[newlib-cygwin] Cygwin: select: Set errno when peek() returns -1

[Takashi Yano]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=6a588c29b562c6da08061f68d71733c78fbfefc3

commit 6a588c29b562c6da08061f68d71733c78fbfefc3
Author: Takashi Yano <takashi.yano@nifty.ne.jp>
Date:   Sat Apr 18 04:31:15 2026 +0900

    Cygwin: select: Set errno when peek() returns -1
    
    Currently, poll() sometimes returns -1 with errno == 0 if the fd is
    not opened. This is due to lack of setting errno when peek() fails.
    This patch ensure to set errno to thread_errno if peek() returns -1.
    
    Addresses: https://cygwin.com/pipermail/cygwin/2026-April/259602.html
    Fixes: 8382778cdb57 ("Cygwin: console: fix select() behaviour")
    Reported-by: Nahor <nahor.j+cygwin@gmail.com>
    Signed-off-by: Takashi Yano <takashi.yano@nifty.ne.jp>
    Reviewed-by: Corinna Vinschen <corinna@vinschen.de>

Diff:
---
 winsup/cygwin/release/3.6.8 | 4 ++++
 winsup/cygwin/select.cc     | 5 ++++-
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/winsup/cygwin/release/3.6.8 b/winsup/cygwin/release/3.6.8
index dc7103c67..5199ad86b 100644
--- a/winsup/cygwin/release/3.6.8
+++ b/winsup/cygwin/release/3.6.8
@@ -22,3 +22,7 @@ Fixes:
   Addresses: https://github.com/git-for-windows/git/issues/5632
 
 - Fix pty bugs regarding GDB with pty.
+
+- Add missing errno setting on error that poll() is called for closed
+  file descriptor.
+  Addresses: https://cygwin.com/pipermail/cygwin/2026-April/259602.html
diff --git a/winsup/cygwin/select.cc b/winsup/cygwin/select.cc
index 8a94ac076..523c46ee6 100644
--- a/winsup/cygwin/select.cc
+++ b/winsup/cygwin/select.cc
@@ -561,7 +561,10 @@ select_stuff::poll (fd_set *readfds, fd_set *writefds, fd_set *exceptfds)
     {
       int ret = s->peek ? s->peek (s, true) : 1;
       if (ret < 0)
-	return -1;
+	{
+	  set_errno (s->thread_errno);
+	  return -1;
+	}
       n += (ret > 0) ?  set_bits (s, readfds, writefds, exceptfds) : 0;
     }
   return n;