Fix enq_work behavior when single-threaded

If there's only one thread in the task's preferred thread pool, use that
thread's work queue.

Author: kpamnany

base/task.jl

@@ -767,22 +767,33 @@ end
 
 function enq_work(t::Task)
     (t._state === task_state_runnable && t.queue === nothing) || error("schedule: Task not runnable")
-    if t.sticky || Threads.threadpoolsize() == 1
+
+    # Sticky tasks go into their thread's work queue.
+    if t.sticky
         tid = Threads.threadid(t)
         if tid == 0
-            # Issue #41324
-            # t.sticky && tid == 0 is a task that needs to be co-scheduled with
-            # the parent task. If the parent (current_task) is not sticky we must
-            # set it to be sticky.
-            # XXX: Ideally we would be able to unset this
-            current_task().sticky = true
+            # The task is not yet stuck to a thread. Stick it to the current
+            # thread and do the same to the parent task (the current task) so
+            # that the tasks are correctly co-scheduled (issue #41324).
+            # XXX: Ideally we would be able to unset this.
             tid = Threads.threadid()
             ccall(:jl_set_task_tid, Cint, (Any, Cint), t, tid-1)
+            current_task().sticky = true
         end
         push!(workqueue_for(tid), t)
     else
-        Partr.multiq_insert(t, t.priority)
-        tid = 0
+        tp = Threads.threadpool(t)
+        if Threads.threadpoolsize(tp) == 1
+            # There's only one thread in the task's assigned thread pool;
+            # use its work queue.
+            tid = (tp === :default) ? 1 : Threads.threadpoolsize(:default)+1
+            ccall(:jl_set_task_tid, Cint, (Any, Cint), t, tid-1)
+            push!(workqueue_for(tid), t)
+        else
+            # Otherwise, put the task in the multiqueue.
+            Partr.multiq_insert(t, t.priority)
+            tid = 0
+        end
     end
     ccall(:jl_wakeup_thread, Cvoid, (Int16,), (tid - 1) % Int16)
     return t