make threadpool care about minPoolSize

lib/pure/concurrency/threadpool.nim

@@ -448,12 +448,22 @@ proc nimSpawn3(fn: WorkerProc; data: pointer) {.compilerProc.} =
     if selectWorker(readyWorker, fn, data): return
     for i in 0.. <currentPoolSize:
       if selectWorker(addr(workersData[i]), fn, data): return
+
     # determine what to do, but keep in mind this is expensive too:
     # state.calls < maxPoolSize: warmup phase
     # (state.calls and 127) == 0: periodic check
     if state.calls < maxPoolSize or (state.calls and 127) == 0:
       # ensure the call to 'advice' is atomic:
       if tryAcquire(stateLock):
+        if currentPoolSize < minPoolSize:
+          if not workersData[currentPoolSize].initialized:
+            activateWorkerThread(currentPoolSize)
+          let w = addr(workersData[currentPoolSize])
+          atomicInc currentPoolSize
+          if selectWorker(w, fn, data):
+            release(stateLock)
+            return
+
         case advice(state)
         of doNothing: discard
         of doCreateThread: