morph branch: Check if the ref already exists before anything else

This was done to ensure tests.branching/branch-fails-if-branch-exists
always passes, but also seems like the right approach in general.

3 files changed, 20 insertions, 6 deletions

diff --git a/morphlib/cachedrepo.py b/morphlib/cachedrepo.py
index 312f580f..0a085bb2 100644
--- a/morphlib/cachedrepo.py
+++ b/morphlib/cachedrepo.py
@@ -104,6 +104,15 @@ class CachedRepo(object):
         self.path = path
         self.is_mirror = not url.startswith('file://')
 
+    def ref_exists(self, ref):
+        '''Returns True if the given ref exists in the repo'''
+
+        try:
+            self._rev_parse(ref)
+        except cliapp.AppException:
+            return False
+        return True
+
     def resolve_ref(self, ref):
         '''Attempts to resolve a ref into its SHA1 and tree SHA1.
 
diff --git a/morphlib/cachedrepo_tests.py b/morphlib/cachedrepo_tests.py
index 0c1ec5de..9251a473 100644
--- a/morphlib/cachedrepo_tests.py
+++ b/morphlib/cachedrepo_tests.py
@@ -129,6 +129,12 @@ class CachedRepoTests(unittest.TestCase):
         self.assertEqual(self.repo.url, self.repo_url)
         self.assertEqual(self.repo.path, self.repo_path)
 
+    def test_ref_exists(self):
+        self.assertEqual(self.repo.ref_exists('master'), True)
+
+    def test_ref_does_not_exist(self):
+        self.assertEqual(self.repo.ref_exists('non-existant-ref'), False)
+
     def test_resolve_named_ref_master(self):
         sha1, tree = self.repo.resolve_ref('master')
         self.assertEqual(sha1, 'e28a23812eadf2fce6583b8819b9c5dbd36b9fb9')
diff --git a/morphlib/plugins/branch_and_merge_plugin.py b/morphlib/plugins/branch_and_merge_plugin.py
index 31b661b6..e6b78475 100644
--- a/morphlib/plugins/branch_and_merge_plugin.py
+++ b/morphlib/plugins/branch_and_merge_plugin.py
@@ -516,11 +516,15 @@ class BranchAndMergePlugin(cliapp.Plugin):
         new_branch = args[1]
         commit = 'master' if len(args) == 2 else args[2]
 
+        self.lrc, self.rrc = morphlib.util.new_repo_caches(self.app)
+        if self.lrc.get_repo(repo).ref_exists(new_branch):
+            raise cliapp.AppException('branch %s already exists in '
+                                      'repository %s' % (new_branch, repo))
+
         # Create the system branch directory.
         workspace = self.deduce_workspace()
         branch_dir = os.path.join(workspace, new_branch)
         os.makedirs(branch_dir)
-        self.lrc, self.rrc = morphlib.util.new_repo_caches(self.app)
 
         try:
             # Create a .morph-system-branch directory to clearly identify
@@ -540,11 +544,6 @@ class BranchAndMergePlugin(cliapp.Plugin):
             repo_dir = os.path.join(branch_dir, self.convert_uri_to_path(repo))
             self.clone_to_directory(repo_dir, repo, commit)
 
-            # Check if branch already exists locally or in a remote
-            if self.resolve_ref(repo_dir, new_branch) is not None:
-                raise cliapp.AppException('branch %s already exists in '
-                                          'repository %s' % (new_branch, repo))
-
             # Create a new branch in the local morphs repository.
             self.app.runcmd(['git', 'checkout', '-b', new_branch, commit],
                             cwd=repo_dir)