Fix "git rebase-update" when multiple branch are stale.

Fix a bug that left branches tracking a dead branch if both their parent
and grand-parent were left with no changes after a "rebase-update" step.

Given the following initial state:
  $ git map-branches -v
  origin/master
    a
      b
        c *        [ ahead 1 ]

without this patch, a "git rebase-update" on this tree state would
leave the branch "c" as tracking a non-existing branch "a":

  $ git recursive-rebase
  a up-to-date
  b up-to-date
  c up-to-date
  Reparented c to track a (was tracking b)
  Deleted branch b (was 448d1da).
  Deleted branch a (was 448d1da).
  $ git map-branches -v
  {a:GONE}
    c *

with the patch, we record that the branch "c" is tracking must be
updated twice and we end up in a state were "c" is correctly tracking
"origin/master":

  $ git recursive-rebase
  a up-to-date
  b up-to-date
  c up-to-date
  Reparented c to track origin/master (was tracking b)
  Deleted branch b (was 448d1da).
  Deleted branch a (was 448d1da).
  $ git map-branches -v
  origin/master
    c *            [ ahead 1 ]

BUG=456806

Review URL: https://codereview.chromium.org/1482753002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@297792 0039d316-1c4b-4281-b951-d872f2087c98

git_rebase_update.py

@@ -87,32 +87,54 @@ def remove_empty_branches(branch_tree):
   tag_set = git.tags()
   ensure_root_checkout = git.once(lambda: git.run('checkout', git.root()))
 
-  deletions = set()
+  deletions = {}
+  reparents = {}
   downstreams = collections.defaultdict(list)
   for branch, parent in git.topo_iter(branch_tree, top_down=False):
     downstreams[parent].append(branch)
 
+    # If branch and parent have the same state, then branch has to be marked
+    # for deletion and its children and grand-children reparented to parent.
     if git.hash_one(branch) == git.hash_one(parent):
       ensure_root_checkout()
 
       logging.debug('branch %s merged to %s', branch, parent)
 
+      # Mark branch for deletion while remembering the ordering, then add all
+      # its children as grand-children of its parent and record reparenting
+      # information if necessary.
+      deletions[branch] = len(deletions)
+
       for down in downstreams[branch]:
         if down in deletions:
           continue
 
-        if parent in tag_set:
-          git.set_branch_config(down, 'remote', '.')
-          git.set_branch_config(down, 'merge', 'refs/tags/%s' % parent)
-          print ('Reparented %s to track %s [tag] (was tracking %s)'
-                 % (down, parent, branch))
+        # Record the new and old parent for down, or update such a record
+        # if it already exists. Keep track of the ordering so that reparenting
+        # happen in topological order.
+        downstreams[parent].append(down)
+        if down not in reparents:
+          reparents[down] = (len(reparents), parent, branch)
         else:
-          git.run('branch', '--set-upstream-to', parent, down)
-          print ('Reparented %s to track %s (was tracking %s)'
-                 % (down, parent, branch))
+          order, _, old_parent = reparents[down]
+          reparents[down] = (order, parent, old_parent)
+
+  # Apply all reparenting recorded, in order.
+  for branch, value in sorted(reparents.iteritems(), key=lambda x:x[1][0]):
+    _, parent, old_parent = value
+    if parent in tag_set:
+      git.set_branch_config(branch, 'remote', '.')
+      git.set_branch_config(branch, 'merge', 'refs/tags/%s' % parent)
+      print ('Reparented %s to track %s [tag] (was tracking %s)'
+             % (branch, parent, old_parent))
+    else:
+      git.run('branch', '--set-upstream-to', parent, branch)
+      print ('Reparented %s to track %s (was tracking %s)'
+             % (branch, parent, old_parent))
 
-      deletions.add(branch)
-      print git.run('branch', '-d', branch)
+  # Apply all deletions recorded, in order.
+  for branch, _ in sorted(deletions.iteritems(), key=lambda x: x[1]):
+    print git.run('branch', '-d', branch)
 
 
 def rebase_branch(branch, parent, start_hash):

tests/git_rebase_update_test.py

@@ -84,6 +84,14 @@ class GitRebaseUpdateTest(git_test_utils.GitRepoReadWriteTestBase):
       f.write('totes the Foobar file')
     self.repo.git_commit('foobar2')
 
+    self.repo.run(self.nb.main, ['--upstream-current', 'int1_foobar'])
+    self.repo.run(self.nb.main, ['--upstream-current', 'int2_foobar'])
+    self.repo.run(self.nb.main, ['--upstream-current', 'sub_foobar'])
+    with self.repo.open('foobar', 'w') as f:
+        f.write('some more foobaring')
+    self.repo.git('add', 'foobar')
+    self.repo.git_commit('foobar3')
+
     self.repo.git('checkout', 'branch_K')
     self.repo.run(self.nb.main, ['--upstream-current', 'sub_K'])
     with self.repo.open('K', 'w') as f:
@@ -103,7 +111,7 @@ class GitRebaseUpdateTest(git_test_utils.GitRepoReadWriteTestBase):
     self.assertSchema("""
     A B H I J K sub_K
             J L
-      B C D E foobar1 foobar2
+      B C D E foobar1 foobar2 foobar3
             E F G
     A old_file
     """)
@@ -131,12 +139,15 @@ class GitRebaseUpdateTest(git_test_utils.GitRepoReadWriteTestBase):
     self.assertIn('Deleted branch branch_G', output)
     self.assertIn('Deleted branch empty_branch', output)
     self.assertIn('Deleted branch empty_branch2', output)
+    self.assertIn('Deleted branch int1_foobar', output)
+    self.assertIn('Deleted branch int2_foobar', output)
     self.assertIn('Reparented branch_K to track origin/master', output)
+    self.assertIn('Reparented sub_foobar to track foobar', output)
 
     self.assertSchema("""
     A B C D E F G M N O H I J K sub_K
                               K L
-                      O foobar1 foobar2
+                      O foobar1 foobar2 foobar3
     A old_file
     """)
 
@@ -167,6 +178,7 @@ class GitRebaseUpdateTest(git_test_utils.GitRepoReadWriteTestBase):
 
     self.assertSchema("""
     A B C D E F G M N O foobar1 foobar2 H I J K L
+                                foobar2 foobar3
                                               K sub_K
     A old_file
     """)
@@ -182,6 +194,7 @@ class GitRebaseUpdateTest(git_test_utils.GitRepoReadWriteTestBase):
 
     self.assertSchema("""
     A B C D E F G M N O foobar1 foobar2 H I J K L
+                                foobar2 foobar3
     A old_file
                                               K sub_K
     """)
@@ -190,14 +203,16 @@ class GitRebaseUpdateTest(git_test_utils.GitRepoReadWriteTestBase):
 
     branches = self.repo.run(set, self.gc.branches())
     self.assertEqual(branches, {'branch_K', 'master', 'sub_K', 'root_A',
-                                'branch_L', 'old_branch', 'foobar'})
+                                'branch_L', 'old_branch', 'foobar',
+                                'sub_foobar'})
 
     self.repo.git('checkout', 'branch_K')
     self.repo.run(self.mv.main, ['special_K'])
 
     branches = self.repo.run(set, self.gc.branches())
     self.assertEqual(branches, {'special_K', 'master', 'sub_K', 'root_A',
-                                'branch_L', 'old_branch', 'foobar'})
+                                'branch_L', 'old_branch', 'foobar',
+                                'sub_foobar'})
 
     self.repo.git('checkout', 'origin/master')
     _, err = self.repo.capture_stdio(self.mv.main, ['special_K', 'cool branch'])
@@ -207,7 +222,8 @@ class GitRebaseUpdateTest(git_test_utils.GitRepoReadWriteTestBase):
     branches = self.repo.run(set, self.gc.branches())
     # This check fails with git 2.4 (see crbug.com/487172)
     self.assertEqual(branches, {'cool_branch', 'master', 'sub_K', 'root_A',
-                                'branch_L', 'old_branch', 'foobar'})
+                                'branch_L', 'old_branch', 'foobar',
+                                'sub_foobar'})
 
     _, branch_tree = self.repo.run(self.gc.get_branch_tree)
     self.assertEqual(branch_tree['sub_K'], 'foobar')