Retire a shared node in use with ?detach=true

DELETE /flows/library/{name} still answers 409 while nodes use the source.
With detach=true each of them keeps a copy of the code, in the published
flow and the draft, the source is deleted and the running users rebuild.

backend/tests/api/routes/test_flows.py

@@ -550,3 +550,34 @@ async def test_the_live_socket_ends_when_its_client_goes(
     await asyncio.sleep(0.05)
     parked = [t for t in asyncio.all_tasks() if "Queue.get" in repr(t.get_coro())]
     assert parked == []
+
+
+def test_a_shared_node_in_use_is_retired_only_with_detach(
+    client: TestClient, superuser_token_headers: dict[str, str]
+) -> None:
+    client.put(
+        f"{PREFIX}/shared", headers=superuser_token_headers, json=a_flow("shared")
+    )
+    client.put(
+        f"{PREFIX}/shared/nodes/sensor/source",
+        headers=superuser_token_headers,
+        json={"code": WORKING_NODE},
+    )
+    client.post(
+        f"{PREFIX}/shared/nodes/sensor/share",
+        headers=superuser_token_headers,
+        json={"lib_name": "reading"},
+    )
+
+    url = f"{PREFIX}/library/reading"
+    assert client.delete(url, headers=superuser_token_headers).status_code == 409
+    response = client.delete(f"{url}?detach=true", headers=superuser_token_headers)
+    assert response.status_code == 200
+
+    assert client.get(f"{PREFIX}/library", headers=superuser_token_headers).json() == []
+    source = client.get(
+        f"{PREFIX}/shared/nodes/sensor/source", headers=superuser_token_headers
+    )
+    assert source.json()["code"] == WORKING_NODE
+
+    client.delete(f"{PREFIX}/shared", headers=superuser_token_headers)

backend/tests/flow/test_library.py

@@ -120,3 +120,27 @@ def test_unsharing_takes_a_private_copy_back(store: FlowStore):
 def test_a_missing_shared_source_is_reported(store: FlowStore):
     with pytest.raises(LibNotFound):
         store.read_lib_source("nothing_here")
+
+
+def test_retiring_a_shared_source_leaves_each_user_a_copy(store: FlowStore):
+    store.write_flow(a_flow())
+    store.write_node_source("heating", "sensor", SOURCE)
+    store.share_node("heating", "sensor", "read_temp")
+    store.publish_flow("heating", store.read_flow("heating", draft=True).version)
+    version = store.read_flow("heating").version
+    # Shared in a draft that was never published.
+    cooling = a_flow("cooling")
+    cooling.nodes[0].source_ref = "read_temp"
+    store.write_draft(cooling, 0)
+
+    store.detach_lib("read_temp")
+
+    assert store.list_lib() == []
+    heating = store.read_flow("heating")
+    assert heating.nodes[0].source_ref is None
+    assert heating.version == version + 1
+    assert store.read_node_source("heating", "sensor") == SOURCE
+    # What runs is the same code, so there is nothing to publish.
+    assert not store.has_draft("heating")
+    assert store.read_flow("cooling", draft=True).nodes[0].source_ref is None
+    assert store.read_node_source("cooling", "sensor", draft=True) == SOURCE