Make sure that SourceStamp.revision=None is not 'None' when using the

latest change's revision to calculate the SourceStamp's revision

Fixes: buildbot#864

master/buildbot/sourcestamp.py

@@ -41,27 +41,29 @@ class SourceStamp(util.ComparableMixin, styles.Versioned):
 
     def __init__(self, branch=None, revision=None, patch=None,
                  changes=None, project='', repository=''):
-        if revision is not None:
-            if isinstance(revision, int):
-                revision = str(revision)
         if patch is not None:
             assert len(patch) == 2
             assert int(patch[0]) != -1
         self.branch = branch
-        self.revision = revision
         self.patch = patch
         self.project = project
         self.repository = repository
         if changes:
             self.changes = tuple(changes)
             # set branch and revision to most recent change
             self.branch = changes[-1].branch
-            self.revision = str(changes[-1].revision)
+            revision = changes[-1].revision
             if not self.project:
                 self.project = changes[-1].project
             if not self.repository:
                 self.repository = changes[-1].repository
 
+        if revision is not None:
+            if isinstance(revision, int):
+                revision = str(revision)
+
+        self.revision = revision
+
     def canBeMergedWith(self, other):
         if other.repository != self.repository:
             return False

master/buildbot/test/regressions/test_sourcestamp_revision.py

@@ -0,0 +1,11 @@
+from twisted.trial import unittest
+
+from buildbot.sourcestamp import SourceStamp
+from buildbot.changes.changes import Change
+
+class TestSourceStampRevision(unittest.TestCase):
+    def testNoRevision(self):
+        c = Change(who="catlee", files=["foo"], comments="", branch="b1", revision=None)
+        ss = SourceStamp(changes=[c])
+
+        self.assertEquals(ss.revision, None)