Change Distributed.cluster_manager from a global non-constant ClusterManager to a global constant Ref{ClusterManager}

src/cluster.jl

@@ -9,6 +9,14 @@ Cluster managers implement how workers can be added, removed and communicated wi
 """
 abstract type ClusterManager end
 
+# cluster_manager is a global constant
+const cluster_manager = Ref{ClusterManager}()
+
+function throw_if_cluster_manager_unassigned()
+    isassigned(cluster_manager) || error("cluster_manager is unassigned")
+    return nothing
+end
+
 """
     WorkerConfig
 
@@ -390,8 +398,7 @@ function init_worker(cookie::AbstractString, manager::ClusterManager=DefaultClus
 
     # On workers, the default cluster manager connects via TCP sockets. Custom
     # transports will need to call this function with their own manager.
-    global cluster_manager
-    cluster_manager = manager
+    cluster_manager[] = manager
 
     # Since our pid has yet to be set, ensure no RemoteChannel / Future  have been created or addprocs() called.
     @assert nprocs() <= 1

src/process_messages.jl

@@ -315,6 +315,8 @@ function handle_msg(msg::ResultMsg, header, r_stream, w_stream, version)
 end
 
 function handle_msg(msg::IdentifySocketMsg, header, r_stream, w_stream, version)
+    throw_if_cluster_manager_unassigned()
+
     # register a new peer worker connection
     w = Worker(msg.from_pid, r_stream, w_stream, cluster_manager; version=version)
     send_connection_hdr(w, false)
@@ -328,6 +330,8 @@ function handle_msg(msg::IdentifySocketAckMsg, header, r_stream, w_stream, versi
 end
 
 function handle_msg(msg::JoinPGRPMsg, header, r_stream, w_stream, version)
+    throw_if_cluster_manager_unassigned()
+
     LPROC.id = msg.self_pid
     controller = Worker(1, r_stream, w_stream, cluster_manager; version=version)
     notify(controller.initialized)
@@ -348,9 +352,9 @@ function handle_msg(msg::JoinPGRPMsg, header, r_stream, w_stream, version)
         let rpid=rpid, wconfig=wconfig
             if lazy
                 # The constructor registers the object with a global registry.
-                Worker(rpid, ()->connect_to_peer(cluster_manager, rpid, wconfig))
+                Worker(rpid, ()->connect_to_peer(cluster_manager[], rpid, wconfig))
             else
-                @async connect_to_peer(cluster_manager, rpid, wconfig)
+                @async connect_to_peer(cluster_manager[], rpid, wconfig)
             end
         end
     end