GitDir: Add resolve_ref_to_{commit,tree} methods

Some APIs should take SHA1 object IDs, there needs to be a way to get
one from a ref. To handle this, we add APIs for getting either the
commit or the tree pointed to by the ref.

Commits are provided because we need to create commits from
existing commits and update refs from existing values.

Trees are provided because we need trees to create commits, and we can
read trees into the index to eventually create another tree.

2 files changed, 22 insertions, 5 deletions

diff --git a/morphlib/gitdir.py b/morphlib/gitdir.py
index e6426e03..b08f69d2 100644
--- a/morphlib/gitdir.py
+++ b/morphlib/gitdir.py
@@ -185,13 +185,18 @@ class GitDirectory(object):
         else:
             return self._list_files_in_ref(ref)
 
-    def _rev_parse_tree(self, ref):
+    def _rev_parse(self, ref):
         try:
-            return self._runcmd(['git', 'rev-parse', '--verify',
-                                 '%s^{tree}' % ref]).strip()
+            return self._runcmd(['git', 'rev-parse', '--verify', ref]).strip()
         except cliapp.AppException as e:
             raise InvalidRefError(self, ref)
 
+    def resolve_ref_to_commit(self, ref):
+        return self._rev_parse('%s^{commit}' % ref)
+
+    def resolve_ref_to_tree(self, ref):
+        return self._rev_parse('%s^{tree}' % ref)
+
     def _list_files_in_work_tree(self):
         for dirpath, subdirs, filenames in os.walk(self.dirname):
             if dirpath == self.dirname and '.git' in subdirs:
@@ -200,7 +205,7 @@ class GitDirectory(object):
                 yield os.path.join(dirpath, filename)[len(self.dirname)+1:]
 
     def _list_files_in_ref(self, ref):
-        tree = self._rev_parse_tree(ref)
+        tree = self.resolve_ref_to_tree(ref)
         output = self._runcmd(['git', 'ls-tree', '--name-only', '-rz', tree])
         # ls-tree appends \0 instead of interspersing, so we need to
         # strip the trailing \0 before splitting
@@ -213,7 +218,7 @@ class GitDirectory(object):
         if ref is None:
             with open(os.path.join(self.dirname, filename)) as f:
                 return f.read()
-        tree = self._rev_parse_tree(ref)
+        tree = self.resolve_ref_to_tree(ref)
         return self.get_file_from_ref(tree, filename)
 
     @property
diff --git a/morphlib/gitdir_tests.py b/morphlib/gitdir_tests.py
index 803f1b3e..a2e530f1 100644
--- a/morphlib/gitdir_tests.py
+++ b/morphlib/gitdir_tests.py
@@ -148,6 +148,18 @@ class GitDirectoryContentsTests(unittest.TestCase):
         gd.checkout('foo')
         self.assertEqual(gd.HEAD, 'foo')
 
+    def test_resolve_ref(self):
+        # Just tests that you get an object IDs back and that the
+        # commit and tree IDs are different, since checking the actual
+        # value of the commit requires foreknowledge of the result or
+        # re-implementing the body in the test.
+        gd = morphlib.gitdir.GitDirectory(self.dirname)
+        commit = gd.resolve_ref_to_commit(gd.HEAD)
+        self.assertEqual(len(commit), 40)
+        tree = gd.resolve_ref_to_tree(gd.HEAD)
+        self.assertEqual(len(tree), 40)
+        self.assertNotEqual(commit, tree)
+
     def test_uncommitted_changes(self):
         gd = morphlib.gitdir.GitDirectory(self.dirname)
         self.assertEqual(sorted(gd.get_uncommitted_changes()),