libxenstore: filter watch events in libxenstore when we unwatch

XenStore puts in queued watch events via a thread and notifies the user.
Sometimes xs_unwatch is called before all related message is read. The use
case is non-threaded libevent, we have two event A and B:
    - Event A will destroy something and call xs_unwatch;
    - Event B is used to notify that a node has changed in XenStore.
As the event is called one by one, event A can be handled before event B.
So on next xs_watch_read the user could retrieve an unwatch token and
a segfault occured if the token store the pointer of the structure
(ie: "backend:0xcafe").

To avoid problem with previous application using libXenStore, this behaviour
will only be enabled if XS_UNWATCH_FILTER is given to xs_open.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Signed-off-by: Julien Grall <julien.grall@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>

1 files changed, 21 insertions, 0 deletions

diff --git a/tools/xenstore/xenstore.h b/tools/xenstore/xenstore.h
index 7259e49cd6..fdf5e764ba 100644
--- a/tools/xenstore/xenstore.h
+++ b/tools/xenstore/xenstore.h
@@ -27,6 +27,27 @@
 #define XS_OPEN_READONLY	1UL<<0
 #define XS_OPEN_SOCKETONLY      1UL<<1
 
+/*
+ * Setting XS_UNWATCH_FILTER arranges that after xs_unwatch, no
+ * related watch events will be delivered via xs_read_watch.  But
+ * this relies on the couple token, subpath is unique.
+ *
+ * XS_UNWATCH_FILTER clear          XS_UNWATCH_FILTER set
+ *
+ * Even after xs_unwatch, "stale"   After xs_unwatch returns, no
+ * instances of the watch event     watch events with the same
+ * may be delivered.                token and with the same subpath
+ *                                  will be delivered.
+ *
+ * A path and a subpath can be      The application must avoid
+ * register with the same token.    registering a path (/foo/) and
+ *                                  a subpath (/foo/bar) with the
+ *                                  same path until a successful
+ *                                  xs_unwatch for the first watch
+ *                                  has returned.
+ */
+#define XS_UNWATCH_FILTER     1UL<<2
+
 struct xs_handle;
 typedef uint32_t xs_transaction_t;