xen: add xenstore watcher infrastructure

A Xen PV frontend communicates its state to the PV backend by writing to
the 'state' key in the frontend area in xenstore. It is therefore
necessary for a XenDevice implementation to be notified whenever the
value of this key changes.

This patch adds code to do this as follows:

- an 'fd handler' is registered on the libxenstore handle which will be
  triggered whenever a 'watch' event occurs
- primitives are added to xen-bus-helper to add or remove watch events
- a list of Notifier objects is added to XenBus to provide a mechanism
  to call the appropriate 'watch handler' when its associated event
  occurs

The xen-block implementation is extended with a 'frontend_changed' method,
which calls as-yet stub 'connect' and 'disconnect' functions when the
relevant frontend state transitions occur. A subsequent patch will supply
a full implementation for these functions.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Anthony Perard <anthony.perard@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>

1 files changed, 34 insertions, 0 deletions

diff --git a/hw/xen/xen-bus-helper.c b/hw/xen/xen-bus-helper.c
index 15b3ad8d78..5f7a4b2612 100644
--- a/hw/xen/xen-bus-helper.c
+++ b/hw/xen/xen-bus-helper.c
@@ -148,3 +148,37 @@ int xs_node_scanf(struct xs_handle *xsh,  xs_transaction_t tid,
 
     return rc;
 }
+
+void xs_node_watch(struct xs_handle *xsh, const char *node, const char *key,
+                   char *token, Error **errp)
+{
+    char *path;
+
+    path = (strlen(node) != 0) ? g_strdup_printf("%s/%s", node, key) :
+        g_strdup(key);
+
+    trace_xs_node_watch(path);
+
+    if (!xs_watch(xsh, path, token)) {
+        error_setg_errno(errp, errno, "failed to watch node '%s'", path);
+    }
+
+    g_free(path);
+}
+
+void xs_node_unwatch(struct xs_handle *xsh, const char *node,
+                     const char *key, const char *token, Error **errp)
+{
+    char *path;
+
+    path = (strlen(node) != 0) ? g_strdup_printf("%s/%s", node, key) :
+        g_strdup(key);
+
+    trace_xs_node_unwatch(path);
+
+    if (!xs_unwatch(xsh, path, token)) {
+        error_setg_errno(errp, errno, "failed to unwatch node '%s'", path);
+    }
+
+    g_free(path);
+}