Service node: a server kept running on a worker (openpanel V2)

- `service` node type: command, port, worker label, health path, env and
  weights (an artifact digest the worker fetches as {weights}). Its flow
  starts it; it restarts with backoff, reads degraded while starting or
  without a worker and down after three starts that never got healthy, and
  publishes http://<worker>:<port> once /health answers 200, "" when it ends.
- Worker protocol 3: a `serve` call, every call in its own process group,
  killed when the engine ends the call, the socket drops or the agent gets
  SIGTERM. `--service-host` overrides the address the engine sees.
- Services and compiles take no `--parallel` slot, and a flow's detail is
  built off the event loop, so a save no longer stalls the engine.

backend/fluksio/api/routes/flows.py

@@ -220,6 +220,11 @@ def _endpoints(controller: FlowController, definition: FlowDef) -> list[Endpoint
 
 
 def _detail(controller: FlowController, definition: FlowDef) -> FlowDetail:
+    """The flow and how its nodes are doing. Blocking: call it off the loop.
+
+    A draft is built to report on it, which asks a worker to compile its
+    nodes, and that waits on the socket the loop serves.
+    """
     name = definition.name
     running = {
         "enabled": controller.is_enabled(name),
@@ -534,7 +539,7 @@ async def save_flow(
             status_code=409,
             detail={"message": str(exc), "current_version": exc.current},
         )
-    return _detail(controller, stored)
+    return await run_in_threadpool(_detail, controller, stored)
 
 
 @router.post("/{name}/publish", response_model=FlowDetail)
@@ -561,7 +566,7 @@ async def publish_flow(
         )
     _audit("published", name, user)
     await controller.reload_flow(name)
-    return _detail(controller, published)
+    return await run_in_threadpool(_detail, controller, published)
 
 
 @router.post("/{name}/discard-draft", response_model=FlowDetail)
@@ -581,7 +586,7 @@ async def discard_draft(name: str, controller: FlowControllerDep) -> Any:
             ),
         )
     published = await run_in_threadpool(controller.store.discard_draft, name)
-    return _detail(controller, published)
+    return await run_in_threadpool(_detail, controller, published)
 
 
 @router.delete("/{name}", response_model=Message)
@@ -644,7 +649,7 @@ async def rename_flow(
     # run, so this is cleanup rather than a migration.
     await run_in_threadpool(controller.forget_flow, name)
     await controller.reload()
-    return _detail(controller, renamed)
+    return await run_in_threadpool(_detail, controller, renamed)
 
 
 @router.post("/{name}/duplicate", response_model=FlowDetail)
@@ -662,7 +667,7 @@ async def duplicate_flow(
         copy = await run_in_threadpool(controller.store.duplicate_flow, name)
     except FlowNotFound:
         raise HTTPException(status_code=404, detail=f"No flow named '{name}'")
-    return _detail(controller, copy)
+    return await run_in_threadpool(_detail, controller, copy)
 
 
 # -----------------------------------------------------------------------------
@@ -810,7 +815,7 @@ async def share_node(
         )
     except LibExists as exc:
         raise HTTPException(status_code=409, detail=str(exc))
-    return _detail(controller, _read_flow(controller, name))
+    return await run_in_threadpool(_detail, controller, _read_flow(controller, name))
 
 
 @router.post("/{name}/nodes/{node_id}/unshare", response_model=FlowDetail)
@@ -825,7 +830,7 @@ async def unshare_node(
         await run_in_threadpool(controller.store.unshare_node, name, node_id)
     except LibNotFound as exc:
         raise HTTPException(status_code=404, detail=str(exc))
-    return _detail(controller, _read_flow(controller, name))
+    return await run_in_threadpool(_detail, controller, _read_flow(controller, name))
 
 
 # -----------------------------------------------------------------------------
@@ -841,7 +846,7 @@ async def start_flow(
     _read_flow(controller, name)
     await controller.set_enabled(name, True)
     _audit("started", name, user)
-    return _detail(controller, _read_flow(controller, name))
+    return await run_in_threadpool(_detail, controller, _read_flow(controller, name))
 
 
 @router.post("/{name}/stop", response_model=FlowDetail)
@@ -850,7 +855,7 @@ async def stop_flow(name: str, controller: FlowControllerDep, user: CurrentUser)
     _read_flow(controller, name)
     await controller.set_enabled(name, False)
     _audit("stopped", name, user)
-    return _detail(controller, _read_flow(controller, name))
+    return await run_in_threadpool(_detail, controller, _read_flow(controller, name))
 
 
 @router.post("/{name}/pause", response_model=Message)

backend/fluksio/api/routes/workers.py

@@ -337,6 +337,10 @@ async def _attached_session(websocket: WebSocket, claims: dict[str, Any]) -> Non
             "version": hello.get("version"),
         },
         inventory=WorkerInventory.from_hello(hello.get("inventory")),
+        host=str(
+            hello.get("service_host")
+            or (websocket.client.host if websocket.client else "")
+        ),
     )
     hub.attach(worker)
     _behind.pop(name, None)