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Practical mod_perl by Eric Cholet, Stas Bekman

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Chapter 23. Getting Help and Online Resources

In this chapter, we propose a way to solve your mod_perl-related problems and provide starting points for information resources related to mod_perl.

If you have any problem with mod_perl itself, be it a build problem or a runtime problem, you should follow the steps below. But before you follow them, think carefully about whether the problem you are experiencing is mod_perl-related. It's quite possible that the problem is in the Perl code, SQL code, Apache itself, or something else entirely. In such cases, you should refer to other resources presented later in this chapter. Remember that although mod_perl resources might help you with many related things, they will never be as detailed as resources devoted to the topic at hand.

If you still think that the problem has something to do with mod_perl, these are the steps to follow:

  1. Try to tackle the problem by yourself for a while. Check that you have the right permissions, that there is enough disk space, etc. Do sanity checks: try to remove the mod_perl source tree, unpack it again, and build from fresh.

    When trying to figure out what the problem is, always run under single-server mode (httpd -X) and always check the error_log file.

    If you still have problems, proceed to step 2.

  2. Reread the documentation (or if you didn't read it yet, do it now). Try to follow the build and usage steps as explained there. This book, Writing Apache Modules with Perl and C (O'Reilly), and the documentation distributed with the mod_perl sources provide in-depth details on this topic. Also, make sure to read Chapter 22 thoroughly. If you are still in trouble, proceed to step 3.

  3. Go to the mod_perl list archives (at http://perl.apache.org/maillist/) and see whether someone has already reported the same problem. If someone did, chances are that a cure to the problem has been posted to the list, be it a source patch or a workaround. If after doing an exhaustive search you haven't come up with any solution, proceed to step 4.

    Notice that sometimes doing this step before step 2 can be a good idea as well—you may happen to have encountered a well-known bug, and if that's the case doing a quick lookup in the mailing-list archives will save you time and frustration.

  4. This step is the last resort. Contact the mod_perl mailing list. You should never abuse this step, and use it only when you have already been through the previous three steps. If you ask FAQ questions unnecessarily, chances are that people will not reply to you. And if you ask more FAQ questions, you might get onto people's blacklists and they will not answer your future questions even if they are relevant. Remember that all the answers that you get are coming from volunteers who, instead of having fun outdoors, try to have fun answering challenging questions. FAQ questions aren't challenging, and few people have fun answering them. See more details about mod_perl list etiquette in the next section.

    It's not enough to just contact the list and ask for help. You have to provide as many details as possible. The next section covers the details you have to provide.

    However, don't be afraid. The mod_perl mailing list is filled with only nice people who can provide much help and guidance, so if you can't figure something out after having followed the above steps, your question is welcome.

    You cannot post to the list without first subscribing to it. To subscribe, send an email to . After you receive a confirmation email, you can start posting to the list. Send your emails to .

    There are other related mailing lists you might want to be on too. See the list of these and subscription instructions in Section 23.3.

How to Report Problems

When reporting a problem to the mod_perl mailing list, always send these details:

  • Anything in the error_log file that looks suspicious and possibly related to the problem

  • Output of perl -V

  • Version of mod_perl

  • Version of Apache

  • Options given to mod_perl's Makefile.PL file

  • Server configuration details

  • If make test fails, the output of make test TEST_VERBOSE=1

Also check whether:

  • make test passes 100%

  • The script works under mod_cgi, if applicable

You should try to isolate the problem and send the smallest possible code snippet that reproduces the problem.

Getting the Backtrace from Core Dumps

If you get a core dump (segmentation fault), send a backtrace if possible. Before you try to produce it, rebuild mod_perl with:

panic% perl Makefile.PL PERL_DEBUG=1

which will:

  • Add -g to EXTRA_CFLAGS

  • Turn on PERL_TRACE

  • Set PERL_DESTRUCT_LEVEL=2 (additional checks during Perl cleanup)

  • Link against libperld, if it exists

You can read a full explanation in Chapter 21, but here is a summary of how to get a backtrace:

panic% cd mod_perl-1.xx
panic% gdb ../apache_1.3.xx/src/httpd
(gdb) run -X -f `pwd`/t/conf/httpd.conf -d `pwd`/t
[now make request that causes core dump]
(gdb) bt

In English: cd to the mod_perl source directory and start gdb with a path to the httpd binary, which is located in the Apache source tree. (Of course, replace x with real version numbers.) Next, start the httpd process from within gdb and issue a request that causes a core dump. When the code has died with the SIGSEGV signal, run bt to get the backtrace.

Alternatively, you can also attach to an already running process like so:

panic% gdb httpd <process id number>

If the dump is happening in libperl, you have to rebuild Perl with -DDEBUGGING enabled during the ./Configure stage. A quick way to this is to go to your Perl source tree and run these commands:

panic% rm *.[oa]
panic% make LIBPERL=libperld.a
panic% cp libperld.a $Config{archlibexp}/CORE

where $Config{archlibexp} is:

% perl -V:archlibexp

Spinning Processes

The gdb attaching to the live process approach is helpful when debugging a spinning process. You can also get a Perl stack trace of a spinning process by installing a $SIG{USR1} handler in your code:

use Carp ( );
$SIG{USR1} = \&Carp::confess;

While the process is spinning, send it a USR1 signal:

panic% kill -USR1 <process id number>

and the Perl stack trace will be printed.

Alternatively, you can use gdb to find which Perl code is causing the spin:

panic% gdb httpd <pid of spinning process>
(gdb) where
(gdb) source mod_perl-x.xx/.gdbinit
(gdb) curinfo

After loading the special macros file (.gdbinit), you can use the curinfo gdb macro to figure out the file and line number in which the code stuck. Chapter 21 talks in more detail about tracing techniques.

Finally, send all these details to .

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