Patch attached. I'm also attaching a test case, that behaves the same on Cygwin
as on Linux.
Ken#include
#include
#include
#include
#include
#include
#include
#define FIFO_PATH "/tmp/myfifo"
int
main ()
{
int fd, tmpfd, nsel;
fd_set readfds;
struct timeval wait_tm = { 0l, 20l }; /* 200 millisecs */
if (unlink (FIFO_PATH) < 0 && errno != ENOENT)
{
perror ("unlink");
exit (1);
}
if (mkfifo (FIFO_PATH, S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH
| S_IWOTH) < 0)
{
perror ("mkfifo");
exit (1);
}
printf ("Opening a FIFO for reading with O_NONBLOCK\n");
if ((fd = open (FIFO_PATH, O_RDONLY | O_NONBLOCK)) < 0)
{
perror ("open");
exit (1);
}
printf ("Testing for read ready...\n");
FD_ZERO (&readfds);
FD_SET (fd, &readfds);
nsel = select (fd + 1, &readfds, NULL, NULL, &wait_tm);
printf (" select returned %d\n", nsel);
printf ("Opening and closing FIFO for writing...\n");
if ((tmpfd = open (FIFO_PATH, O_WRONLY)) < 0)
{
perror ("open");
exit (1);
}
if (close (tmpfd) < 0)
{
perror ("close");
exit (1);
}
FD_ZERO (&readfds);
FD_SET (fd, &readfds);
nsel = select (fd + 1, &readfds, NULL, NULL, &wait_tm);
printf (" now select returned %d\n", nsel);
}
From f4a92734eac17dbfc7ff3541eef9611c87184ed0 Mon Sep 17 00:00:00 2001
From: Ken Brown
Date: Fri, 23 Sep 2022 10:24:04 -0400
Subject: [PATCH] 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
---
winsup/cygwin/fhandler/fifo.cc | 2 ++
winsup/cygwin/local_includes/fhandler.h | 9 +
winsup/cygwin/release/3.3.4 | 3 +++
winsup/cygwin/select.cc | 12 +++-
4 files changed, 25 insertions(+), 1 deletion(-)
diff --git a/winsup/cygwin/fhandler/fifo.cc b/winsup/cygwin/fhandler/fifo.cc
index 1d3e42908..ecfb33bff 100644
--- a/winsup/cygwin/fhandler/fifo.cc
+++ b/winsup/cygwin/fhandler/fifo.cc
@@ -1043,6 +1043,8 @@ writer_shmem:
set_pipe_non_blocking (get_handle (), flags & O_NONBLOCK);
nwriters_lock ();
inc_nwriters ();
+ if (!writer_opened () )
+set_writer_opened ();
SetEvent (write_ready);
ResetEvent (writer_opening);
nwriters_unlock ();
diff --git a/winsup/cygwin/local_includes/fhandler.h
b/winsup/cygwin/local_includes/fhandler.h
index aad7f4c37..b012c6e8f 100644
--- a/winsup/cygwin/local_includes/fhandler.h
+++ b/winsup/cygwin/local_includes/fhandler.h
@@ -1327,6 +1327,8 @@ struct fifo_reader_id_t
class fifo_shmem_t
{
LONG _nreaders, _nwriters;
+ /* Set to 1 the first time a writer opens. */
+ LONG _writer_opened;
fifo_reader_id_t _owner, _prev_owner, _pending_owner;
af_unix_spinlock_t _owner_lock, _reading_lock, _nreaders_lock,
_nwriters_lock;
@@ -1343,6 +1345,9 @@ public:
int inc_nwriters () { return (int) InterlockedIncrement (&_nwriters); }
int dec_nwriters () { return (int) InterlockedDecrement (&_nwriters); }
+ bool writer_opened () const { return (bool) _writer_opened; }
+ void set_writer_opened () { InterlockedExchange (&_writer_opened, 1); }
+
fifo_reader_id_t get_owner () const { return _owner; }
void set_owner (fifo_reader_id_t fr_id) { _owner = fr_id; }
fifo_reader_id_t get_prev_owner () const { return _prev_owner; }
@@ -1425,6 +1430,8 @@ class fhandler_fifo: public fhandler_pipe_fifo
int nwriters () const { return shmem->nwriters (); }
int inc_nwriters () { return shmem->inc_nwriters (); }
int dec_nwriters () { return shmem->dec_nwriters (); }
+ bool writer_opened () const { return shmem->writer_opened (); }
+ void set_writer_opened () { shmem->set_writer_opened (); }
void nreaders_lock () { shmem->nreaders_lock (); }
void nreaders_unlock () { shmem->nreaders_unlock (); }
void
