I am comparing current pipe code with the one in 8.3-STABLE r236165,

I found 8.3 is a history BSD version using socket to implement FIFO
pipe, it uses per-file seqcount to compare with writer generation
stored in per-pipe object. The concept is after all writers are gone,
the pipe enters next generation, all old readers have not closed the
pipe should get the indication that the pipe is disconnected, result
is they should get EPIPE, SIGPIPE or get POLLHUP in poll().
But newcomer should not know that previous writters were gone, it
should treat it as a fresh session.
I am trying to bring back FIFO pipe to history behavior. It is still
unclear that if single EOF flag can represent SBS_CANTSENDMORE and
SBS_CANTRCVMORE which socket-based version is using, but I have run
the poll regression test in tool directory, output is same as the one
on 8.3-STABLE now.
I think the output "not ok 18 FIFO state 6b: poll result 0 expected 1.
expected POLLHUP; got 0" might be bogus, because newcomer should not
know that old writers were gone. I got the same behavior on Linux.
Our implementation always return POLLIN for disconnected pipe even it
should return POLLHUP, but I think it is not wise to remove POLLIN for
compatible reason, this is our history behavior.

Regression test: /usr/src/tools/regression/poll

3 files changed, 6 insertions, 19 deletions

diff --git a/sys/fs/fifofs/fifo_vnops.c b/sys/fs/fifofs/fifo_vnops.c
index 859f68c..bdb1367 100644
--- a/sys/fs/fifofs/fifo_vnops.c
+++ b/sys/fs/fifofs/fifo_vnops.c
@@ -59,23 +59,13 @@
  * Notes about locking:
  *   - fi_pipe is invariant since init time.
  *   - fi_readers and fi_writers are protected by the vnode lock.
- *   - fi_wgen and fi_seqcount are protected by the pipe mutex.
  */
 struct fifoinfo {
 	struct pipe *fi_pipe;
 	long	fi_readers;
 	long	fi_writers;
-	int	fi_wgen;
-	int	fi_seqcount;
 };
 
-#define FIFO_UPDWGEN(fip, pip)	do { \
-	if ((fip)->fi_wgen == (fip)->fi_seqcount) 			\
-		(pip)->pipe_state |= PIPE_SAMEWGEN;			\
-	else								\
-		(pip)->pipe_state &= ~PIPE_SAMEWGEN;			\
-} while (0)
-
 static vop_print_t	fifo_print;
 static vop_open_t	fifo_open;
 static vop_close_t	fifo_close;
@@ -161,7 +151,7 @@ fifo_open(ap)
 			return (error);
 		fip = malloc(sizeof(*fip), M_VNODE, M_WAITOK);
 		fip->fi_pipe = fpipe;
-		fip->fi_wgen = fip->fi_readers = fip->fi_writers = 0;
+		fpipe->pipe_wgen = fip->fi_readers = fip->fi_writers = 0;
  		KASSERT(vp->v_fifoinfo == NULL, ("fifo_open: v_fifoinfo race"));
 		vp->v_fifoinfo = fip;
 	}
@@ -181,8 +171,7 @@ fifo_open(ap)
 			if (fip->fi_writers > 0)
 				wakeup(&fip->fi_writers);
 		}
-		fip->fi_seqcount = fip->fi_wgen - fip->fi_writers;
-		FIFO_UPDWGEN(fip, fpipe);
+		fp->f_seqcount = fpipe->pipe_wgen - fip->fi_writers;
 	}
 	if (ap->a_mode & FWRITE) {
 		if ((ap->a_mode & O_NONBLOCK) && fip->fi_readers == 0) {
@@ -235,8 +224,7 @@ fifo_open(ap)
 					fpipe->pipe_state |= PIPE_EOF;
 					if (fpipe->pipe_state & PIPE_WANTR)
 						wakeup(fpipe);
-					fip->fi_wgen++;
-					FIFO_UPDWGEN(fip, fpipe);
+					fpipe->pipe_wgen++;
 					PIPE_UNLOCK(fpipe);
 					fifo_cleanup(vp);
 				}
@@ -300,8 +288,7 @@ fifo_close(ap)
 				cpipe->pipe_state &= ~PIPE_WANTR;
 				wakeup(cpipe);
 			}
-			fip->fi_wgen++;
-			FIFO_UPDWGEN(fip, cpipe);
+			cpipe->pipe_wgen++;
 			pipeselwakeup(cpipe);
 			PIPE_UNLOCK(cpipe);
 		}
diff --git a/sys/kern/sys_pipe.c b/sys/kern/sys_pipe.c
index 02664e9..338256c 100644
--- a/sys/kern/sys_pipe.c
+++ b/sys/kern/sys_pipe.c
@@ -1442,7 +1442,7 @@ pipe_poll(fp, events, active_cred, td)
 	levents = events &
 	    (POLLIN | POLLINIGNEOF | POLLPRI | POLLRDNORM | POLLRDBAND);
 	if (rpipe->pipe_state & PIPE_NAMED && fp->f_flag & FREAD && levents &&
-	    rpipe->pipe_state & PIPE_SAMEWGEN)
+	    fp->f_seqcount == rpipe->pipe_wgen)
 		events |= POLLINIGNEOF;
 
 	if ((events & POLLINIGNEOF) == 0) {
diff --git a/sys/sys/pipe.h b/sys/sys/pipe.h
index f0029a8..dfe7f2f 100644
--- a/sys/sys/pipe.h
+++ b/sys/sys/pipe.h
@@ -96,7 +96,6 @@ struct pipemapping {
 #define PIPE_DIRECTW	0x400	/* Pipe direct write active. */
 #define PIPE_DIRECTOK	0x800	/* Direct mode ok. */
 #define PIPE_NAMED	0x1000	/* Is a named pipe. */
-#define PIPE_SAMEWGEN	0x2000	/* same write generation for named pipes. */
 
 /*
  * Per-pipe data structure.
@@ -115,6 +114,7 @@ struct pipe {
 	u_int	pipe_state;		/* pipe status info */
 	int	pipe_busy;		/* busy flag, mostly to handle rundown sanely */
 	int	pipe_present;		/* still present? */
+	int	pipe_wgen;		/* writer generation for named pipe */
 	ino_t	pipe_ino;		/* fake inode for stat(2) */
 };