Only compute root_tree_key's directory hash when needed

swh/model/git.py

@@ -187,7 +187,8 @@ def compute_tree_metadata(dirname, ls_hashes):
 
 
 def walk_and_compute_sha1_from_directory(rootdir,
-                                         dir_ok_fn=lambda dirpath: True):
+                                         dir_ok_fn=lambda dirpath: True,
+                                         with_root_tree=True):
     """Compute git sha1 from directory rootdir.
 
     Args:
@@ -197,6 +198,10 @@ def walk_and_compute_sha1_from_directory(rootdir,
         defined in the function. By default, all folders are ok.
         Example override: dir_ok_fn = lambda dirpath: b'svn' not in dirpath
 
+        - with_root_tree: Determine if we compute the upper root tree's
+          checksums. As a default, we want it. One possible use case where this
+          is not useful is the update (cf. `update_checksums_from`)
+
     Returns:
         Dictionary of entries with keys <path-name> and as values a list of
         directory entries.
@@ -260,15 +265,16 @@ def walk_and_compute_sha1_from_directory(rootdir,
 
         ls_hashes[dirpath].extend(dir_hashes)
 
-    # compute the current directory hashes
-    root_hash = {
-        'sha1_git': compute_directory_git_sha1(rootdir, ls_hashes),
-        'path': rootdir,
-        'name': os.path.basename(rootdir),
-        'perms': GitPerm.TREE,
-        'type': GitType.TREE
-    }
-    ls_hashes[ROOT_TREE_KEY] = [root_hash]
+    if with_root_tree:
+        # compute the current directory hashes
+        root_hash = {
+            'sha1_git': compute_directory_git_sha1(rootdir, ls_hashes),
+            'path': rootdir,
+            'name': os.path.basename(rootdir),
+            'perms': GitPerm.TREE,
+            'type': GitType.TREE
+        }
+        ls_hashes[ROOT_TREE_KEY] = [root_hash]
 
     return ls_hashes
 
@@ -371,11 +377,11 @@ def update_checksums_from(changed_paths, objects,
             continue
 
         # recompute from disk the checksums
-        hashes = walk_and_compute_sha1_from_directory(rootdir, dir_ok_fn)
+        hashes = walk_and_compute_sha1_from_directory(rootdir, dir_ok_fn,
+                                                      with_root_tree=False)
         # update the objects with new checksums for the arborescence tree below
         # rootdir
-        for d in (k for k in hashes.keys() if k != ROOT_TREE_KEY):
-            objects[d] = hashes[d]
+        objects.update(hashes)
 
         # now recompute the hashes in memory from deeper_rootdir to root
         objects = recompute_sha1_in_memory(root, rootdir, objects)

swh/model/tests/test_git.py

@@ -216,6 +216,22 @@ class GitHashArborescenceTree(unittest.TestCase):
 
         self.assertEquals(actual_walk1, expected_checksums)
 
+    @istest
+    def walk_and_compute_sha1_from_directory_without_root_tree(self):
+        # compute the full checksums
+        expected_hashes = git.walk_and_compute_sha1_from_directory(
+            self.tmp_root_path)
+
+        # except for the key on that round
+        actual_hashes = git.walk_and_compute_sha1_from_directory(
+            self.tmp_root_path,
+            with_root_tree=False)
+
+        # then, removing the root tree hash from the first round
+        del expected_hashes[git.ROOT_TREE_KEY]
+
+        # should give us the same checksums as the second round
+        self.assertEquals(actual_hashes, expected_hashes)
 
 
 class GitHashUpdate(GitHashArborescenceTree):