server bootstrapping upon connection (WARNING: LONG)

[Benjamin Han]

On 2004-02-10 13:46:37 -0500, ralf at brainbot.com said:

> fortepianissimo at yahoo.com.tw (Fortepianissimo) writes:
>> The problem: I need to prevent multiple copies of the server being
>> started. I did this by using file locking (fcntl.lockf()). However,
>> not every time the code successfully prevented the server being
>> started up more than one time. Here is the relevant code:
> 
> When using fcntl.lockf different FooClient instances in the same
> process will be able lock the file and start another server. You could
> either use fcntl.flock to prevent that or use some global flag.

Hm... I didn't know there's a difference between flock() and lockf(), 
and I didn't get much info from the document either. Could you explain 
a bit on why lockf() would not lock the file?

Actually I wrote a tiny script just to test if lockf() does what it 
claims to do:

--- CODE STARTS ---
#!/usr/bin/env python

import os,fcntl,sys

print "* about to open flock.txt"
f=open('flock.txt','w')
print "* opened the file"
fcntl.lockf(f.fileno(),fcntl.LOCK_EX|fcntl.LOCK_NB)
print "* obtained the lock, enter your line below:"
l=sys.stdin.readline()
f.truncate()
f.write(l)
f.flush()
sys.stdin.readline()
f.close()

--- CODE ENDS ---

It seems it does lock the file? (Mac OS X 10.3.2).

> Also be sure to keep the file open by keeping a reference to it
> (i.e. self.serverStartLock=open(...)). For debugging purposes, remove 
> that 'if self.connect(): return' and
> I think you'll see much more servers being started.

--- CODE SNIPPET STARTS ---
class FooClient:
    def __init__ (self, startServer=True):
        """Connects to FooServer if it exists, otherwise starts it and 
connects to it"""
        self.connected=True
        if self.connect(): return
        elif not startServer:
            if FOO_CLIENT_DEBUG: log('connection failed 1')
            self.connected=False
            return
..
--- CODE SNIPPET ENDS ---

Well in that case every connection will try to start a server. Good 
point on keeping a reference to the lock file though - I'll add to it 
and see what happens.

>> 
>> From the log (when problem occurred) I see even *AFTER* the server was
>> started and accepted connections (several connections came and went
>> happily), a connection would come in and hit the "connection failed 1"
>> log line. This shouldn't have happened as the default value of
>> startServer for FooClient.__init__() is True. In the very same
> 
> Well, maybe too many connection attempts are pending...

I failed to see why this should affect the default value of the 
argument... if startServer is True (default), that log line should have 
never been reached.

Thanks!

Ben