Yaws Server and the Process Dictionary

[Trevor Woollacott [ MTN - Innovation Centre ]]

Hi all,

I have setup an HTTPS server using Yaws-1.75 in Erlang R12-B5. While the clients are connected I'm seeing the following error every few seconds:
{{badmatch,undefined},[{proc_lib,exit_p,2}]}

I realise that this occurs in the following case (in R12-B5):
- A process is spawned using proc_lib:spawn_link,
- $initial_call is erased from the dictionary of that process,
- Then the process exits or crashes

When I check the dictionary of some of the Yaws Acceptors they are missing '$ancestors' and '$initial_call', as shown below (with the actual values removed by me):
dictionary: [
  {gc, ...},
  {init_db,[{gc, ...}]}]

However the other Acceptors still have $ancestors and $initial_call:
dictionary: [
  {gc, ...},
  {'$ancestors',[<5368.75.0>,yaws_server,yaws_sup,<5368.58.0>]},
  {'$initial_call',{yaws_server,acceptor0,2}},
  {init_db,[{gc, ...},
            {'$ancestors',[<5368.75.0>,yaws_server,yaws_sup,<5368.58.0>]},
            {'$initial_call',{yaws_server,acceptor0,2}]}]

So I assume that when the Yaws Acceptor exits normally, proc_lib:exit_p/2 crashes when it tries to match {m, f, a} with the value in '$initial_call'.

Has anyone seen this occur before with Yaws?

Thanks,
Trevor

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[Claes Wikström]

Trevor Woollacott [ MTN - Innovation Centre ] wrote:
> Hi all,
>
> I have setup an HTTPS server using Yaws-1.75

1.75 is pretty old, and Yaws is using the proc dict in
fairly ugly ways. I think that you're seeing old (fixed) bugs
in how the proc dict is handled by Yaws.

Maybe

  - proc dict/proc_lib patch by Magnus F  (in 1.78)


is the fix.

/klacke

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[Trevor Woollacott [ MTN - Innovation Centre ]]

> From: Claes Wikström [mailto:klacke@...]
> Sent: Wednesday, 16 September 2009 12:31 PM
> To: Trevor Woollacott [ MTN - Innovation Centre ]
> Cc: erlyaws-list@...
> Subject: Re: [Erlyaws-list] Yaws Server and the Process Dictionary
>
> Trevor Woollacott [ MTN - Innovation Centre ] wrote:
> > Hi all,
> >
> > I have setup an HTTPS server using Yaws-1.75
>
> 1.75 is pretty old, and Yaws is using the proc dict in
> fairly ugly ways. I think that you're seeing old (fixed) bugs
> in how the proc dict is handled by Yaws.
>
> Maybe
>
>   - proc dict/proc_lib patch by Magnus F  (in 1.78)
>
>
> is the fix.
>
> /klacke

Thanks Klacke.

It definitely looks like a newer version will fix our problem. Here's a copy & paste from the yaws_server:erase_transients/0 (Yaws-1.79):

    if I == undefined ->
            ok;
       list(I) ->
           %% Need to keep init_db in case we do not enter aloop (i.e. init:db)
           %% again as R12B-5 requires proc_lib keys in dict while exiting...
           put(init_db, I),
            lists:foreach(fun({K,V}) -> put(K,V) end, I)
    end.

The line containing put(init_db, I) was added, which will most likely fix our problem where the dictionary had been erased before the Acceptor exits. There are a couple other fixes which will benefit us also.

Regards,
Trevor

NOTE: This e-mail message is subject to the MTN Group disclaimer see http://www.mtn.co.za/SUPPORT/LEGAL/Pages/EmailDisclaimer.aspx

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