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Subject: Re: Mutex protection of output bucket brigade
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From: Alex Bligh <alex@alex.org.uk>
In-Reply-To: <51B83D4F.60401@gmail.com>
Date: Wed, 12 Jun 2013 10:48:54 +0100
Cc: Alex Bligh <alex@alex.org.uk>
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On 12 Jun 2013, at 10:20, Sorin Manolache wrote:

> If I understand correctly, the main thread belongs to your module, =
i.e. it is not a concise pseudo-code of the request processing in =
apache's code.

The main thread is the (presumably single) thread of the prefork mpm =
process, created (I assume) by a fork() in apache's worker code.

The pseudo code was what my long-running request handler does (after =
creating the other thread). IE, broadly speaking my request handler (the =
main thread if you like) does this:

 apr_thread_create; /* create the spawned thread */
 while (!done)
  {
    /* Blocking read */
    apr_brigade_create;
    ap_get_brigade;
    apr_brigade_flatten;

    /* Do stuff with the data */
    blocking_socket_write;
  }
 apr_thread_join; /* wait until the spawned thread has executed */

> I don't see where the output brigade appears in the main thread. I =
think this is critical, as the output_bucket_brigade is the data item =
shared between the two threads. ap_get_brigade triggers the execution of =
the chain of input filters. One of these input filters writes to the =
output brigade?

ap_get_brigade is called with APR_BLOCK_READ.

What I now /believe/ this does (because it's the only way data would =
actually get written) is write the post processed output bucket brigade =
to the client too. If this were not the case, it's difficult to see how =
a single threaded application would every write the output bucket =
brigade.

Or are you saying the output bucket brigade is only actually written to =
the client during an ap_fwrite()? In which case are all the filters =
(primarily mod_ssl) guaranteed to be thread safe if a different thread =
is doing input from that doing output?

--=20
Alex Bligh