fix non-sticky tasks with nthreads==1

base/task.jl

@@ -412,9 +412,15 @@ end
 function enq_work(t::Task)
     (t.state == :runnable && t.queue === nothing) || error("schedule: Task not runnable")
     tid = Threads.threadid(t)
+    # Note there are three reasons a Task might be put into a sticky queue
+    # even if t.sticky == false:
+    # 1. The Task's stack is currently being used by the scheduler for a certain thread.
+    # 2. There is only 1 thread.
+    # 3. The multiq is full (can be fixed by making it growable).
     if t.sticky || tid != 0 || Threads.nthreads() == 1
         if tid == 0
             tid = Threads.threadid()
+            ccall(:jl_set_task_tid, Cvoid, (Any, Cint), t, tid-1)
         end
         push!(Workqueues[tid], t)
     else

test/threads.jl

@@ -640,3 +640,16 @@ let t = Timer(identity, 0.025, interval=0.025)
         close(t)
     end
 end
+
+# shared workqueue
+
+function pfib(n::Int)
+    if n <= 1
+        return n
+    end
+    t = @task pfib(n-2)
+    t.sticky = false
+    schedule(t)
+    return pfib(n-1) + fetch(t)::Int
+end
+@test pfib(20) == 6765