Safely close mirrors

We need to use the mirrors when using LineProcBuffer and LineHostBuffer. In the previous article, we have explained how to use the derived class of LineProcBuffer to catch the progress. In that example, we need to move the pbuf.wait method to the background like this:

...
thd = threading.Thread(target=pbuf.wait, daemon=True)
thd.start()
print("Start to wait.")
while pbuf.progress < 100:
    ...
    time.sleep(0.1)
print("Progress is 100%.")
thd.join()
...

To make sure that all sub-processes are finished safely, we need to use thd.join to wait the pbuf.wait method finishes. However, in some cases, we need to terminate the sub-processes before they are finished. Use terminate method of the process is a possible solution. However, such kind of termination is not safe enough. A better way is to send a closing signal to the sub-processes. When each sub-process intends to write new messages, it needs to check the closing signal. If the closing signal is detected, a StopIteration exception would be raised. For example, if we change the above part like this:

thd = threading.Thread(target=pbuf.wait, daemon=True)
thd.start()
print("Start to wait.")
prog = pbuf.progress
while prog < 100:
    time.sleep(0.1)
    prog = pbuf.progress
    if prog > 10:
        pbuf.stop_all_mirrors()
        break
print("Progress is not 100%.")
thd.join()
print("Final progress: {0}%".format(pbuf.progress))

The sub-process would be terminated in advance. Consequently, the buffer would not be able to receive the finishing message from the sub-process. Instead, the StopIteration exceptions from the sub-processes would be catched.