Fix select/poll/kqueue for write on reverse direction before first write.

The reverse direction of a pipe is lazily allocated on the first write in
that direction (because pipes are usually used in one direction only).  A
special case is needed to ensure the pipe appears writable before the first
write because there are 0 bytes of pending data in 0 bytes of buffer space
at that point, leaving 0 bytes of data that can be written with the normal
code.

Note that the first write returns [ENOMEM] if kern.ipc.maxpipekva is
exceeded and does not block or return [EAGAIN], so selecting true for write
is correct even in that case.

PR:		kern/93685
Submitted by:	gianni
MFC after:	2 weeks

1 files changed, 4 insertions, 2 deletions

diff --git a/sys/kern/sys_pipe.c b/sys/kern/sys_pipe.c
index 9a4448a..9edcb74 100644
--- a/sys/kern/sys_pipe.c
+++ b/sys/kern/sys_pipe.c
@@ -1349,7 +1349,8 @@ pipe_poll(fp, events, active_cred, td)
 		if (wpipe->pipe_present != PIPE_ACTIVE ||
 		    (wpipe->pipe_state & PIPE_EOF) ||
 		    (((wpipe->pipe_state & PIPE_DIRECTW) == 0) &&
-		     (wpipe->pipe_buffer.size - wpipe->pipe_buffer.cnt) >= PIPE_BUF))
+		     ((wpipe->pipe_buffer.size - wpipe->pipe_buffer.cnt) >= PIPE_BUF ||
+			 wpipe->pipe_buffer.size == 0)))
 			revents |= events & (POLLOUT | POLLWRNORM);
 
 	if ((events & POLLINIGNEOF) == 0) {
@@ -1660,7 +1661,8 @@ filt_pipewrite(struct knote *kn, long hint)
 		PIPE_UNLOCK(rpipe);
 		return (1);
 	}
-	kn->kn_data = wpipe->pipe_buffer.size - wpipe->pipe_buffer.cnt;
+	kn->kn_data = (wpipe->pipe_buffer.size > 0) ?
+	    (wpipe->pipe_buffer.size - wpipe->pipe_buffer.cnt) : PIPE_BUF;
 	if (wpipe->pipe_state & PIPE_DIRECTW)
 		kn->kn_data = 0;