Resolve tree SHA1 along with commit SHA1 when resolving refs

Adjust all other parts and the tests to work with this.

13 files changed, 109 insertions, 82 deletions

diff --git a/morphlib/app.py b/morphlib/app.py
index 9135e440..b60a11a7 100755
--- a/morphlib/app.py
+++ b/morphlib/app.py
@@ -179,8 +179,8 @@ class Morph(cliapp.Application):
     def create_source_pool(self, lrc, rrc, triplet):
         pool = morphlib.sourcepool.SourcePool()
 
-        def add_to_pool(reponame, ref, filename, absref, morphology):
-            source = morphlib.source.Source(reponame, ref, absref,
+        def add_to_pool(reponame, ref, filename, absref, tree, morphology):
+            source = morphlib.source.Source(reponame, ref, absref, tree,
                                             morphology, filename)
             pool.add(source)
 
@@ -196,7 +196,7 @@ class Morph(cliapp.Application):
         return self.create_source_pool(*args)
 
     def _resolveref(self, lrc, rrc, reponame, ref, update=True):
-        '''Resolves the sha1 of the ref in reponame and returns it.
+        '''Resolves commit and tree sha1s of the ref in a repo and returns it.
 
         If update is True then this has the side-effect of updating
         or cloning the repository into the local repo cache.
@@ -209,10 +209,10 @@ class Morph(cliapp.Application):
                 self.status(msg='Updating cached git repository %(reponame)s',
                             reponame=reponame)
                 repo.update()
-            absref = repo.resolve_ref(ref)
+            absref, tree = repo.resolve_ref(ref)
         elif rrc is not None:
             try:
-                absref = rrc.resolve_ref(reponame, ref)
+                absref, tree = rrc.resolve_ref(reponame, ref)
             except:
                 pass
         if absref is None:
@@ -223,8 +223,8 @@ class Morph(cliapp.Application):
                 repo.update()
             else:
                 repo = lrc.get_repo(reponame)
-            absref = repo.resolve_ref(ref)
-        return absref
+            absref, tree = repo.resolve_ref(ref)
+        return absref, tree
 
     def traverse_morphs(self, triplets, lrc, rrc, update=True,
                         visit=lambda rn, rf, fn, arf, m: None):
@@ -233,10 +233,10 @@ class Morph(cliapp.Application):
 
         while queue:
             reponame, ref, filename = queue.popleft()
-            absref = self._resolveref(lrc, rrc, reponame, ref, update)
+            absref, tree = self._resolveref(lrc, rrc, reponame, ref, update)
             morphology = morph_factory.get_morphology(
                 reponame, absref, filename)
-            visit(reponame, ref, filename, absref, morphology)
+            visit(reponame, ref, filename, absref, tree, morphology)
             if morphology['kind'] == 'system':
                 queue.extend((s['repo'], s['ref'], '%s.morph' % s['morph'])
                              for s in morphology['strata'])
diff --git a/morphlib/artifact_tests.py b/morphlib/artifact_tests.py
index a8637fd0..1d9e6cca 100644
--- a/morphlib/artifact_tests.py
+++ b/morphlib/artifact_tests.py
@@ -41,7 +41,7 @@ class ArtifactTests(unittest.TestCase):
             }
             ''')
         self.source = morphlib.source.Source(
-            'repo', 'ref', 'sha1', morph, 'chunk.morph')
+            'repo', 'ref', 'sha1', 'tree', morph, 'chunk.morph')
         self.artifact_name = 'chunk-runtime'
         self.artifact = morphlib.artifact.Artifact(
             self.source, self.artifact_name)
diff --git a/morphlib/artifactresolver_tests.py b/morphlib/artifactresolver_tests.py
index 1371162c..5510b1cf 100644
--- a/morphlib/artifactresolver_tests.py
+++ b/morphlib/artifactresolver_tests.py
@@ -109,7 +109,7 @@ class ArtifactResolverTests(unittest.TestCase):
 
         morph = FakeChunkMorphology('chunk')
         source = morphlib.source.Source(
-            'repo', 'ref', 'sha1', morph, 'chunk.morph')
+            'repo', 'ref', 'sha1', 'tree', morph, 'chunk.morph')
         pool.add(source)
 
         artifacts = self.resolver.resolve_artifacts(pool)
@@ -126,7 +126,7 @@ class ArtifactResolverTests(unittest.TestCase):
 
         morph = FakeChunkMorphology('chunk', ['chunk-runtime'])
         source = morphlib.source.Source(
-            'repo', 'ref', 'sha1', morph, 'chunk.morph')
+            'repo', 'ref', 'sha1', 'tree', morph, 'chunk.morph')
         pool.add(source)
 
         artifacts = self.resolver.resolve_artifacts(pool)
@@ -142,7 +142,7 @@ class ArtifactResolverTests(unittest.TestCase):
 
         morph = FakeChunkMorphology('chunk', ['chunk-runtime', 'chunk-devel'])
         source = morphlib.source.Source(
-            'repo', 'ref', 'sha1', morph, 'chunk.morph')
+            'repo', 'ref', 'sha1', 'tree', morph, 'chunk.morph')
         pool.add(source)
 
         artifacts = self.resolver.resolve_artifacts(pool)
@@ -172,7 +172,7 @@ class ArtifactResolverTests(unittest.TestCase):
             ''')
         morph.builds_artifacts = ['foo']
         stratum = morphlib.source.Source(
-            'repo', 'original/ref', 'sha1', morph, 'foo.morph')
+            'repo', 'original/ref', 'sha1', 'tree', morph, 'foo.morph')
         pool.add(stratum)
 
         artifacts = self.resolver.resolve_artifacts(pool)
@@ -194,7 +194,7 @@ class ArtifactResolverTests(unittest.TestCase):
             ''')
         morph.builds_artifacts = ['foo-rootfs']
         system = morphlib.source.Source(
-            'repo', 'original/ref', 'sha1', morph, 'foo.morph')
+            'repo', 'original/ref', 'sha1', 'tree', morph, 'foo.morph')
         pool.add(system)
 
         artifacts = self.resolver.resolve_artifacts(pool)
@@ -217,7 +217,7 @@ class ArtifactResolverTests(unittest.TestCase):
             ''')
         morph.builds_artifacts = ['foo-rootfs', 'foo-kernel']
         system = morphlib.source.Source(
-            'repo', 'original/ref', 'sha1', morph, 'foo.morph')
+            'repo', 'original/ref', 'sha1', 'tree', morph, 'foo.morph')
         pool.add(system)
 
         artifacts = self.resolver.resolve_artifacts(pool)
@@ -234,13 +234,13 @@ class ArtifactResolverTests(unittest.TestCase):
 
         morph = FakeChunkMorphology('chunk')
         chunk = morphlib.source.Source(
-            'repo', 'ref', 'sha1', morph, 'chunk.morph')
+            'repo', 'ref', 'sha1', 'tree', morph, 'chunk.morph')
         pool.add(chunk)
 
         morph = FakeStratumMorphology(
             'stratum', chunks=[('chunk', 'chunk', 'repo', 'ref')])
         stratum = morphlib.source.Source(
-            'repo', 'ref', 'sha1', morph, 'stratum.morph')
+            'repo', 'ref', 'sha1', 'tree', morph, 'stratum.morph')
         pool.add(stratum)
 
         artifacts = self.resolver.resolve_artifacts(pool)
@@ -262,7 +262,7 @@ class ArtifactResolverTests(unittest.TestCase):
 
         morph = FakeChunkMorphology('chunk', ['chunk-devel', 'chunk-runtime'])
         chunk = morphlib.source.Source(
-            'repo', 'ref', 'sha1', morph, 'chunk.morph')
+            'repo', 'ref', 'sha1', 'tree', morph, 'chunk.morph')
         pool.add(chunk)
 
         morph = FakeStratumMorphology(
@@ -272,7 +272,7 @@ class ArtifactResolverTests(unittest.TestCase):
                 ('chunk-runtime', 'chunk', 'repo', 'ref')
             ])
         stratum = morphlib.source.Source(
-            'repo', 'ref', 'sha1', morph, 'stratum.morph')
+            'repo', 'ref', 'sha1', 'tree', morph, 'stratum.morph')
         pool.add(stratum)
 
         artifacts = self.resolver.resolve_artifacts(pool)
@@ -300,14 +300,14 @@ class ArtifactResolverTests(unittest.TestCase):
 
         morph = FakeChunkMorphology('chunk', ['chunk-devel', 'chunk-runtime'])
         chunk = morphlib.source.Source(
-            'repo', 'ref', 'sha1', morph, 'chunk.morph')
+            'repo', 'ref', 'sha1', 'tree', morph, 'chunk.morph')
         pool.add(chunk)
 
         morph = FakeStratumMorphology(
             'stratum',
             chunks=[('chunk-runtime', 'chunk', 'repo', 'ref')])
         stratum = morphlib.source.Source(
-            'repo', 'ref', 'sha1', morph, 'stratum.morph')
+            'repo', 'ref', 'sha1', 'tree', morph, 'stratum.morph')
         pool.add(stratum)
 
         artifacts = self.resolver.resolve_artifacts(pool)
@@ -329,12 +329,12 @@ class ArtifactResolverTests(unittest.TestCase):
 
         morph = FakeChunkMorphology('foo')
         foo_chunk = morphlib.source.Source(
-            'repo', 'ref', 'sha1', morph, 'foo.morph')
+            'repo', 'ref', 'sha1', 'tree', morph, 'foo.morph')
         pool.add(foo_chunk)
 
         morph = FakeChunkMorphology('bar')
         bar_chunk = morphlib.source.Source(
-            'repo', 'ref', 'sha1', morph, 'bar.morph')
+            'repo', 'ref', 'sha1', 'tree', morph, 'bar.morph')
         pool.add(bar_chunk)
 
         morph = FakeStratumMorphology(
@@ -344,7 +344,7 @@ class ArtifactResolverTests(unittest.TestCase):
                 ('bar', 'bar', 'repo', 'ref')
             ])
         stratum = morphlib.source.Source(
-            'repo', 'ref', 'sha1', morph, 'stratum.morph')
+            'repo', 'ref', 'sha1', 'tree', morph, 'stratum.morph')
         pool.add(stratum)
 
         artifacts = self.resolver.resolve_artifacts(pool)
@@ -372,7 +372,7 @@ class ArtifactResolverTests(unittest.TestCase):
 
         morph = FakeStratumMorphology('stratum1')
         stratum1 = morphlib.source.Source(
-            'repo', 'ref', 'sha1', morph, 'stratum1.morph')
+            'repo', 'ref', 'sha1', 'tree', morph, 'stratum1.morph')
         pool.add(stratum1)
 
         morph = FakeStratumMorphology(
@@ -380,7 +380,7 @@ class ArtifactResolverTests(unittest.TestCase):
             chunks=[],
             build_depends=[('stratum1', 'repo', 'ref')])
         stratum2 = morphlib.source.Source(
-            'repo', 'ref', 'sha1', morph, 'stratum2.morph')
+            'repo', 'ref', 'sha1', 'tree', morph, 'stratum2.morph')
         pool.add(stratum2)
 
         artifacts = self.resolver.resolve_artifacts(pool)
@@ -402,7 +402,7 @@ class ArtifactResolverTests(unittest.TestCase):
 
         morph = FakeStratumMorphology('stratum1')
         stratum1 = morphlib.source.Source(
-            'repo', 'original/ref', 'sha1', morph, 'stratum1.morph')
+            'repo', 'original/ref', 'sha1', 'tree', morph, 'stratum1.morph')
         pool.add(stratum1)
 
         morph = FakeStratumMorphology(
@@ -413,17 +413,17 @@ class ArtifactResolverTests(unittest.TestCase):
             ],
             build_depends=[('stratum1', 'repo', 'original/ref')])
         stratum2 = morphlib.source.Source(
-            'repo', 'original/ref', 'sha1', morph, 'stratum2.morph')
+            'repo', 'original/ref', 'sha1', 'tree', morph, 'stratum2.morph')
         pool.add(stratum2)
 
         morph = FakeChunkMorphology('chunk1')
         chunk1 = morphlib.source.Source(
-            'repo', 'original/ref', 'sha1', morph, 'chunk1.morph')
+            'repo', 'original/ref', 'sha1', 'tree', morph, 'chunk1.morph')
         pool.add(chunk1)
 
         morph = FakeChunkMorphology('chunk2')
         chunk2 = morphlib.source.Source(
-            'repo', 'original/ref', 'sha1', morph, 'chunk2.morph')
+            'repo', 'original/ref', 'sha1', 'tree', morph, 'chunk2.morph')
         pool.add(chunk2)
 
         artifacts = self.resolver.resolve_artifacts(pool)
@@ -458,7 +458,7 @@ class ArtifactResolverTests(unittest.TestCase):
 
         morph = FakeStratumMorphology('stratum1')
         stratum1 = morphlib.source.Source(
-            'repo', 'ref', 'sha1', morph, 'stratum1.morph')
+            'repo', 'ref', 'sha1', 'tree', morph, 'stratum1.morph')
         pool.add(stratum1)
 
         morph = morphlib.morph2.Morphology(
@@ -482,13 +482,13 @@ class ArtifactResolverTests(unittest.TestCase):
             ''')
         morph.builds_artifacts = ['system-rootfs']
         system = morphlib.source.Source(
-            'repo', 'ref', 'sha1', morph, 'system.morph')
+            'repo', 'ref', 'sha1', 'tree', morph, 'system.morph')
         pool.add(system)
 
         morph = FakeStratumMorphology(
             'stratum2', chunks=[], build_depends=[('stratum1', 'repo', 'ref')])
         stratum2 = morphlib.source.Source(
-            'repo', 'ref', 'sha1', morph, 'stratum2.morph')
+            'repo', 'ref', 'sha1', 'tree', morph, 'stratum2.morph')
         pool.add(stratum2)
 
         artifacts = self.resolver.resolve_artifacts(pool)
@@ -546,22 +546,22 @@ class ArtifactResolverTests(unittest.TestCase):
             ''')
         morph.builds_artifacts = ['stratum']
         stratum = morphlib.source.Source(
-            'repo', 'original/ref', 'sha1', morph, 'stratum.morph')
+            'repo', 'original/ref', 'sha1', 'tree', morph, 'stratum.morph')
         pool.add(stratum)
 
         morph = FakeChunkMorphology('chunk1')
         chunk1 = morphlib.source.Source(
-            'repo', 'original/ref', 'sha1', morph, 'chunk1.morph')
+            'repo', 'original/ref', 'sha1', 'tree', morph, 'chunk1.morph')
         pool.add(chunk1)
 
         morph = FakeChunkMorphology('chunk2')
         chunk2 = morphlib.source.Source(
-            'repo', 'original/ref', 'sha1', morph, 'chunk2.morph')
+            'repo', 'original/ref', 'sha1', 'tree', morph, 'chunk2.morph')
         pool.add(chunk2)
 
         morph = FakeChunkMorphology('chunk3')
         chunk3 = morphlib.source.Source(
-            'repo', 'original/ref', 'sha1', morph, 'chunk3.morph')
+            'repo', 'original/ref', 'sha1', 'tree', morph, 'chunk3.morph')
         pool.add(chunk3)
 
         artifacts = self.resolver.resolve_artifacts(pool)
@@ -596,14 +596,14 @@ class ArtifactResolverTests(unittest.TestCase):
 
         morph = FakeChunkMorphology('chunk')
         chunk = morphlib.source.Source(
-            'repo', 'ref', 'sha1', morph, 'chunk.morph')
+            'repo', 'ref', 'sha1', 'tree', morph, 'chunk.morph')
         pool.add(chunk)
 
         morph = FakeStratumMorphology(
             'stratum',
             chunks=[('chunk-runtime', 'chunk', 'repo', 'ref')])
         stratum = morphlib.source.Source(
-            'repo', 'ref', 'sha1', morph, 'stratum.morph')
+            'repo', 'ref', 'sha1', 'tree', morph, 'stratum.morph')
         pool.add(stratum)
 
         self.assertRaises(
@@ -618,7 +618,7 @@ class ArtifactResolverTests(unittest.TestCase):
             chunks=[],
             build_depends=[('stratum2', 'repo', 'original/ref')])
         stratum1 = morphlib.source.Source(
-            'repo', 'original/ref', 'sha1', morph, 'stratum1.morph')
+            'repo', 'original/ref', 'sha1', 'tree', morph, 'stratum1.morph')
         pool.add(stratum1)
 
         morph = FakeStratumMorphology(
@@ -626,7 +626,7 @@ class ArtifactResolverTests(unittest.TestCase):
             chunks=[],
             build_depends=[('stratum1', 'repo', 'original/ref')])
         stratum2 = morphlib.source.Source(
-            'repo', 'original/ref', 'sha1', morph, 'stratum2.morph')
+            'repo', 'original/ref', 'sha1', 'tree', morph, 'stratum2.morph')
         pool.add(stratum2)
 
         self.assertRaises(morphlib.artifactresolver.MutualDependencyError,
@@ -642,7 +642,7 @@ class ArtifactResolverTests(unittest.TestCase):
                 ('chunk2', 'chunk2', 'repo', 'original/ref')
             ])
         stratum1 = morphlib.source.Source(
-            'repo', 'original/ref', 'sha1', morph, 'stratum1.morph')
+            'repo', 'original/ref', 'sha1', 'tree', morph, 'stratum1.morph')
         pool.add(stratum1)
 
         morph = FakeStratumMorphology(
@@ -653,17 +653,17 @@ class ArtifactResolverTests(unittest.TestCase):
             ],
             build_depends=[('stratum1', 'repo', 'original/ref')])
         stratum2 = morphlib.source.Source(
-            'repo', 'original/ref', 'sha1', morph, 'stratum2.morph')
+            'repo', 'original/ref', 'sha1', 'tree', morph, 'stratum2.morph')
         pool.add(stratum2)
 
         morph = FakeChunkMorphology('chunk1')
         chunk1 = morphlib.source.Source(
-            'repo', 'original/ref', 'sha1', morph, 'chunk1.morph')
+            'repo', 'original/ref', 'sha1', 'tree', morph, 'chunk1.morph')
         pool.add(chunk1)
 
         morph = FakeChunkMorphology('chunk2')
         chunk2 = morphlib.source.Source(
-            'repo', 'original/ref', 'sha1', morph, 'chunk2.morph')
+            'repo', 'original/ref', 'sha1', 'tree', morph, 'chunk2.morph')
         pool.add(chunk2)
 
         self.assertRaises(morphlib.artifactresolver.MutualDependencyError,
@@ -701,12 +701,12 @@ class ArtifactResolverTests(unittest.TestCase):
                 ''')
             morph.builds_artifacts = ['stratum']
             stratum = morphlib.source.Source(
-                'repo', 'original/ref', 'sha1', morph, 'stratum.morph')
+                'repo', 'original/ref', 'sha1', 'tree', morph, 'stratum.morph')
             pool.add(stratum)
 
             morph = FakeChunkMorphology('chunk')
             chunk = morphlib.source.Source(
-                'repo', 'original/ref', 'sha1', morph, 'chunk.morph')
+                'repo', 'original/ref', 'sha1', 'tree', morph, 'chunk.morph')
             pool.add(chunk)
 
             artifacts = self.resolver.resolve_artifacts(pool)
@@ -750,17 +750,17 @@ class ArtifactResolverTests(unittest.TestCase):
             ''')
         morph.builds_artifacts = ['stratum']
         stratum = morphlib.source.Source(
-            'repo', 'original/ref', 'sha1', morph, 'stratum.morph')
+            'repo', 'original/ref', 'sha1', 'tree', morph, 'stratum.morph')
         pool.add(stratum)
 
         morph = FakeChunkMorphology('chunk1')
         chunk1 = morphlib.source.Source(
-            'repo', 'original/ref', 'sha1', morph, 'chunk1.morph')
+            'repo', 'original/ref', 'sha1', 'tree', morph, 'chunk1.morph')
         pool.add(chunk1)
 
         morph = FakeChunkMorphology('chunk2')
         chunk2 = morphlib.source.Source(
-            'repo', 'original/ref', 'sha1', morph, 'chunk2.morph')
+            'repo', 'original/ref', 'sha1', 'tree', morph, 'chunk2.morph')
         pool.add(chunk2)
 
         self.assertRaises(morphlib.artifactresolver.DependencyOrderError,
@@ -786,12 +786,12 @@ class ArtifactResolverTests(unittest.TestCase):
             ''')
         morph.builds_artifacts = ['stratum']
         stratum = morphlib.source.Source(
-            'repo', 'original/ref', 'sha1', morph, 'stratum.morph')
+            'repo', 'original/ref', 'sha1', 'tree', morph, 'stratum.morph')
         pool.add(stratum)
 
         morph = FakeChunkMorphology('chunk')
         chunk = morphlib.source.Source(
-            'repo', 'original/ref', 'sha1', morph, 'chunk.morph')
+            'repo', 'original/ref', 'sha1', 'tree', morph, 'chunk.morph')
         pool.add(chunk)
 
         self.assertRaises(morphlib.artifactresolver.DependencyFormatError,
diff --git a/morphlib/cachedrepo.py b/morphlib/cachedrepo.py
index f647102f..eef49ffb 100644
--- a/morphlib/cachedrepo.py
+++ b/morphlib/cachedrepo.py
@@ -91,27 +91,29 @@ class CachedRepo(object):
         return len(ref) == 40 and all([x in valid_chars for x in ref])
 
     def resolve_ref(self, ref):
-        '''Attempts to resolve a Git ref into its corresponding SHA1.
+        '''Attempts to resolve a ref into its SHA1 and tree SHA1.
 
         Raises an InvalidReferenceError if the ref is not found in the
         repository.
 
         '''
 
-        try:
-            refs = self._show_ref(ref).split('\n')
-            # split each ref line into an array, drop non-origin branches
-            refs = [x.split() for x in refs if 'origin' in x]
-            return refs[0][0]
-        except cliapp.AppException:
-            pass
-
         if not self.is_valid_sha1(ref):
-            raise InvalidReferenceError(self, ref)
-        try:
-            return self._rev_list(ref).strip()
-        except cliapp.AppException:
-            raise InvalidReferenceError(self, ref)
+            try:
+                refs = self._show_ref(ref).split('\n')
+                # split each ref line into an array, drop non-origin branches
+                refs = [x.split() for x in refs if 'origin' in x]
+                absref = refs[0][0]
+            except cliapp.AppException:
+                raise InvalidReferenceError(self, ref)
+        else:
+            try:
+                absref = self._rev_list(ref).strip()
+            except cliapp.AppException:
+                raise InvalidReferenceError(self, ref)
+
+        tree = self._show_tree_hash(absref)
+        return absref, tree
 
     def cat(self, ref, filename):
         '''Attempts to read a file given a SHA1 ref.
@@ -197,6 +199,10 @@ class CachedRepo(object):
     def _show_ref(self, ref):  # pragma: no cover
         return self._runcmd(['git', 'show-ref', ref])
 
+    def _show_tree_hash(self, absref):  # pragma: no cover
+        return self._runcmd(
+                ['git', 'log', '-1', '--format=format:%T', absref]).strip()
+
     def _rev_list(self, ref):  # pragma: no cover
         return self._runcmd(['git', 'rev-list', '--no-walk', ref])
 
diff --git a/morphlib/cachedrepo_tests.py b/morphlib/cachedrepo_tests.py
index fa660010..14c5daac 100644
--- a/morphlib/cachedrepo_tests.py
+++ b/morphlib/cachedrepo_tests.py
@@ -40,6 +40,19 @@ class CachedRepoTests(unittest.TestCase):
         except:
             raise cliapp.AppException('git show-ref %s' % ref)
 
+    def show_tree_hash(self, absref):
+        output = {
+            'e28a23812eadf2fce6583b8819b9c5dbd36b9fb9':
+              'eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee',
+            '8b780e2e6f102fcf400ff973396566d36d730501':
+              'ffffffffffffffffffffffffffffffffffffffff'
+        }
+        try:
+            return output[absref]
+        except:
+            raise cliapp.AppException('git log -1 --format=format:%T %s' %
+                                      absref)
+
     def rev_list(self, ref):
         output = {
             'e28a23812eadf2fce6583b8819b9c5dbd36b9fb9':
@@ -102,6 +115,7 @@ class CachedRepoTests(unittest.TestCase):
         self.repo = cachedrepo.CachedRepo(
             object(), self.repo_name, self.repo_url, self.repo_path)
         self.repo._show_ref = self.show_ref
+        self.repo._show_tree_hash = self.show_tree_hash
         self.repo._rev_list = self.rev_list
         self.repo._cat_file = self.cat_file
         self.repo._copy_repository = self.copy_repository
@@ -118,21 +132,24 @@ class CachedRepoTests(unittest.TestCase):
         self.assertEqual(self.repo.path, self.repo_path)
 
     def test_resolve_named_ref_master(self):
-        sha1 = self.repo.resolve_ref('master')
+        sha1, tree = self.repo.resolve_ref('master')
         self.assertEqual(sha1, 'e28a23812eadf2fce6583b8819b9c5dbd36b9fb9')
+        self.assertEqual(tree, 'eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee')
 
     def test_resolve_named_ref_baserock_morph(self):
-        sha1 = self.repo.resolve_ref('baserock/morph')
+        sha1, tree = self.repo.resolve_ref('baserock/morph')
         self.assertEqual(sha1, '8b780e2e6f102fcf400ff973396566d36d730501')
+        self.assertEqual(tree, 'ffffffffffffffffffffffffffffffffffffffff')
 
     def test_fail_resolving_invalid_named_ref(self):
         self.assertRaises(cachedrepo.InvalidReferenceError,
                           self.repo.resolve_ref, 'foo/bar')
 
     def test_resolve_sha1_ref(self):
-        sha1 = self.repo.resolve_ref(
+        sha1, tree = self.repo.resolve_ref(
             'e28a23812eadf2fce6583b8819b9c5dbd36b9fb9')
         self.assertEqual(sha1, 'e28a23812eadf2fce6583b8819b9c5dbd36b9fb9')
+        self.assertEqual(tree, 'eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee')
 
     def test_fail_resolving_an_invalid_sha1_ref(self):
         self.assertRaises(cachedrepo.InvalidReferenceError,
diff --git a/morphlib/cachekeycomputer_tests.py b/morphlib/cachekeycomputer_tests.py
index 7ace9c36..7077a313 100644
--- a/morphlib/cachekeycomputer_tests.py
+++ b/morphlib/cachekeycomputer_tests.py
@@ -91,7 +91,7 @@ class CacheKeyComputerTests(unittest.TestCase):
             }''',
         }.iteritems():
             source = morphlib.source.Source(
-                'repo', 'original/ref', 'sha',
+                'repo', 'original/ref', 'sha', 'tree',
                 morphlib.morph2.Morphology(text), name)
             self.source_pool.add(source)
             # FIXME: This should use MorphologyFactory
@@ -171,7 +171,7 @@ class CacheKeyComputerTests(unittest.TestCase):
         old_artifact = self._find_artifact('system-rootfs')
         morphology = old_artifact.source.morphology
         new_source = morphlib.source.Source('repo', 'original/ref', 'newsha',
-                                            morphology,
+                                            'tree', morphology,
                                             old_artifact.source.filename)
         self.source_pool.add(new_source)
         artifacts = self.artifact_resolver.resolve_artifacts(self.source_pool)
diff --git a/morphlib/localartifactcache_tests.py b/morphlib/localartifactcache_tests.py
index aacba8fb..c3deb9dc 100644
--- a/morphlib/localartifactcache_tests.py
+++ b/morphlib/localartifactcache_tests.py
@@ -43,7 +43,7 @@ class LocalArtifactCacheTests(unittest.TestCase):
             }
             ''')
         self.source = morphlib.source.Source(
-            'repo', 'ref', 'sha1', morph, 'chunk.morph')
+            'repo', 'ref', 'sha1', 'tree', morph, 'chunk.morph')
         self.runtime_artifact = morphlib.artifact.Artifact(
             self.source, 'chunk-runtime')
         self.devel_artifact = morphlib.artifact.Artifact(
diff --git a/morphlib/plugins/branch_and_merge_plugin.py b/morphlib/plugins/branch_and_merge_plugin.py
index e7294cc7..07f1a154 100644
--- a/morphlib/plugins/branch_and_merge_plugin.py
+++ b/morphlib/plugins/branch_and_merge_plugin.py
@@ -306,7 +306,7 @@ class BranchAndMergePlugin(cliapp.Plugin):
                            ref=ref)
                 assert cache.has_repo(reponame)
                 repo = cache.get_repo(reponame)
-                source['ref'] = repo.resolve_ref(ref)
+                source['ref'], tree = repo.resolve_ref(ref)
 
             self.write_morphology(filename, morph)
 
diff --git a/morphlib/plugins/update_gits_plugin.py b/morphlib/plugins/update_gits_plugin.py
index ac7beaa8..92a687a3 100644
--- a/morphlib/plugins/update_gits_plugin.py
+++ b/morphlib/plugins/update_gits_plugin.py
@@ -51,7 +51,7 @@ class UpdateGitsPlugin(cliapp.Plugin):
 
         subs_to_process = set()
 
-        def visit(reponame, ref, filename, absref, morphology):
+        def visit(reponame, ref, filename, absref, tree, morphology):
             app.status(msg='Updating %(repo_name)s %(ref)s %(filename)s',
                        repo_name=reponame, ref=ref, filename=filename)
             assert cache.has_repo(reponame)
diff --git a/morphlib/remoteartifactcache_tests.py b/morphlib/remoteartifactcache_tests.py
index d77f8f53..a4ec9370 100644
--- a/morphlib/remoteartifactcache_tests.py
+++ b/morphlib/remoteartifactcache_tests.py
@@ -46,7 +46,7 @@ class RemoteArtifactCacheTests(unittest.TestCase):
             }
             ''')
         self.source = morphlib.source.Source(
-            'repo', 'ref', 'sha1', morph, 'chunk.morph')
+            'repo', 'ref', 'sha1', 'tree', morph, 'chunk.morph')
         self.runtime_artifact = morphlib.artifact.Artifact(
             self.source, 'chunk-runtime')
         self.runtime_artifact.cache_key = 'CHUNK-RUNTIME'
diff --git a/morphlib/remoterepocache.py b/morphlib/remoterepocache.py
index c40057b1..5c2017de 100644
--- a/morphlib/remoterepocache.py
+++ b/morphlib/remoterepocache.py
@@ -74,7 +74,7 @@ class RemoteRepoCache(object):
     def _resolve_ref_for_repo_url(self, repo_url, ref):  # pragma: no cover
         data = self._make_request('sha1s?repo=%s&ref=%s' % (repo_url, ref))
         info = json.loads(data)
-        return info['sha1']
+        return info['sha1'], info['tree']
 
     def _cat_file_for_repo_url(self, repo_url, ref,
                                filename):  # pragma: no cover
diff --git a/morphlib/source.py b/morphlib/source.py
index fceed9ef..ed62ea4a 100644
--- a/morphlib/source.py
+++ b/morphlib/source.py
@@ -27,6 +27,7 @@ class Source(object):
     * ``repo_name`` -- name of the git repository which contains the source
     * ``original_ref`` -- the git ref provided by the user or a morphology
     * ``sha1`` -- the absolute git commit id for the revision we use
+    * ``tree`` -- the SHA1 of the tree corresponding to the commit
     * ``morphology`` -- the in-memory representation of the morphology we use
     * ``filename`` -- basename of the morphology filename
     * ``dependencies`` -- list of Sources for build dependencies for us
@@ -37,11 +38,13 @@ class Source(object):
 
     '''
 
-    def __init__(self, repo_name, original_ref, sha1, morphology, filename):
+    def __init__(self, repo_name, original_ref, sha1, tree, morphology,
+            filename):
         self.repo = None
         self.repo_name = repo_name
         self.original_ref = original_ref
         self.sha1 = sha1
+        self.tree = tree
         self.morphology = morphology
         self.filename = filename
 
diff --git a/morphlib/source_tests.py b/morphlib/source_tests.py
index 7ad5d370..6643f0fc 100644
--- a/morphlib/source_tests.py
+++ b/morphlib/source_tests.py
@@ -32,14 +32,15 @@ class SourceTests(unittest.TestCase):
         self.repo_name = 'foo.repo'
         self.original_ref = 'original/ref'
         self.sha1 = 'CAFEF00D'
+        self.tree = 'F000000D'
         self.morphology = morphlib.morph2.Morphology(self.morphology_text)
         self.filename = 'foo.morph'
         self.source = morphlib.source.Source(
-            self.repo_name, self.original_ref, self.sha1, self.morphology,
-            self.filename)
+            self.repo_name, self.original_ref, self.sha1, self.tree,
+            self.morphology, self.filename)
         self.other = morphlib.source.Source(
-            self.repo_name, self.original_ref, self.sha1, self.morphology,
-            self.filename)
+            self.repo_name, self.original_ref, self.sha1, self.tree,
+            self.morphology, self.filename)
 
     def test_sets_repo_name(self):
         self.assertEqual(self.source.repo_name, self.repo_name)