syscall,internal/poll: move pipe check from syscall.Seek to callers
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>