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/fluksio/api/routes/flows.py

@@ -336,8 +336,23 @@ def read_library(controller: FlowControllerDep) -> Any:
 
 
 @router.delete("/library/{lib_name}", response_model=Message)
-async def delete_shared_node(lib_name: str, controller: FlowControllerDep) -> Any:
-    """Remove a shared source, as long as no flow still runs it."""
+async def delete_shared_node(
+    lib_name: str, controller: FlowControllerDep, detach: bool = False
+) -> Any:
+    """Remove a shared source, as long as no flow still runs it.
+
+    With ``detach``, every node using it keeps a copy of the code as its own
+    first, so the source can be retired while flows still use it.
+    """
+    if detach:
+        running = await run_in_threadpool(controller.store.usages, lib_name, False)
+        try:
+            await run_in_threadpool(controller.store.detach_lib, lib_name)
+        except LibNotFound:
+            raise HTTPException(status_code=404, detail=f"No shared node '{lib_name}'")
+        for flow in sorted({usage.split(".", 1)[0] for usage in running}):
+            await controller.reload_flow(flow)
+        return Message(message=f"Deleted shared node '{lib_name}'")
     used_by = controller.store.usages(lib_name)
     if used_by:
         raise HTTPException(

backend/fluksio/flow/store.py

@@ -400,6 +400,37 @@ class FlowStore:
             self._set_source_ref(flow, node_id, None)
             self._commit(f"Unshare node '{flow}.{node_id}'")
 
+    def detach_lib(self, name: str) -> None:
+        """Give every node running this shared source a copy, then delete it.
+
+        Published flows and drafts alike, so a running flow keeps the same code
+        and nothing waits on a publish. Each document changed goes up a version,
+        so an editor holding the old one gets a conflict on its next save
+        instead of pointing a node back at a source that is gone.
+        """
+        with self._write_lock:
+            code = self.read_lib_source(name)
+            for flow in self.list_flows():
+                for path, node_file in (
+                    (self._flow_file(flow), self._node_file),
+                    (self._draft_file(flow), self._draft_node_file),
+                ):
+                    if not path.exists():
+                        continue
+                    definition = FlowDef.model_validate_json(path.read_text())
+                    users = [n for n in definition.nodes if n.source_ref == name]
+                    if not users:
+                        continue
+                    for node in users:
+                        target = node_file(flow, node.id)
+                        target.parent.mkdir(parents=True, exist_ok=True)
+                        target.write_text(code)
+                        node.source_ref = None
+                    definition.version += 1
+                    path.write_text(definition.model_dump_json(indent=2) + "\n")
+            self._lib_file(name).unlink()
+            self._commit(f"Detach and delete shared node '{name}'")
+
     def usages(self, lib_name: str, draft: bool = True) -> list[str]:
         """Every ``flow.node`` pointing at this shared source.