Builder: Re-use the old buffer after trimming #581
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@clyring could you please rebase to get a clean CI run? |
Bodigrim
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May 20, 2023
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Thanks! |
Bodigrim
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May 28, 2023
adamgundry
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`toLazyByteString :: Builder -> LazyByteString` had a race condition that could generate wrong results if two threads concurrently evaluated the result. This bug was introduced in haskell#581 (5c4d236) and first present in release 0.11.5.0 (as 0c030bb). Due to the use of `unsafeDupablePerformIO` for performance, it is critical that any IO actions executed when running a `Builder` can be interrupted or executed multiple times. In principle, filling a buffer is safe provided the buffer is used only once and the same bytes are written each time. However, `wrapChunk` in `buildStepToCIOS` would re-use a buffer in the trimming case after copying its contents to produce a new trimmed chunk. This is safe when run in a single thread, but if two threads simultaneously execute the code, one of them may still be copying the contents while the other starts overwriting the buffer. This patch fixes `wrapChunk` to unconditionally allocate a new buffer after trimming, rather than re-using the old buffer. This will presumably come at a slight performance cost for builders inserting many trimmed chunks.
clyring
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Sep 18, 2024
`toLazyByteString :: Builder -> LazyByteString` had a race condition that could generate wrong results if two threads concurrently evaluated the result. This bug was introduced in #581 (5c4d236) and first present in release 0.11.5.0 (as 0c030bb). Due to the use of `unsafeDupablePerformIO` for performance, it is critical that any IO actions executed when running a `Builder` can be interrupted or executed multiple times. In principle, filling a buffer is safe provided the buffer is used only once and the same bytes are written each time. However, `wrapChunk` in `buildStepToCIOS` would re-use a buffer in the trimming case after copying its contents to produce a new trimmed chunk. This is safe when run in a single thread, but if two threads simultaneously execute the code, one of them may still be copying the contents while the other starts overwriting the buffer. This patch fixes `wrapChunk` to unconditionally allocate a new buffer after trimming, rather than re-using the old buffer. This will presumably come at a slight performance cost for builders inserting many trimmed chunks.
Bodigrim
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Oct 9, 2024
`toLazyByteString :: Builder -> LazyByteString` had a race condition that could generate wrong results if two threads concurrently evaluated the result. This bug was introduced in #581 (5c4d236) and first present in release 0.11.5.0 (as 0c030bb). Due to the use of `unsafeDupablePerformIO` for performance, it is critical that any IO actions executed when running a `Builder` can be interrupted or executed multiple times. In principle, filling a buffer is safe provided the buffer is used only once and the same bytes are written each time. However, `wrapChunk` in `buildStepToCIOS` would re-use a buffer in the trimming case after copying its contents to produce a new trimmed chunk. This is safe when run in a single thread, but if two threads simultaneously execute the code, one of them may still be copying the contents while the other starts overwriting the buffer. This patch fixes `wrapChunk` to unconditionally allocate a new buffer after trimming, rather than re-using the old buffer. This will presumably come at a slight performance cost for builders inserting many trimmed chunks.
Bodigrim
pushed a commit
that referenced
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Oct 15, 2024
`toLazyByteString :: Builder -> LazyByteString` had a race condition that could generate wrong results if two threads concurrently evaluated the result. This bug was introduced in #581 (5c4d236) and first present in release 0.11.5.0 (as 0c030bb). Due to the use of `unsafeDupablePerformIO` for performance, it is critical that any IO actions executed when running a `Builder` can be interrupted or executed multiple times. In principle, filling a buffer is safe provided the buffer is used only once and the same bytes are written each time. However, `wrapChunk` in `buildStepToCIOS` would re-use a buffer in the trimming case after copying its contents to produce a new trimmed chunk. This is safe when run in a single thread, but if two threads simultaneously execute the code, one of them may still be copying the contents while the other starts overwriting the buffer. This patch fixes `wrapChunk` to unconditionally allocate a new buffer after trimming, rather than re-using the old buffer. This will presumably come at a slight performance cost for builders inserting many trimmed chunks.
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It might be nice to work out how much this affects the balance of power between trimming and not trimming. But it's a nice incremental improvement without that.