python: Extract registered watch search logic from xspy_read_watch()

When a watch fires, xspy_read_watch() checks whether the client has
registered interest in the path which changed and, if so, returns the
path and a client-supplied token.   The binding for xs_check_watch()
needs to do the same, so this patch extracts the search code into a
separate function.

Signed-off-by: Euan Harris <euan.harris@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>

tools/python/xen/lowlevel/xs/xs.c

@@ -77,6 +77,8 @@ static inline struct xs_handle *xshandle(XsHandle *self)
 
 static void remove_watch(XsHandle *xsh, PyObject *token);
 
+static PyObject *match_watch_by_token(XsHandle *self, char **xsval);
+
 static PyObject *none(bool result);
 
 static int parse_transaction_path(XsHandle *self, PyObject *args,
@@ -484,8 +486,6 @@ static PyObject *xspy_read_watch(XsHandle *self, PyObject *args)
     struct xs_handle *xh = xshandle(self);
     PyObject *val = NULL;
     char **xsval;
-    PyObject *token;
-    int i;
     unsigned int num;
 
     if (!xh)
@@ -497,29 +497,16 @@ again:
     Py_END_ALLOW_THREADS
     if (!xsval) {
         PyErr_SetFromErrno(xs_error);
-        goto exit;
+        return val;
     }
-    if (sscanf(xsval[XS_WATCH_TOKEN], "%li", (unsigned long *)&token) != 1) {
-	xs_set_error(EINVAL);
-        goto exit;
-    }
-    for (i = 0; i < PyList_Size(self->watches); i++) {
-        if (token == PyList_GetItem(self->watches, i))
-            break;
-    }
-    if (i == PyList_Size(self->watches)) {
-      /* We do not have a registered watch for the one that has just fired.
-         Ignore this -- a watch that has been recently deregistered can still
-         have watches in transit.  This is a blocking method, so go back to
-         read again.
-      */
-      free(xsval);
-      goto again;
-    }
-    /* Create tuple (path, token). */
-    val = Py_BuildValue("(sO)", xsval[XS_WATCH_PATH], token);
- exit:
+
+    val = match_watch_by_token(self, xsval);
     free(xsval);
+
+    if (!val && errno == EAGAIN) {
+        goto again;
+    }
+
     return val;
 }
 
@@ -868,6 +855,33 @@ static int parse_transaction_path(XsHandle *self, PyObject *args,
 }
 
 
+static PyObject *match_watch_by_token(XsHandle *self, char **xsval)
+{
+    PyObject *token;
+    int i;
+
+    if (sscanf(xsval[XS_WATCH_TOKEN], "%li", (unsigned long *)&token) != 1) {
+        xs_set_error(EINVAL);
+        return NULL;
+    }
+    for (i = 0; i < PyList_Size(self->watches); i++) {
+        if (token == PyList_GetItem(self->watches, i))
+            break;
+    }
+    if (i == PyList_Size(self->watches)) {
+        /* We do not have a registered watch for the one that has just fired.
+           Ignore this -- a watch that has been recently deregistered can still
+           have watches in transit.
+        */
+        xs_set_error(EAGAIN);
+        return NULL;
+    }
+
+    /* Create tuple (path, token). */
+    return Py_BuildValue("(sO)", xsval[XS_WATCH_PATH], token);
+}
+
+
 static PyObject *none(bool result)
 {
     if (result) {