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The bug is that the check `if (size - w > count)` should be `if (size - w >= count)` instead (`>=` instead of `>`), because we can do the write all fine if it fits exactly. This allows us to do 1 instead of 2 write syscalls the case it fits. An example of when this matters is when an application writes output in chunks that are a fraction of the handle buffer size. For example, assume the buffer size is 8 KB, and the application writes four 2 KB chunks. Until now, this would result in 3 copies to the handle buffer, but the 4th one would not be allowed in by `size - w > count` (because `size - w == count` is the case), so we'd end up with a write syscall of only 6 KB data instead of 8 KB, thus creating more syscalls overall. Implementing this fix (switching to `size - w >= count`), we also have to flush the buffer if we fill it completely. If we made only the changes described so far, that would have the unintended side effect that writes of the size equal to the handle buffer size (`count == size`) suddenly also go to the handle buffer first: The data would first be copied to the handle buffer, and then immediately get flushed to the underlying FD. We don't want that extra `memcpy`, because it'd be unnecessary: The point of handle buffering is to coalesce smaller writes, and there are no smaller writes in this case. For example, if you specify 8 KB buffers (which menas you want your data to be written out in 8 KB blocks), and you get data that's already 8 KB in size, you can write that out as an 8 KB straight away, zero-copy fashion. For this reason, adding to the handle buffer now got an additional condition `count < size`. That way, writes equal to the buffer size go straight to the FD, as they did before this commit. Reviewers: simonmar, austin, hvr, bgamari Reviewed By: simonmar Subscribers: mpickering, thomie Differential Revision: https://phabricator.haskell.org/D3117
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