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From Justin Erenkrantz <jerenkra...@ebuilt.com>
Subject [PATCH] POD and Threaded MPM...
Date Wed, 11 Jul 2001 08:07:05 GMT
I'd wager that threaded MPM should probably use the calls in
mpm_common.c.  But, maybe there is a reason that it doesn't.
It's 1AM here.  I've been staring at this code all night.  My brain
isn't working too well right now.  (My whiteboard is covered with notes 
about threaded MPM.)

Anyway, consider the following case in the threaded MPM:

- parent writes a ! to the POD signaling ONE child process to die
- around line 585, ALL of the children processes pick up on this write 
  via select
- a representative thread for each child process now calls c_p_o_d
  (if not more than one thread).
- Acquires a mutex (locking all other threads in same process, but not 
  the other processes - not a big deal - just keep that in mind...).
- One of the children (with the lock per process) calls apr_recv and is 
  the lucky winner who receives the !.  He and all of his sibling 
  worker threads now need to exit (since workers_may_exit is defined 
  for that process and should be).

Now, what about the other child processes that *shouldn't* exit?  
They potentially received the select as well - they are going to do
the apr_recv.  They won't receive EAGAIN, but rather it'll be 
returned with n == 0.  But, we don't check for that - so, all of 
those threads in that other process now exits (workers_may_exit now 
defined).  Oops.

I've got a local program that does a bunch of forks with pipe reads
with one writer to prove that EAGAIN isn't returned - just that the 
length is 0 when there is nothing more to be read at that point.

My thought is this patch needs to be applied.  Similar logic to this 
is in ap_mpm_pod_check (len == 1).  This may fix some of the 
wackiness seen with threaded MPM, if I'm right (I may well not be 
right).  Thoughts?  -- justin

P.S. What is supposed to happen when APR_FILES_AS_SOCKETS is not
defined?  Threaded MPM would never read the POD?

Index: threaded.c
===================================================================
RCS file: /home/cvs/httpd-2.0/server/mpm/threaded/threaded.c,v
retrieving revision 1.44
diff -u -r1.44 threaded.c
--- threaded.c	2001/07/03 13:58:10	1.44
+++ threaded.c	2001/07/11 07:44:30
@@ -511,7 +511,9 @@
         else {
             /* It won the lottery (or something else is very
              * wrong). Embrace death with open arms. */
-            workers_may_exit = 1;
+            /* Only if we read a character can we exit. */
+            if (n == 1)
+                workers_may_exit = 1;
         }
     }
     apr_lock_release(pipe_of_death_mutex);


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