aa4d5e739f
On Windows, syscall.Seek is a thin wrapper over SetFilePointerEx [1], which does not work on pipes, although it doesn't return an error on that case. To avoid this undefined behavior, Seek defensively calls GetFileType and errors if the type is FILE_TYPE_PIPE. The problem with this approach is that Seek is a low level foundational function that can be called many times for the same file, and the additional cgo call (GetFileType) will artificially slow down seek operations. I've seen GetFileType to account for 10% of cpu time in seek-intensive workloads. A better approach, implemented in this CL, would be to move the check one level up, where many times the file type is already known so the GetFileType is unnecessary. The drawback is that syscall.Seek has had this behavior since pipes where first introduced to Windows in https://codereview.appspot.com/1715046 and someone could be relying on it. On the other hand, this behavior is not documented, so we couldn't be breaking any contract. [1] https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-setfilepointerex Change-Id: I7602182f9d08632e22a8a1635bc8ad9ad35a5056 Reviewed-on: https://go-review.googlesource.com/c/go/+/493626 Run-TryBot: Quim Muntal <quimmuntal@gmail.com> Reviewed-by: Alex Brainman <alex.brainman@gmail.com> TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Bryan Mills <bcmills@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com> |
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