Validate a branch that we parse when running cherry_picker --continue

When running cherry_picker --continue we count on being able to get
certain information from the branch's name which cherry_picker
constructed earlier.  Verify as best we can that the branch name is one
which cherry_picker could have constructed.

Relies on the changes here: python#265
(which does the work of one of the validations)

cherry_picker/cherry_picker/cherry_picker.py

@@ -74,12 +74,7 @@ def upstream(self):
 
     @property
     def sorted_branches(self):
-        def version_from_branch(branch):
-            try:
-                return tuple(map(int, re.match(r'^.*(?P<version>\d+(\.\d+)+).*$', branch).groupdict()['version'].split('.')))
-            except AttributeError as attr_err:
-                raise ValueError(f'Branch {branch} seems to not have a version in its name.') from attr_err
-
+        """Return the branches to cherry-pick to, sorted by version"""
         return sorted(
             self.branches,
             reverse=True,
@@ -423,9 +418,28 @@ def get_base_branch(cherry_pick_branch):
     return '2.7' from 'backport-sha-2.7'
     """
     prefix, sha, base_branch = cherry_pick_branch.split('-', 2)
+
+    if prefix != 'backport':
+        raise ValueError('branch name is not prefixed with "backport-".  Is this a cherry_picker branch?')
+    if not re.match('[0-9a-f]{7,40}', sha):
+        raise ValueError('branch name has an invalid sha')
+    # Subject the parsed base_branch to the same tests as when we generated it
+    # This throws a ValueError if the base_branch doesn't need our requirements
+    version_from_branch(base_branch)
+
     return base_branch
 
 
+def version_from_branch(branch):
+    """
+    return version information from a git branch name
+    """
+    try:
+        return tuple(map(int, re.match(r'^.*(?P<version>\d+(\.\d+)+).*$', branch).groupdict()['version'].split('.')))
+    except AttributeError as attr_err:
+        raise ValueError(f'Branch {branch} seems to not have a version in its name.') from attr_err
+
+
 def get_current_branch():
     """
     Return the current branch

cherry_picker/cherry_picker/test.py

@@ -33,18 +33,29 @@ def changedir(d):
 
 
 def test_get_base_branch():
-    # The format of cherry-pick branches we create are "backport-{SHA}-{base_branch}"
+    # The format of cherry-pick branches we create are::
+    #     backport-{SHA}-{base_branch}
     cherry_pick_branch = 'backport-afc23f4-2.7'
     result = get_base_branch(cherry_pick_branch)
     assert result == '2.7'
 
 
 def test_get_base_branch_which_has_dashes():
-    cherry_pick_branch ='backport-afc23f4-baseprefix-2.7-basesuffix'
+    cherry_pick_branch = 'backport-afc23f4-baseprefix-2.7-basesuffix'
     result = get_base_branch(cherry_pick_branch)
     assert result == 'baseprefix-2.7-basesuffix'
 
 
+@pytest.mark.parametrize('cherry_pick_branch', ['backport-afc23f4',  # Not enough fields
+                                                'prefix-afc23f4-2.7',  # Not the prefix we were expecting
+                                                'backport-afc23f4-base',  # No version info in the base branch
+                                                ]
+                         )
+def test_get_base_branch_invalid(cherry_pick_branch):
+    with pytest.raises(ValueError):
+        get_base_branch(cherry_pick_branch)
+
+
 @mock.patch('subprocess.check_output')
 def test_get_current_branch(subprocess_check_output):
     subprocess_check_output.return_value = b'master'