Add support for poll(2) on files. vop_nopoll() now returns POLLNVAL

if one of the new poll types is requested; hopefully this will not break
any existing code.  (This is done so that programs have a dependable
way of determining whether a filesystem supports the extended poll types
or not.)

The new poll types added are:

	POLLWRITE - file contents may have been modified
	POLLNLINK - file was linked, unlinked, or renamed
	POLLATTRIB - file's attributes may have been changed
	POLLEXTEND - file was extended

Note that the internal operation of poll() means that it is impossible
for two processes to reliably poll for the same event (this could
be fixed but may not be worth it), so it is not possible to rewrite
`tail -f' to use poll at this time.

1 files changed, 85 insertions, 1 deletions

diff --git a/sys/kern/vfs_export.c b/sys/kern/vfs_export.c
index 254e9a7..c978bbf 100644
--- a/sys/kern/vfs_export.c
+++ b/sys/kern/vfs_export.c
@@ -36,7 +36,7 @@
  * SUCH DAMAGE.
  *
  *	@(#)vfs_subr.c	8.31 (Berkeley) 5/26/95
- * $Id: vfs_subr.c,v 1.113 1997/11/12 05:42:15 julian Exp $
+ * $Id: vfs_subr.c,v 1.114 1997/11/22 08:35:39 bde Exp $
  */
 
 /*
@@ -1207,6 +1207,7 @@ vclean(vp, flags, p)
 	 * Done with purge, notify sleepers of the grim news.
 	 */
 	vp->v_op = dead_vnodeop_p;
+	vn_pollgone(vp);
 	vp->v_tag = VT_NON;
 	vp->v_flag &= ~VXLOCK;
 	if (vp->v_flag & VXWANT) {
@@ -2111,3 +2112,86 @@ vbusy(vp)
 	simple_unlock(&vnode_free_list_slock);
 	vp->v_flag &= ~VFREE;
 }
+
+/*
+ * Record a process's interest in events which might happen to
+ * a vnode.  Because poll uses the historic select-style interface
+ * internally, this routine serves as both the ``check for any
+ * pending events'' and the ``record my interest in future events''
+ * functions.  (These are done together, while the lock is held,
+ * to avoid race conditions.)
+ */
+int
+vn_pollrecord(vp, p, events)
+	struct vnode *vp;
+	struct proc *p;
+	short events;
+{
+	simple_lock(&vp->v_pollinfo.vpi_lock);
+	if (vp->v_pollinfo.vpi_revents & events) {
+		/*
+		 * This leaves events we are not interested
+		 * in available for the other process which
+		 * which presumably had requested them
+		 * (otherwise they would never have been
+		 * recorded).
+		 */
+		events &= vp->v_pollinfo.vpi_revents;
+		vp->v_pollinfo.vpi_revents &= ~events;
+
+		simple_unlock(&vp->v_pollinfo.vpi_lock);
+		return events;
+	}
+	vp->v_pollinfo.vpi_events |= events;
+	selrecord(p, &vp->v_pollinfo.vpi_selinfo);
+	simple_unlock(&vp->v_pollinfo.vpi_lock);
+	return 0;
+}
+
+/*
+ * Note the occurrence of an event.  If the VN_POLLEVENT macro is used,
+ * it is possible for us to miss an event due to race conditions, but
+ * that condition is expected to be rare, so for the moment it is the
+ * preferred interface.
+ */
+void
+vn_pollevent(vp, events)
+	struct vnode *vp;
+	short events;
+{
+	simple_lock(&vp->v_pollinfo.vpi_lock);
+	if (vp->v_pollinfo.vpi_events & events) {
+		/*
+		 * We clear vpi_events so that we don't
+		 * call selwakeup() twice if two events are
+		 * posted before the polling process(es) is
+		 * awakened.  This also ensures that we take at
+		 * most one selwakeup() if the polling process
+		 * is no longer interested.  However, it does
+		 * mean that only one event can be noticed at
+		 * a time.  (Perhaps we should only clear those
+		 * event bits which we note?) XXX
+		 */
+		vp->v_pollinfo.vpi_events = 0;	/* &= ~events ??? */
+		vp->v_pollinfo.vpi_revents |= events;
+		selwakeup(&vp->v_pollinfo.vpi_selinfo);
+	}
+	simple_unlock(&vp->v_pollinfo.vpi_lock);
+}
+
+/*
+ * Wake up anyone polling on vp because it is being revoked.
+ * This depends on dead_poll() returning POLLHUP for correct
+ * behavior.
+ */
+void
+vn_pollgone(vp)
+	struct vnode *vp;
+{
+	simple_lock(&vp->v_pollinfo.vpi_lock);
+	if (vp->v_pollinfo.vpi_events) {
+		vp->v_pollinfo.vpi_events = 0;
+		selwakeup(&vp->v_pollinfo.vpi_selinfo);
+	}
+	simple_unlock(&vp->v_pollinfo.vpi_lock);
+}