Need documentation describing how to configure VirtualHost with mod_pagespeed

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What steps will reproduce the problem?
1. Comment out all of /etc/apache2/mods-enabled/pagespeed.conf, replacing with 
<IfModule pagespeed_module>ModPagespeed off</IfModule>
2. Include the former contents of pagespeed.conf into a single virtualhost
3. Attempt to access a non-pagespeed'd virtualhost

What is the expected output? What do you see instead?
I would expect modpagespeed to be disabled globally, but enabled only on 
specifically-configured virtualhosts

What version of the product are you using (please check X-Mod-Pagespeed
header)?
0.9.10.1-250

On what operating system?
Ubuntu Karmic (9.10)

Which version of Apache?
2.2.12-1ubuntu2.4

Which MPM?
mpm-worker

Please provide any additional information below, especially a URL or an
HTML file that exhibits the problem.
When I don't have "ModPagespeed off" in the pagespeed.conf, Apache fails to 
start with:

[Thu Nov 25 17:22:02 2010] [error] [mod_pagespeed 0.9.10.1-250] mod_pagespeed 
is enabled.  The following directives must not be 
NULL\nModPagespeedFileCachePath=\nModPagespeedGeneratedFilePrefix=\n

When I do have "ModPagespeed off" in the pagespeed.conf, Apache starts, but 
fails to serve pages for non-Pagespeed'd virtualhosts, with:

[Thu Nov 25 17:22:28 2010] [alert] [mod_pagespeed 0.9.10.1-250] 
[1125/172228:FATAL:net/instaweb/rewriter/rewrite_driver_factory.cc(205)] Check 
failed: !filename_prefix_.empty(). Must specify --filename_prefix or call 
RewriteDriverFactory::set_filename_prefix.\nBacktrace:\n\t/usr/lib/apache2/modul
es/mod_pagespeed.so 
[0x7fbfe7487dda]\n\t/usr/lib/apache2/modules/mod_pagespeed.so 
[0x7fbfe74796ad]\n\t/usr/lib/apache2/modules/mod_pagespeed.so 
[0x7fbfe7488c5d]\n\t/usr/lib/apache2/modules/mod_pagespeed.so 
[0x7fbfe74cec23]\n\t/usr/lib/apache2/modules/mod_pagespeed.so 
[0x7fbfe747c779]\n\t/usr/lib/apache2/modules/mod_pagespeed.so 
[0x7fbfe74cdf40]\n\t/usr/lib/apache2/modules/mod_pagespeed.so 
[0x7fbfe74ce03c]\n\t/usr/lib/apache2/modules/mod_pagespeed.so 
[0x7fbfe7478e76]\n\t/usr/sbin/apache2(ap_run_handler+0x70) 
[0x7fbfeb755310]\n\t/usr/sbin/apache2(ap_invoke_handler+0x98) 
[0x7fbfeb758c58]\n\t/usr/sbin/apache2(ap_process_request+0x1c8) 
[0x7fbfeb7664e8]\n\t/usr/sbin/apache2 
[0x7fbfeb7633c8]\n\t/usr/sbin/apache2(ap_run_process_connection+0x68) 
[0x7fbfeb75cea8]\n\t/usr/sbin/apache2 
[0x7fbfeb76bdd2]\n\t/usr/lib/libapr-1.so.0 
[0x7fbfeae942ff]\n\t/lib/libpthread.so.0 
[0x7fbfeac55a04]\n\t/lib/libc.so.6(clone+0x6d) [0x7fbfea9bed4d]

Original issue reported on code.google.com by rtuc...@gmail.com on 25 Nov 2010 at 5:36