Bug#656137: readme.txt: asynchronous usage is a misleading lie

[Enrico Zini]

On Mon, Jan 16, 2012 at 08:53:06PM +0100, Enrico Zini wrote:

>  - Actions are synchronous. After sending a command to asterisk, a
>    readline() is issued right away. Again, this blocks the event loop
>    until asterisk replies, if it ever does.

This is in fact a bigger issue than just documentation. Suppose you want
to detect if the network connection to asterisk went down, and do
something like this:

class ResilientManager(Asterisk.Manager):
    [...]

    def read_loop(self, idle_timeout=5*60.0):
        """
        Read loop that will quit if there has been no input from asterisk for a
        configurable amount of seconds
        """
        while True:
            rlist, wlist, xlist = select.select([self], [], [], idle_timeout)
            if not rlist:
                # It timed out
                return
            else:
                self.read()

    def read_loop_with_pings(self, idle_timeout=5*60.0):
        """
        Read loop that, if nothing is heard from Asterisk after a given idle
        timeout, performs a ping to see if Asterks is still there.
        """
        while True:
            self.read_loop(idle_timeout)
            log.info("%s: no news from asterisk after %f seconds: pinging", self.name, idle_timeout)
            event = self.Ping()
            log.info("%s: asterisk replied to ping", self.name)

In this case, since Ping reads its reply with a readline(), if the
network went down then readline() waits indefinitely, defeating the
purpose of doing a Ping in the first place. I tested it here,
disconnecting the VPN to the asterisk server: a Ping was issued by the
process has been stuck in the Ping() method since 15 minutes and
counting.

At least for Ping, you *definitely* need at least a timeout.

As a workaround, one needs to wrap the above code in a
multiprocessing.Process, and then implement a further watchdog in the
parent process.


Ciao,

Enrico

-- 
GPG key: 4096R/E7AD5568 2009-05-08 Enrico Zini <enrico at enricozini.org>
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