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, 23 insertions, 2 deletions

diff --git a/sys/kern/vfs_default.c b/sys/kern/vfs_default.c
index adebcc7..59f6af4 100644
--- a/sys/kern/vfs_default.c
+++ b/sys/kern/vfs_default.c
@@ -231,13 +231,34 @@ vop_nopoll(ap)
 		struct proc *a_p;
 	} */ *ap;
 {
-
 	/*
-	 * Just return what we were asked for.
+	 * Return true for read/write.  If the user asked for something
+	 * special, return POLLNVAL, so that clients have a way of
+	 * determining reliably whether or not the extended
+	 * functionality is present without hard-coding knowledge
+	 * of specific filesystem implementations.
 	 */
+	if (ap->a_events & ~POLLSTANDARD)
+		return (POLLNVAL);
+
 	return (ap->a_events & (POLLIN | POLLOUT | POLLRDNORM | POLLWRNORM));
 }
 
+/*
+ * Implement poll for local filesystems that support it.
+ */
+int
+vop_stdpoll(ap)
+	struct vop_poll_args /* {
+		struct vnode *a_vp;
+		int  a_events;
+		struct ucred *a_cred;
+		struct proc *a_p;
+	} */ *ap;
+{
+	return (vn_pollrecord(ap->a_vp, ap->a_p, ap->a_events));
+}
+
 int
 vop_stdbwrite(ap)
 	struct vop_bwrite_args *ap;