Improve worker launch thread-safety

The `launched_q` array is filled concurrently by the launcher tasks for each
worker. Now it's wrapped inside `Base.Lockable` for thread-safety.

src/cluster.jl

@@ -475,7 +475,7 @@ function addprocs_locked(manager::ClusterManager; kwargs...)
 
     # References to launched workers, filled when each worker is fully initialized and
     # has connected to all nodes.
-    launched_q = Int[]   # Asynchronously filled by the launch method
+    launched_q = Base.Lockable(Int[])   # Asynchronously filled by the launch method
 
     # The `launch` method should add an object of type WorkerConfig for every
     # worker launched. It provides information required on how to connect
@@ -522,7 +522,7 @@ function addprocs_locked(manager::ClusterManager; kwargs...)
         remote_do(set_valid_processes, pid, all_w)
     end
 
-    sort!(launched_q)
+    sort!(@lock launched_q launched_q[])
 end
 
 function set_valid_processes(plist::Array{Int})
@@ -551,7 +551,7 @@ default_addprocs_params() = Dict{Symbol,Any}(
 
 function setup_launched_worker(manager, wconfig, launched_q)
     pid = create_worker(manager, wconfig)
-    push!(launched_q, pid)
+    @lock launched_q push!(launched_q[], pid)
 
     # When starting workers on remote multi-core hosts, `launch` can (optionally) start only one
     # process on the remote machine, with a request to start additional workers of the
@@ -589,7 +589,7 @@ function launch_n_additional_processes(manager, frompid, fromconfig, cnt, launch
                 Threads.@spawn Threads.threadpool() begin
                     pid = create_worker(manager, wconfig)
                     remote_do(redirect_output_from_additional_worker, frompid, pid, port)
-                    push!(launched_q, pid)
+                    @lock launched_q push!(launched_q[], pid)
                 end
             end
         end