Add cross-source tests

Change-Id: Iaf31211d12a2c8ce3b4a2860e079748f7e705aba
Story: 2001334
Task: 5885

zuul/driver/gerrit/gerritsource.py

@@ -70,13 +70,15 @@ class GerritSource(BaseSource):
         return change
 
     def getChangesDependingOn(self, change, projects):
+        changes = []
+        if not change.uris:
+            return changes
         queries = set()
         for uri in change.uris:
             queries.add('message:%s' % uri)
         query = '(' + ' OR '.join(queries) + ')'
         results = self.connection.simpleQuery(query)
         seen = set()
-        changes = []
         for result in results:
             for match in find_dependency_headers(result['commitMessage']):
                 found = False

zuul/driver/github/githubconnection.py

@@ -646,9 +646,12 @@ class GithubConnection(BaseConnection):
         return self._github
 
     def maintainCache(self, relevant):
+        remove = set()
         for key, change in self._change_cache.items():
             if change not in relevant:
-                del self._change_cache[key]
+                remove.add(key)
+        for key in remove:
+            del self._change_cache[key]
 
     def getChange(self, event, refresh=False):
         """Get the change representing an event."""
@@ -661,7 +664,6 @@ class GithubConnection(BaseConnection):
             change.uris = [
                 '%s/%s/pull/%s' % (self.server, project, change.number),
             ]
-            change.updated_at = self._ghTimestampToDate(event.updated_at)
             change.source_event = event
             change.is_current_patchset = (change.pr.get('head').get('sha') ==
                                           event.patch_number)
@@ -789,6 +791,8 @@ class GithubConnection(BaseConnection):
         change.labels = change.pr.get('labels')
         # ensure message is at least an empty string
         change.message = change.pr.get('body') or ''
+        change.updated_at = self._ghTimestampToDate(
+            change.pr.get('updated_at'))
 
         if history is None:
             history = []

zuul/driver/github/githubsource.py

@@ -46,6 +46,8 @@ class GithubSource(BaseSource):
         if not change.number:
             # Not a pull request, considering merged.
             return True
+        # We don't need to perform another query because the API call
+        # to perform the merge will ensure this is updated.
         return change.is_merged
 
     def canMerge(self, change, allow_needs):