fix a problem scanning for the base of a branch with no tag

When there is no tag at the base of a branch, the scanner cannot figure
out where to stop scanning. Avoid an exception and allow the scanner to
continue to the end of the history.

Change-Id: I9bed42724d5ab0e8d11ab0c781e215a71af5e99a
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

reno/scanner.py

@@ -385,7 +385,15 @@ class Scanner(object):
                 # on master, so this is the base.
                 tags = self._repo.get_tags_on_commit(
                     c.commit.sha().hexdigest().encode('ascii'))
-                return tags[-1]
+                if tags:
+                    return tags[-1]
+                else:
+                    # Naughty, naughty, branching without tagging.
+                    LOG.error(
+                        ('There is no tag on commit %s at the base of %s. '
+                         'Branch scan short-cutting is disabled.'),
+                        c.commit.sha().hexdigest(), branch)
+                    return None
         return None
 
     def _topo_traversal(self, branch):

reno/tests/test_scanner.py

@@ -933,6 +933,15 @@ class BranchBaseTest(Base):
             self.scanner._get_branch_base('not-master'),
         )
 
+    def test_no_tag_at_base(self):
+        # remove the tag at the branch point
+        self.repo.git('tag', '-d', '2.0.0')
+        self._add_notes_file('slug4')
+        self.repo.git('checkout', 'master')
+        self.assertIsNone(
+            self.scanner._get_branch_base('not-master')
+        )
+
 
 class BranchTest(Base):