From 4a54322f0b8dcd72b1946004f9b964ef0093af1a Mon Sep 17 00:00:00 2001
From: ed <ed@FreeBSD.org>
Date: Wed, 5 Aug 2015 07:34:29 +0000
Subject: [PATCH] Make it possible to implement poll(2) on top of kqueue(2).

It looks like EVFILT_READ and EVFILT_WRITE trigger under the same
conditions as poll()'s POLLRDNORM and POLLWRNORM as described by POSIX.
The only difference is that POLLRDNORM has to be triggered on regular
files unconditionally, whereas EVFILT_READ only triggers when not EOF.

Introduce a new flag, NOTE_FILE_POLL, that can be used to make
EVFILT_READ and EVFILT_WRITE behave identically to poll(). This flag
will be used by cloudlibc's poll() function.

Reviewed by:	jmg
Differential Revision:	https://reviews.freebsd.org/D3303
---
 lib/libc/sys/kqueue.2 | 10 +++++++++-
 sys/kern/vfs_subr.c   |  2 +-
 sys/sys/event.h       |  1 +
 3 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/lib/libc/sys/kqueue.2 b/lib/libc/sys/kqueue.2
index c5e8cafd5579..ca94852cba29 100644
--- a/lib/libc/sys/kqueue.2
+++ b/lib/libc/sys/kqueue.2
@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd March 29, 2015
+.Dd August 4, 2015
 .Dt KQUEUE 2
 .Os
 .Sh NAME
@@ -288,6 +288,14 @@ Returns when the file pointer is not at the end of file.
 .Va data
 contains the offset from current position to end of file,
 and may be negative.
+.Pp
+This behavior is different from
+.Xr poll 2 ,
+where read events are triggered for regular files unconditionally.
+This event can be triggered unconditionally by setting the
+.Dv NOTE_FILE_POLL
+flag in
+.Va fflags .
 .It "Fifos, Pipes"
 Returns when the there is data to read;
 .Va data
diff --git a/sys/kern/vfs_subr.c b/sys/kern/vfs_subr.c
index 42cab157573a..3c189d1a2df5 100644
--- a/sys/kern/vfs_subr.c
+++ b/sys/kern/vfs_subr.c
@@ -4622,7 +4622,7 @@ filt_vfsread(struct knote *kn, long hint)
 
 	VI_LOCK(vp);
 	kn->kn_data = va.va_size - kn->kn_fp->f_offset;
-	res = (kn->kn_data != 0);
+	res = (kn->kn_sfflags & NOTE_FILE_POLL) != 0 || kn->kn_data != 0;
 	VI_UNLOCK(vp);
 	return (res);
 }
diff --git a/sys/sys/event.h b/sys/sys/event.h
index 089c9b3874f0..0f132318c827 100644
--- a/sys/sys/event.h
+++ b/sys/sys/event.h
@@ -108,6 +108,7 @@ struct kevent {
  * data/hint flags for EVFILT_{READ|WRITE}, shared with userspace
  */
 #define NOTE_LOWAT	0x0001			/* low water mark */
+#define NOTE_FILE_POLL	0x0002			/* behave like poll() */
 
 /*
  * data/hint flags for EVFILT_VNODE, shared with userspace