[newlib-cygwin] Cygwin: pipe: Do not call PeekNamedPipe() if it is not necessary.

Corinna Vinschen corinna@sourceware.org
Wed Sep 15 09:56:36 GMT 2021


https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=00cbbaa33e7183a6b414cc0f36bc6cbf0b22d8ce

commit 00cbbaa33e7183a6b414cc0f36bc6cbf0b22d8ce
Author: Takashi Yano <takashi.yano@nifty.ne.jp>
Date:   Wed Sep 15 09:55:57 2021 +0900

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

Diff:
---
 winsup/cygwin/select.cc | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/winsup/cygwin/select.cc b/winsup/cygwin/select.cc
index ac2f3a9e0..fd2312298 100644
--- a/winsup/cygwin/select.cc
+++ b/winsup/cygwin/select.cc
@@ -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)
 	{


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