repo: open: handle bare git repos

If we are asked to open a potentially unitialized repo, we need to make sure
it is not a bare git repo, so that we don't accidentally start putting dvc
files inside git control dir.

Discovered while working on #8789

dvc/repo/__init__.py

@@ -125,6 +125,8 @@ class Repo:
             if not scm:
                 try:
                     scm = SCM(root_dir or os.curdir)
+                    if scm.dulwich.repo.bare:
+                        raise NotDvcRepoError(f"{scm.root_dir} is a bare git repo")
                 except SCMError:
                     scm = SCM(os.curdir, no_scm=True)
 

tests/func/test_repo.py

@@ -1,6 +1,32 @@
+import os
+
 from dvc.cachemgr import CacheManager
 from dvc.dvcfile import LOCK_FILE, PROJECT_FILE
 from dvc.fs import system
+from dvc.repo import Repo
+from dvc.scm import Git
+
+
+def test_open_bare(tmp_dir, scm, dvc, tmp_path_factory):
+    tmp_dir.dvc_gen(
+        {
+            "dir123": {"foo": "foo content"},
+            "dirextra": {"extrafoo": "extra foo content"},
+        },
+        commit="initial",
+    )
+
+    url = os.fspath(tmp_path_factory.mktemp("bare"))
+    Git.init(url, bare=True).close()
+
+    scm.gitpython.repo.create_remote("origin", url)
+    scm.gitpython.repo.remote("origin").push("master")
+
+    with Repo.open(url) as repo:
+        assert repo.scm.root_dir != url
+
+    with Repo.open(url, uninitialized=True) as repo:
+        assert repo.scm.root_dir != url
 
 
 def test_destroy(tmp_dir, dvc, run_copy):