Fire watches once immediately upon registration.

It matches the model of programming used to avoid races with watches,
and also makes re-establishing watches on daemon restart easier.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Christian Limpach <Christian.Limpach@cl.cam.ac.uk>

linux-2.6-xen-sparse/drivers/xen/blkback/xenbus.c

@@ -228,6 +228,7 @@ static int blkback_probe(struct xenbus_device *dev,
 	be->dev = dev;
 	be->backend_watch.node = dev->nodename;
 	be->backend_watch.callback = backend_changed;
+	/* Will implicitly call backend_changed once. */
 	err = register_xenbus_watch(&be->backend_watch);
 	if (err) {
 		be->backend_watch.node = NULL;
@@ -249,8 +250,6 @@ static int blkback_probe(struct xenbus_device *dev,
 	}
 
 	dev->data = be;
-
-	backend_changed(&be->backend_watch, dev->nodename);
 	return 0;
 
  free_be:

linux-2.6-xen-sparse/drivers/xen/blktap/xenbus.c

@@ -172,6 +172,7 @@ static int blkback_probe(struct xenbus_device *dev,
 	be->dev = dev;
 	be->backend_watch.node = dev->nodename;
 	be->backend_watch.callback = backend_changed;
+	/* Registration implicitly fires backend_changed once */
 	err = register_xenbus_watch(&be->backend_watch);
 	if (err) {
 		be->backend_watch.node = NULL;
@@ -193,8 +194,6 @@ static int blkback_probe(struct xenbus_device *dev,
 	}
 
 	dev->data = be;
-
-	backend_changed(&be->backend_watch, dev->nodename);
 	return 0;
 
  free_be:

linux-2.6-xen-sparse/drivers/xen/netback/xenbus.c

@@ -242,6 +242,7 @@ static int netback_probe(struct xenbus_device *dev,
 	be->dev = dev;
 	be->backend_watch.node = dev->nodename;
 	be->backend_watch.callback = backend_changed;
+	/* Registration implicitly calls backend_changed. */
 	err = register_xenbus_watch(&be->backend_watch);
 	if (err) {
 		be->backend_watch.node = NULL;
@@ -263,8 +264,6 @@ static int netback_probe(struct xenbus_device *dev,
 	}
 
 	dev->data = be;
-
-	backend_changed(&be->backend_watch, dev->nodename);
 	return 0;
 
  free_be:

linux-2.6-xen-sparse/drivers/xen/netfront/netfront.c

@@ -1284,10 +1284,6 @@ static int netfront_probe(struct xenbus_device *dev,
 		return err;
 	}
 
-
-	/* Call once in case entries already there. */
-	watch_for_status(&info->watch, info->watch.node);
-
 	return 0;
 }
 

linux-2.6-xen-sparse/drivers/xen/tpmback/xenbus.c

@@ -213,6 +213,7 @@ static int tpmback_probe(struct xenbus_device *dev,
 
 	be->dev = dev;
 	be->backend_watch.node     = dev->nodename;
+	/* Implicitly calls backend_changed() once. */
 	be->backend_watch.callback = backend_changed;
 	be->instance = -1;
 	err = register_xenbus_watch(&be->backend_watch);
@@ -236,8 +237,6 @@ static int tpmback_probe(struct xenbus_device *dev,
 	}
 
 	dev->data = be;
-
-	backend_changed(&be->backend_watch, dev->nodename);
 	return err;
 
 free_be:

linux-2.6-xen-sparse/drivers/xen/tpmfront/tpmfront.c

@@ -411,7 +411,6 @@ static int tpmfront_probe(struct xenbus_device *dev,
 		return err;
 	}
 
-	watch_for_status(&info->watch, info->watch.node);
 	return 0;
 }
 

tools/xenstore/testsuite/07watch.test

@@ -168,10 +168,12 @@ expect 1: waitwatch failed: Connection timed out
 # Watch event must have happened before we registered interest.
 1 watch / token
 2 write /test/subnode contents2
-1 watch / token2 0
+1 watchnoack / token2 0
 expect 1:/test/subnode:token
 1 waitwatch
 1 ackwatch token
+expect 1:/:token2
+1 waitwatch
 expect 1: waitwatch failed: Connection timed out
 1 waitwatch
 1 close

tools/xenstore/testsuite/14complexperms.test

@@ -23,7 +23,8 @@ expect *Permission denied
 getperm /dir/file 
 expect *Permission denied
 setperm /dir/file 0 NONE 
-watch /dir/file token 
+# We get no watch event when there's no permission.  It's a corner case.
+watchnoack /dir/file token 
 1 write /dir/file contents
 1 rm /dir/file
 expect waitwatch failed: Connection timed out
@@ -63,7 +64,7 @@ expect *Permission denied
 getperm /dir/file 
 expect *Permission denied
 setperm /dir/file 0 NONE 
-watch /dir/file token 
+watchnoack /dir/file token 
 1 write /dir/file contents
 1 rm /dir/file
 expect waitwatch failed: Connection timed out

tools/xenstore/xenstored_watch.c

@@ -236,6 +236,9 @@ void do_watch(struct connection *conn, struct buffered_data *in)
 	trace_create(watch, "watch");
 	talloc_set_destructor(watch, destroy_watch);
 	send_ack(conn, XS_WATCH);
+
+	/* We fire once up front: simplifies clients and restart. */
+	add_event(conn, watch, watch->node);
 }
 
 void do_watch_ack(struct connection *conn, const char *token)

tools/xenstore/xs_test.c

@@ -200,6 +200,7 @@ static void __attribute__((noreturn)) usage(void)
 	     "  setperm <path> <id> <flags> ...\n"
 	     "  shutdown\n"
 	     "  watch <path> <token>\n"
+	     "  watchnoack <path> <token>\n"
 	     "  waitwatch\n"
 	     "  ackwatch <token>\n"
 	     "  unwatch <path> <token>\n"
@@ -480,10 +481,20 @@ static void do_shutdown(unsigned int handle)
 		failed(handle);
 }
 
-static void do_watch(unsigned int handle, const char *node, const char *token)
+static void do_watch(unsigned int handle, const char *node, const char *token,
+		     bool swallow_event)
 {
 	if (!xs_watch(handles[handle], node, token))
 		failed(handle);
+
+	/* Convenient for testing... */
+	if (swallow_event) {
+		char **vec = xs_read_watch(handles[handle]);
+		if (!vec || !streq(vec[0], node) || !streq(vec[1], token))
+			failed(handle);
+		if (!xs_acknowledge_watch(handles[handle], token))
+			failed(handle);
+	}
 }
 
 static void set_timeout(void)
@@ -767,7 +778,9 @@ static void do_command(unsigned int default_handle, char *line)
 	else if (streq(command, "shutdown"))
 		do_shutdown(handle);
 	else if (streq(command, "watch"))
-		do_watch(handle, arg(line, 1), arg(line, 2));
+		do_watch(handle, arg(line, 1), arg(line, 2), true);
+	else if (streq(command, "watchnoack"))
+		do_watch(handle, arg(line, 1), arg(line, 2), false);
 	else if (streq(command, "waitwatch"))
 		do_waitwatch(handle);
 	else if (streq(command, "ackwatch"))