. Add an else-if branch to the finally block that advances past all buffered data (_readTail) when the segment loop completes normally. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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Pull request overview
Fixes a StreamPipeReader.CopyToAsync buffering bug where successfully draining all buffered segments could leave _readHead/_readIndex unchanged, causing the same buffered data to be returned again on subsequent reads.
Changes:
- Update
StreamPipeReader.CopyToAsync(bothPipeWriterandStreamdestinations) to advance past all buffered data when the buffered-segment loop completes normally. - Add regression tests ensuring
CopyToAsyncadvances buffered data for both destination overloads.
Reviewed changes
Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.
| File | Description |
|---|---|
src/libraries/System.IO.Pipelines/src/System/IO/Pipelines/StreamPipeReader.cs |
Ensures buffered segments are fully advanced/cleared after a successful buffered drain in both CopyToAsync overloads. |
src/libraries/System.IO.Pipelines/tests/PipeReaderCopyToAsyncTests.cs |
Adds tests validating that buffered data is not re-observed after CopyToAsync completes. |
stephentoub
approved these changes
Feb 27, 2026
…ests.cs Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
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Both CopyToAsync overloads on StreamPipeReader (PipeWriter and Stream destinations) iterate buffered segments, but on successful drain the loop exits with segment set to null. The finally block only called AdvanceTo when segment was non-null, so buffered data remained in _readHead/_readIndex and would be returned again on subsequent reads.
Add an else-if branch to the finally block that advances past all buffered data (_readTail) when the segment loop completes normally.