fix(repo): fail loudly if worktrees are used

As GitPython is in maintenance mode, there will be no new features.
However, I believe it's good idea to explicitly state we do not support
certain things if this is the case.

Therefore, when worktrees are encountered, we will throw an specific
exception to indicate that.

The current implementation is hacky to speed up development,
and increases the risk of failing due to false-positive worktree
directories.

Related to #344

3 files changed, 39 insertions, 10 deletions

diff --git a/git/exc.py b/git/exc.py
index f5b52374..34382ecd 100644
--- a/git/exc.py
+++ b/git/exc.py
@@ -14,6 +14,10 @@ class InvalidGitRepositoryError(Exception):
     """ Thrown if the given repository appears to have an invalid format.  """
 
 
+class WorkTreeRepositoryUnsupported(InvalidGitRepositoryError):
+    """ Thrown to indicate we can't handle work tree repositories """
+
+
 class NoSuchPathError(OSError):
     """ Thrown if a path could not be access by the system. """
 
diff --git a/git/repo/fun.py b/git/repo/fun.py
index 049800b9..6b06663a 100644
--- a/git/repo/fun.py
+++ b/git/repo/fun.py
@@ -4,7 +4,7 @@ from string import digits
 
 from gitdb.exc import (
     BadObject,
-    BadName
+    BadName,
 )
 from git.refs import SymbolicReference
 from git.objects import Object
@@ -16,6 +16,7 @@ from gitdb.util import (
     hex_to_bin,
     bin_to_hex
 )
+from git.exc import WorkTreeRepositoryUnsupported
 from git.compat import xrange
 
 
@@ -31,14 +32,20 @@ def touch(filename):
 
 def is_git_dir(d):
     """ This is taken from the git setup.c:is_git_directory
-    function."""
-    if isdir(d) and \
-            isdir(join(d, 'objects')) and \
-            isdir(join(d, 'refs')):
-        headref = join(d, 'HEAD')
-        return isfile(headref) or \
-            (os.path.islink(headref) and
-             os.readlink(headref).startswith('refs'))
+    function.
+
+    @throws WorkTreeRepositoryUnsupported if it sees a worktree directory. It's quite hacky to do that here,
+            but at least clearly indicates that we don't support it.
+            There is the unlikely danger to throw if we see directories which just look like a worktree dir,
+            but are none."""
+    if isdir(d):
+        if isdir(join(d, 'objects')) and isdir(join(d, 'refs')):
+            headref = join(d, 'HEAD')
+            return isfile(headref) or \
+                (os.path.islink(headref) and
+                 os.readlink(headref).startswith('refs'))
+        elif isfile(join(d, 'gitdir')) and isfile(join(d, 'commondir')) and isfile(join(d, 'gitfile')):
+            raise WorkTreeRepositoryUnsupported(d)
     return False
 
 
diff --git a/git/test/test_repo.py b/git/test/test_repo.py
index c95592ea..2e44f0aa 100644
--- a/git/test/test_repo.py
+++ b/git/test/test_repo.py
@@ -33,7 +33,10 @@ from git import (
 )
 from git.repo.fun import touch
 from git.util import join_path_native
-from git.exc import BadObject
+from git.exc import (
+    BadObject,
+    WorkTreeRepositoryUnsupported
+)
 from gitdb.util import bin_to_hex
 from git.compat import string_types
 from gitdb.test.lib import with_rw_directory
@@ -45,6 +48,8 @@ import shutil
 import itertools
 from io import BytesIO
 
+from nose import SkipTest
+
 
 class TestRepo(TestBase):
 
@@ -779,3 +784,16 @@ class TestRepo(TestBase):
         self.assertFalse(repo.is_ancestor("master", c1))
         for i, j in itertools.permutations([c1, 'ffffff', ''], r=2):
             self.assertRaises(GitCommandError, repo.is_ancestor, i, j)
+
+    @with_rw_directory
+    def test_work_tree_unsupported(self, rw_dir):
+        git = Git(rw_dir)
+        if git.version_info[:3] < (2, 5, 1):
+            raise SkipTest("worktree feature unsupported")
+
+        rw_master = self.rorepo.clone(join_path_native(rw_dir, 'master_repo'))
+        rw_master.git.checkout('HEAD~10')
+        worktree_path = join_path_native(rw_dir, 'worktree_repo')
+        rw_master.git.worktree('add', worktree_path, 'master')
+
+        self.failUnlessRaises(WorkTreeRepositoryUnsupported, Repo, worktree_path)