subprocess on UNIX may lead to process hang

[axw]

When calling subprocess.Popen via Pushy on UNIX, the server process may hang. I believe this occurs when the process forks, and the child and parent processes acquire different values for the message reading locks/conditions, etc., in BaseConnection.

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Imported from Launchpad using lp2gh.

• date created: 2009-05-26T00:40:07Z
• owner: axwalk
• assignee: axwalk
• the launchpad url was https://bugs.launchpad.net/bugs/380433

[axw]

(by axwalk) The problem was largely due to the use of the original sys.stdin when creating the pushy.protocol.Connection in client.py. Since the subprocess will replace file descriptor 0 (stdin), any attempts to access a proxy object will lead to reading from the new pipe, rather than the original stdin.

A few changes were made. First and foremost, I have dup'd sys.stdin in client.py and passed that to pushy.protocol.Connection instead. Second, although unnecessary to fix this bug, I've removed calls to self.send_response out of Connection, and have request handlers simply return the result. BaseConnection.__handle will now call self.send_response.

There is one more thing that needs to be done before this can be closed. If the child process that results from the fork in subprocess.Popen attempts to access a proxy object, it may receive asynchronous requests while waiting for a response. We need these messages need to be handled in the parent process, and so we need a way of asking the peer to resend a message.

[axw]

(by axwalk) I have decided to omit the "message resending" referred to above, as this problem has remained a theoretical one thus far. I have not been able to produce this defect. If it becomes problematic, a new bug may be raised later for it to be dealt with. Otherwise it's just holding up the 0.3 release, and complicating the code.