[asterisk-users] Help with semaphores

[Trevor Peirce]

Mitch Thompson wrote:
> [SATX_555_Extensions]
>
> exten => 1212,1,System(cat /tmp/{orig_num})  ; ${orig_num} is set at 
> the beginning of [from-trunk-custom] to the full dialed digits in 
> ${EXTEN}, before I break it down.
> exten => 1212,n,Busy(); if the file exists, someone else has already 
> called this number, return busy
>
> exten => 1212,102,System(echo ${UNIQUEID} > /tmp/${orig_num}) ; 
> basically, create a file in /tmp whose name is the full number from 
> the beginning.  In this case, the full
>                                                                                                              
> ; filename would be /tmp/2105551212.  I don't really care about the 
> contents, though.
> exten => 1212,103, Goto(Idle,1) ; from here, we jump to a new 
> extension called Idle, where we do a Random to decide whether to 
> simulate no one home (ring no answer) or
>                                                    ; we send ring for 
> about 10 seconds, then Answer() and play some .wav files, then 
> hangup.  The last thing we do in either case is to delete
>                                                    ; the 
> /tmp/${orig_num} file.
>
> The above code works very well at low call volumes.  However, I'm 
> running into race conditions at high call volumes where several calls 
> are getting through the test in priority 1 before the file is created 
> in priority 102 (n+101).
>

Here's what I would do...

First, no System calls.  Stay within asterisk.

I doubt this will get rid of all race conditions, but I imagine it would 
at least reduce them.

exten => 1212,1,GotoIf($["${DB(/tmp/${orig_num})}" != "${UNIQUEID}"]?abort)
exten => 1212,1,Set(${DB(/tmp/${orig_num})}=${UNIQUEID})
exten => 1212,n,GotoIf($["${DB(/tmp/${orig_num})}" != "${UNIQUEID}"]?abort)
exten => 1212,n, Goto(Idle,1)

exten => 1212,n(abort),Busy()

Regards,
Trevor