fixes slice logic for fetcher

Author: allenwang28

src/forge/controller/provisioner.py

@@ -48,7 +48,13 @@ async def get_remote_info(host_mesh: HostMesh) -> tuple[str, str]:
     """Returns the host name and port of the host mesh."""
     throwaway_procs = host_mesh.spawn_procs(per_host={"procs": 1})
     fetcher = throwaway_procs.spawn("_fetcher", _RemoteInfoFetcher)
-    fetcher = fetcher.slice(procs=0)
+
+    # This will reduce something like extent = {"hosts": 2, "procs": 1} to
+    # {"hosts": 1, "procs": 1}.
+    singleton_slice = {k: slice(0, 1) for k in fetcher.extent.keys()}
+    fetcher = fetcher.slice(**singleton_slice)
+    # Fetcher should be a singleton at this point - call_one() will fail otherwise
+
     host, port = await fetcher.get_info.call_one()
     await throwaway_procs.stop()
     return host, port