worker: add threading lock

add lock to avoid same proxy to be scanned multiple time when
a small number a jobs is handed to worker

proxywatchd.py

@@ -149,15 +149,18 @@ class WorkerThread():
 		self.thread = None
 		self.workqueue = []
 		self.workdone = []
+		self.lock = threading.Lock()
 	def stop(self):
 		self.done.set()
 	def term(self):
 		if self.thread: self.thread.join()
 	def add_jobs(self, jobs):
-		self.workqueue.extend(jobs)
+		with self.lock:
+			self.workqueue.extend(jobs)
 	def return_jobs(self):
-		jobs = self.workqueue
-		self.workqueue = []
+		with self.lock:
+			jobs = self.workqueue
+			self.workqueue = []
 		return jobs
 	def jobcount(self):
 		return len(self.workqueue)
@@ -168,14 +171,21 @@ class WorkerThread():
 	def start_thread(self):
 		self.thread = threading.Thread(target=self.workloop)
 		self.thread.start()
+	def pop_if_possible(self):
+		with self.lock:
+			if len(self.workqueue):
+				job = self.workqueue.pop()
+			else:
+				job = None
+		return job
 	def workloop(self):
 		success_count = 0
 		job_count = 0
 		duration_total = 0
 		duration_success_total = 0
 		while True:
-			if len(self.workqueue):
-				job = self.workqueue.pop()
+			job = self.pop_if_possible()
+			if job:
 				nao = time.time()
 				job.run()
 				spent = time.time() - nao