experiments: support cloning parent in detached-head state (#4611)

* experiments: account for cloning detached-head parent repo

* add test for detached-head parent

dvc/repo/experiments/__init__.py

@@ -178,11 +178,26 @@ class Experiments:
         self.scm.checkout(rev)
 
     def _checkout_default_branch(self):
+        from git.refs.symbolic import SymbolicReference
+
         # switch to default branch
         git_repo = self.scm.repo
         origin_refs = git_repo.remotes["origin"].refs
-        ref = origin_refs["HEAD"].reference
+
+        # origin/HEAD will point to tip of the default branch unless we
+        # initially cloned a repo that was in a detached-HEAD state.
+        #
+        # If we are currently detached because we cloned a detached
+        # repo, we can't actually tell what branch should be considered
+        # default, so we just fall back to the first available reference.
+        if "HEAD" in origin_refs:
+            ref = origin_refs["HEAD"].reference
+        else:
+            ref = origin_refs[0]
+            if not isinstance(ref, SymbolicReference):
+                ref = ref.reference
         branch_name = ref.name.split("/")[-1]
+
         if branch_name in git_repo.heads:
             branch = git_repo.heads[branch_name]
         else:

tests/func/experiments/test_experiments.py

@@ -208,3 +208,31 @@ def test_extend_branch(tmp_dir, scm, dvc):
     dvc.experiments.checkout(exp_b)
     assert (tmp_dir / "params.yaml").read_text().strip() == "foo: 3"
     assert (tmp_dir / "metrics.yaml").read_text().strip() == "foo: 3"
+
+
+def test_detached_parent(tmp_dir, scm, dvc, mocker):
+    tmp_dir.gen("copy.py", COPY_SCRIPT)
+    tmp_dir.gen("params.yaml", "foo: 1")
+    stage = dvc.run(
+        cmd="python copy.py params.yaml metrics.yaml",
+        metrics_no_cache=["metrics.yaml"],
+        params=["foo"],
+        name="copy-file",
+    )
+    scm.add(["dvc.yaml", "dvc.lock", "copy.py", "params.yaml", "metrics.yaml"])
+    scm.commit("v1")
+    detached_rev = scm.get_rev()
+
+    tmp_dir.gen("params.yaml", "foo: 2")
+    dvc.reproduce(stage.addressing)
+    scm.add(["dvc.yaml", "dvc.lock", "copy.py", "params.yaml", "metrics.yaml"])
+    scm.commit("v2")
+
+    scm.checkout(detached_rev)
+    assert scm.repo.head.is_detached
+    dvc.reproduce(stage.addressing, experiment=True, params=["foo=3"])
+
+    exp_rev = dvc.experiments.scm.get_rev()
+    assert dvc.experiments.get_baseline(exp_rev) == detached_rev
+    assert (tmp_dir / "params.yaml").read_text().strip() == "foo: 3"
+    assert (tmp_dir / "metrics.yaml").read_text().strip() == "foo: 3"