prune: handle branches that track missing branches

Series of steps:
* Create a local "b1" branch with `repo start b1` that tracks a remote
  branch (totally fine)
* Manually create a local "b2" branch with `git branch --track b1 b2`
  that tracks the local "b1" (uh-oh...)
* Delete the local "b1" branch manually or via `repo prune` (....)
* Try to process the "b2" branch with `repo prune`

Since b2 tracks a branch that no longer exists, everything blows up
at this point as we try to probe the non-existent ref.  Instead, we
should flag this as unknown and leave it up to the user to resolve.

This probably could come up if a local branch was tracking a remote
branch that was deleted from the server, and users ran something like
`repo sync --prune` which cleaned up the remote refs.

Bug: https://crbug.com/gerrit/11485
Change-Id: I6b6b6041943944b8efa6e2ad0b8b10f13a75a5c2
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/236793
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Reviewed-by: Kirtika Ruchandani <kirtika@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

tests/test_project.py

@@ -18,11 +18,30 @@
 
 from __future__ import print_function
 
+import contextlib
+import os
+import shutil
+import subprocess
+import tempfile
 import unittest
 
+import git_config
 import project
 
 
+@contextlib.contextmanager
+def TempGitTree():
+  """Create a new empty git checkout for testing."""
+  # TODO(vapier): Convert this to tempfile.TemporaryDirectory once we drop
+  # Python 2 support entirely.
+  try:
+    tempdir = tempfile.mkdtemp(prefix='repo-tests')
+    subprocess.check_call(['git', 'init'], cwd=tempdir)
+    yield tempdir
+  finally:
+    shutil.rmtree(tempdir)
+
+
 class RepoHookShebang(unittest.TestCase):
   """Check shebang parsing in RepoHook."""
 
@@ -60,3 +79,58 @@ class RepoHookShebang(unittest.TestCase):
     for shebang, interp in DATA:
       self.assertEqual(project.RepoHook._ExtractInterpFromShebang(shebang),
                        interp)
+
+
+class FakeProject(object):
+  """A fake for Project for basic functionality."""
+
+  def __init__(self, worktree):
+    self.worktree = worktree
+    self.gitdir = os.path.join(worktree, '.git')
+    self.name = 'fakeproject'
+    self.work_git = project.Project._GitGetByExec(
+        self, bare=False, gitdir=self.gitdir)
+    self.bare_git = project.Project._GitGetByExec(
+        self, bare=True, gitdir=self.gitdir)
+    self.config = git_config.GitConfig.ForRepository(gitdir=self.gitdir)
+
+
+class ReviewableBranchTests(unittest.TestCase):
+  """Check ReviewableBranch behavior."""
+
+  def test_smoke(self):
+    """A quick run through everything."""
+    with TempGitTree() as tempdir:
+      fakeproj = FakeProject(tempdir)
+
+      # Generate some commits.
+      with open(os.path.join(tempdir, 'readme'), 'w') as fp:
+        fp.write('txt')
+      fakeproj.work_git.add('readme')
+      fakeproj.work_git.commit('-mAdd file')
+      fakeproj.work_git.checkout('-b', 'work')
+      fakeproj.work_git.rm('-f', 'readme')
+      fakeproj.work_git.commit('-mDel file')
+
+      # Start off with the normal details.
+      rb = project.ReviewableBranch(
+          fakeproj, fakeproj.config.GetBranch('work'), 'master')
+      self.assertEqual('work', rb.name)
+      self.assertEqual(1, len(rb.commits))
+      self.assertIn('Del file', rb.commits[0])
+      d = rb.unabbrev_commits
+      self.assertEqual(1, len(d))
+      short, long = next(iter(d.items()))
+      self.assertTrue(long.startswith(short))
+      self.assertTrue(rb.base_exists)
+      # Hard to assert anything useful about this.
+      self.assertTrue(rb.date)
+
+      # Now delete the tracking branch!
+      fakeproj.work_git.branch('-D', 'master')
+      rb = project.ReviewableBranch(
+          fakeproj, fakeproj.config.GetBranch('work'), 'master')
+      self.assertEqual(0, len(rb.commits))
+      self.assertFalse(rb.base_exists)
+      # Hard to assert anything useful about this.
+      self.assertTrue(rb.date)