PUT /panels/ carries a version and refuses a stale one with 409

The Panels dialog re-reads the list on 409. A panels.json without a
version reads as 0.

backend/tests/api/routes/test_panels.py

@@ -11,7 +11,11 @@ DASHBOARDS = f"{settings.API_V1_STR}/dashboards"
 
 
 def _panels(client: TestClient, headers: dict[str, str], config: dict) -> None:
-    response = client.put(f"{PREFIX}/", headers=headers, json=config)
+    """Replace the panels, based on whatever is stored now."""
+    version = client.get(f"{PREFIX}/", headers=headers).json()["version"]
+    response = client.put(
+        f"{PREFIX}/", headers=headers, json={**config, "version": version}
+    )
     assert response.status_code == 200, response.text
 
 
@@ -130,6 +134,39 @@ def test_duplicate_panel_is_refused(
     assert response.status_code == 422
 
 
+def test_a_stale_save_is_refused(
+    client: TestClient, superuser_token_headers: dict[str, str]
+) -> None:
+    """Two clients writing back the whole list must not drop each other's panels."""
+    read = client.get(f"{PREFIX}/", headers=superuser_token_headers).json()
+    base = read["version"]
+
+    saved = client.put(
+        f"{PREFIX}/",
+        headers=superuser_token_headers,
+        json={"panels": [*read["panels"], {"id": "mine"}], "version": base},
+    )
+    assert saved.status_code == 200, saved.text
+    assert saved.json()["version"] == base + 1
+
+    stale = client.put(
+        f"{PREFIX}/",
+        headers=superuser_token_headers,
+        json={"panels": [*read["panels"], {"id": "theirs"}], "version": base},
+    )
+    assert stale.status_code == 409
+    assert stale.json()["detail"]["current_version"] == base + 1
+
+    ids = [
+        p["id"]
+        for p in client.get(f"{PREFIX}/", headers=superuser_token_headers).json()[
+            "panels"
+        ]
+    ]
+    assert "mine" in ids
+    assert "theirs" not in ids
+
+
 def test_polling_needs_the_secret(client: TestClient) -> None:
     started = client.post(f"{PREFIX}/pair").json()
     assert len(started["code"]) == 6