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Cygwin: pipe: Do not call PeekNamedPipe() if it is not necessary.

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- In pipe_data_available() in select.cc, PeekNamedPipe() call is
  not needed if WriteQuotaAvailable is non-zero because we already
  know the write pipe has a space. Therefore, with this patch,
  PeekNamedPipe() is called only when WriteQuotaAvailable is zero.
  This makes select() on pipe faster a bit.
This commit is contained in:
Takashi Yano 2021-09-15 09:55:57 +09:00 committed by Corinna Vinschen
parent 593a86f9b0
commit 00cbbaa33e
1 changed files with 6 additions and 8 deletions
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14
winsup/cygwin/select.cc
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@ -633,17 +633,15 @@ pipe_data_available (int fd, fhandler_base *fh, HANDLE h, bool writing)
/* Note: Do not use NtQueryInformationFile() for query_hdl because
NtQueryInformationFile() seems to interfere with reading pipes
in non-cygwin apps. Instead, use PeekNamedPipe() here. */
if (fh->get_device () == FH_PIPEW)
if (fh->get_device () == FH_PIPEW && fpli.WriteQuotaAvailable == 0)
{
HANDLE query_hdl = ((fhandler_pipe *) fh)->get_query_handle ();
if (query_hdl)
{
DWORD nbytes_in_pipe;
PeekNamedPipe (query_hdl, NULL, 0, NULL, &nbytes_in_pipe, NULL);
fpli.WriteQuotaAvailable = fpli.InboundQuota - nbytes_in_pipe;
}
else
if (!query_hdl)
return 1; /* We cannot know actual write pipe space. */
DWORD nbytes_in_pipe;
if (!PeekNamedPipe (query_hdl, NULL, 0, NULL, &nbytes_in_pipe, NULL))
return 1;
fpli.WriteQuotaAvailable = fpli.InboundQuota - nbytes_in_pipe;
}
if (fpli.WriteQuotaAvailable > 0)
{