Prune branches that no longer exist on remote

If a local branch tracks removed remote tracking branch (e.g. foo) and
remote has a branch that starts with such name (e.g. foo/bar), git fetch
will fail. --prune flag removes any remote-tracking branches that no
longer exist.

R=apolito@google.com, ehmaldonado@chromium.org

Bug: 1079483
Change-Id: I9bc31bf961d52a86b6fa2342249971b99a003666
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2190341
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>

git_cache.py

@@ -529,18 +529,26 @@ class Mirror(object):
             'but failed. Continuing with non-optimized repository.'
             % len(pack_files))
 
-  def _fetch(self, rundir, verbose, depth, no_fetch_tags, reset_fetch_config):
+  def _fetch(self,
+             rundir,
+             verbose,
+             depth,
+             no_fetch_tags,
+             reset_fetch_config,
+             prune=True):
     self.config(rundir, reset_fetch_config)
-    v = []
-    d = []
-    t = []
+
+    fetch_cmd = ['fetch']
     if verbose:
-      v = ['-v', '--progress']
+      fetch_cmd.extend(['-v', '--progress'])
     if depth:
-      d = ['--depth', str(depth)]
+      fetch_cmd.extend(['--depth', str(depth)])
     if no_fetch_tags:
-      t = ['--no-tags']
-    fetch_cmd = ['fetch'] + v + d + t + ['origin']
+      fetch_cmd.append('--no-tags')
+    if prune:
+      fetch_cmd.append('--prune')
+    fetch_cmd.append('origin')
+
     fetch_specs = subprocess.check_output(
         [self.git_exe, 'config', '--get-all', 'remote.origin.fetch'],
         cwd=rundir).decode('utf-8', 'ignore').strip().splitlines()
@@ -574,16 +582,16 @@ class Mirror(object):
 
     try:
       self._ensure_bootstrapped(depth, bootstrap, reset_fetch_config)
-      self._fetch(
-          self.mirror_path, verbose, depth, no_fetch_tags, reset_fetch_config)
+      self._fetch(self.mirror_path, verbose, depth, no_fetch_tags,
+                  reset_fetch_config)
     except ClobberNeeded:
       # This is a major failure, we need to clean and force a bootstrap.
       gclient_utils.rmtree(self.mirror_path)
       self.print(GIT_CACHE_CORRUPT_MESSAGE)
       self._ensure_bootstrapped(
           depth, bootstrap, reset_fetch_config, force=True)
-      self._fetch(
-          self.mirror_path, verbose, depth, no_fetch_tags, reset_fetch_config)
+      self._fetch(self.mirror_path, verbose, depth, no_fetch_tags,
+                  reset_fetch_config)
     finally:
       if not ignore_lock:
         lockfile.unlock()

tests/git_cache_test.py

@@ -12,6 +12,13 @@ import sys
 import tempfile
 import unittest
 
+if sys.version_info.major == 2:
+  from StringIO import StringIO
+  import mock
+else:
+  from io import StringIO
+  from unittest import mock
+
 DEPOT_TOOLS_ROOT = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
 sys.path.insert(0, DEPOT_TOOLS_ROOT)
 
@@ -96,6 +103,22 @@ class GitCacheTest(unittest.TestCase):
 
     mirror.populate()
 
+  @mock.patch('sys.stdout', StringIO())
+  def testPruneRequired(self):
+    self.git(['init', '-q'])
+    with open(os.path.join(self.origin_dir, 'foo'), 'w') as f:
+      f.write('touched\n')
+    self.git(['checkout', '-b', 'foo'])
+    self.git(['add', 'foo'])
+    self.git(['commit', '-m', 'foo'])
+    mirror = git_cache.Mirror(self.origin_dir)
+    mirror.populate()
+    self.git(['checkout', '-b', 'foo_tmp', 'foo'])
+    self.git(['branch', '-D', 'foo'])
+    self.git(['checkout', '-b', 'foo/bar', 'foo_tmp'])
+    mirror.populate()
+    self.assertNotIn(git_cache.GIT_CACHE_CORRUPT_MESSAGE, sys.stdout.getvalue())
+
   def _makeGitRepoWithTag(self):
     self.git(['init', '-q'])
     with open(os.path.join(self.origin_dir, 'foo'), 'w') as f: