Cygwin: select: don't report read ready on a FIFO never opened for writing

According to POSIX and the Linux man page, select(2) is supposed to
report read ready if a file is at EOF.  In the case of a FIFO, this
means that the pipe is empty and there are no writers.  But there
seems to be an undocumented exception, observed on Linux and other
platforms:  If no writer has ever been opened, then select(2) does not
report read ready.  This can happen if a reader is opened with
O_NONBLOCK before any writers have opened.

This commit makes Cygwin consistent with those other platforms by
introducing a special EOF test, fhandler_fifo::select_hit_eof, which
returns false if there's never been a writer opened.

To implement this we use a new variable '_writer_opened' in the FIFO's
shared memory, which is set to 1 the first time a writer opens.  New
methods writer_opened() and set_writer_opened() are used to test and
set this variable.

Addresses: https://cygwin.com/pipermail/cygwin/2022-September/252223.html

1 files changed, 1 insertions, 0 deletions

diff --git a/winsup/cygwin/fhandler/fifo.cc b/winsup/cygwin/fhandler/fifo.cc
index 1d3e42908..efea508ae 100644
--- a/winsup/cygwin/fhandler/fifo.cc
+++ b/winsup/cygwin/fhandler/fifo.cc
@@ -1043,6 +1043,7 @@ writer_shmem:
   set_pipe_non_blocking (get_handle (), flags & O_NONBLOCK);
   nwriters_lock ();
   inc_nwriters ();
+  set_writer_opened ();
   SetEvent (write_ready);
   ResetEvent (writer_opening);
   nwriters_unlock ();