experiments: fix bug where wrong git branch checked-out before pull (#4450)

dvc/repo/experiments/__init__.py

@@ -162,13 +162,25 @@ class Experiments:
     def _scm_checkout(self, rev):
         self.scm.repo.git.reset(hard=True)
         if self.scm.repo.head.is_detached:
-            # switch back to default branch
-            self.scm.repo.heads[0].checkout()
+            self._checkout_default_branch()
         if not Git.is_sha(rev) or not self.scm.has_rev(rev):
             self.scm.pull()
         logger.debug("Checking out experiment commit '%s'", rev)
         self.scm.checkout(rev)
 
+    def _checkout_default_branch(self):
+        # switch to default branch
+        git_repo = self.scm.repo
+        origin_refs = git_repo.remotes["origin"].refs
+        ref = origin_refs["HEAD"].reference
+        branch_name = ref.name.split("/")[-1]
+        if branch_name in git_repo.heads:
+            branch = git_repo.heads[branch_name]
+        else:
+            branch = git_repo.create_head(branch_name, ref)
+            branch.set_tracking_branch(ref)
+        branch.checkout()
+
     def _stash_exp(self, *args, params: Optional[dict] = None, **kwargs):
         """Stash changes from the current (parent) workspace as an experiment.
 

tests/func/experiments/test_experiments.py

@@ -22,3 +22,30 @@ def test_new_simple(tmp_dir, scm, dvc, mocker):
     assert (
         tmp_dir / ".dvc" / "experiments" / "metrics.yaml"
     ).read_text() == "foo: 2"
+
+
+def test_update_with_pull(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("init")
+    expected_revs = [scm.get_rev()]
+
+    tmp_dir.gen("params.yaml", "foo: 2")
+    dvc.reproduce(stage.addressing, experiment=True)
+    scm.add(["dvc.yaml", "dvc.lock", "params.yaml", "metrics.yaml"])
+    scm.commit("promote experiment")
+    expected_revs.append(scm.get_rev())
+
+    tmp_dir.gen("params.yaml", "foo: 3")
+    dvc.reproduce(stage.addressing, experiment=True)
+
+    exp_scm = dvc.experiments.scm
+    for rev in expected_revs:
+        assert exp_scm.has_rev(rev)