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Subject mutual exclusion Q
Date Mon, 01 Dec 2008 02:49:59 GMT
My Q today may be affected by Apache and/or Perl, so it seems  
appropriate for this mailing list.

I've written a mod-perl module that needs mutual exclusion among  
Apache child processes using the prefork MPM, so I decided to use a  
file as a semaphore using Perl's 'flock'.

1. The mutual exclusion doesn't work (in the sense that multiple  
processes are allowed to execute mutually-excluded code  
simultaneously) if the post-config phase (before forking) just  
sysopens the file for write with permissions 0666, and the child  
processes flock the resulting filehandle ($outer_lock_fh) when they  
need to.

2. It works if the post-config phase does the same sysopen (to ensure  
the file exists) and then immediately closes the file, and each child  
process opens the file for input in its child-init phase.

3. Mutual exclusion does not work (in the same way as in 1) if the  
post-config phase sysopens for write, closes, and then opens for  
reading in the same way that the child-init phases do in case 2.

This suggests that the forking operation (with demotion to a lower  
priority user) prevents flock from working properly on the inherited  
filehandle.  Yet my Apache child processes can inherit other open  
filehandles and tied hashes from the original process, and do I/O via  
them just fine, without having to re-open them.

Can anyone this difference in use of these filehandles?  If I could  
understood why 1-3 act as they do, this would probably help me in the  


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