Fix Vast results push from merge commits

devops/vast/self_destruct.py

@@ -102,8 +102,15 @@ def push_results(
     for i in range(1, attempts + 1):
         fetched = _run(["git", "fetch", "origin", branch], cwd=repo)
         if fetched.returncode == 0:
-            # Rebase our disjoint per-run folder onto the current branch tip.
-            rebased = _run(["git", "rebase", "--autostash", "FETCH_HEAD"], cwd=repo)
+            # Replay only the tip results commit onto the current branch tip.
+            # Boxes often train from merge commits (merged PRs); rebasing the
+            # full history onto the orphan `results` branch fails trying to
+            # recreate those merges. Per-run folders are disjoint, so only the
+            # tip commit needs to move.
+            rebased = _run(
+                ["git", "rebase", "--autostash", "--onto", "FETCH_HEAD", "HEAD~1"],
+                cwd=repo,
+            )
             if rebased.returncode != 0:
                 _run(["git", "rebase", "--abort"], cwd=repo)
                 _log(f"rebase failed (attempt {i}): {rebased.stderr.strip()}", log)

tests/test_infra_safeguards.py

@@ -584,6 +584,33 @@ def fake_run(args, cwd=None):
     ) in calls
 
 
+def test_self_destruct_rebases_only_tip_results_commit(tmp_path, monkeypatch):
+    calls = []
+
+    def fake_run(args, cwd=None):
+        calls.append(args)
+        if args[:3] == ["git", "diff", "--cached"]:
+            return SimpleNamespace(returncode=1, stdout="", stderr="")
+        return SimpleNamespace(returncode=0, stdout="", stderr="")
+
+    monkeypatch.setattr("devops.vast.self_destruct._run", fake_run)
+
+    assert push_results(
+        branch="results",
+        run_name="test",
+        instance_id="1",
+        repo=tmp_path,
+    )
+    assert [
+        "git",
+        "rebase",
+        "--autostash",
+        "--onto",
+        "FETCH_HEAD",
+        "HEAD~1",
+    ] in calls
+
+
 def test_failed_results_push_preserves_box_for_recovery(tmp_path, monkeypatch):
     messages = []
     destroyed = []