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/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