Re: ANNOUNCE: Debug::SIGSEGVTrace

[Stas Bekman <st...@stason.org>]

Stas Bekman wrote:
> For all those who have troubles getting a backtrace or the core file, 
> when a mod_perl server (or a perl program) segfaults, here is a module 
> that attempts to do all the work for you. The rest of the doc follows. 
> Please read the TODO section, and help me with things that need to be 
> done if you know how. Thanks.
> 
> I've just uploaded it on CPAN, so it'll take a while before it gets 
> mirrored, but you can get it from here:
> https://pause.perl.org/pub/PAUSE/authors/id/S/ST/STAS/Debug-SIGSEGVTrace-0.01.tar.gz 

Oops, I haven't realized that this URL is passwd protected. Anyway it's 
on CPAN now: http://search.cpan.org/search?mode=module&query=SIGSEGVTrace


-- 


__________________________________________________________________
Stas Bekman            JAm_pH ------> Just Another mod_perl Hacker
http://stason.org/     mod_perl Guide ---> http://perl.apache.org
mailto:stas@stason.org http://use.perl.org http://apacheweek.com
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ANNOUNCE: Debug::FaultAutoBT v0.02

[Stas Bekman <st...@stason.org>]

As suggested by Tim Bunce at modules@perl.org I've renamed the 
Debug::SIGSEGVTrace package to be Debug::FaultAutoBT and it currently 
handles more signals than just SIGSEGV.

It's on its way to CPAN now. Meanwhile you can get it from:
http://www.cpan.org/authors/id/S/ST/STAS/Debug-FaultAutoBT-0.02.tar.gz

-- Changes:

v0.02 Wed Aug  7 11:57:15 SGT 2002

- added another test with SIGABRT

- now supporting the following signals SIGQUIT SIGILL SIGTRAP SIGABRT
   SIGEMT SIGFPE SIGBUS SIGSEGV SIGSYS

- backported to 5.005_03 (what a pain!)

- renamed Debug::SIGSEGVFault to Debug::DumpCore

- renamed the Debug::SIGSEGVTrace package to be Debug::FaultAutoBT,
   planning to trap other signals that result in a core dump.

- changed the default backtrace base filename to be "core.backtrace."
   rather than "core." (which was misleading).

- split SIGSEGVTrace.xs into SIGSEGVFault.xs and SIGSEGVTrace.xs

-- README:

NAME
     Debug::FaultAutoBT - Automatic Backtrace Extractor on SIGSEGV, SIGBUS,
     etc.

SYNOPSIS
       use Debug::FaultAutoBT;

       use File::Spec::Functions;
       my $tmp_dir = File::Spec::Functions::tmpdir;

       my $trace = Debug::FaultAutoBT->new(
           dir            => "$tmp_dir",
           #verbose        => 1,
           #exec_path      => '/home/stas/perl/bin/perl',
           #core_path_base => catfile($tmp_dir, "mycore"),
           #command_path   => catfile($tmp_dir, "my-gdb-command"),
           #debugger       => "gdb",
       );

       # enable the sighandler
       $trace->ready();

       # or simply:
       Debug::FaultAutoBT->new(dir => "$tmp_dir")->ready;

DESCRIPTION
     When a signal, that normally causes a coredump, is delivered This 
module
     attempts to automatically extract a backtrace, rather than letting the
     core file be dumped. This has the following benefits:

     *   no need to setup the environment to allow core file dumped.
         Sometimes people just don't know how to set it up. Sometimes you
         aren't allowed to set it up (e.g., when the webserver 
environment is
         not under your control).

     *   if many Perl programs are run in a row and more than one program
         segfaults it's possible to collect all backtraces, rathen then
         aborting the run on the first segfault or staying with only the 
last
         core file, which will overwrite all the previous ones. For example
         consider a live webserver or a test suite which may segfault many
         times for different reasons.

     *   for huge core files, this approach saves disk space. And can be a
         saver when you don't have disk space left for various reasons
         (passed the quota?), but still have a few kilo-bytes left.

     Currently the following signals are trapped:

          SIGQUIT
          SIGILL
          SIGTRAP
          SIGABRT
          SIGEMT
          SIGFPE
          SIGBUS
          SIGSEGV
          SIGSYS

     (If you know of other signals that should be trapped let me know.
     thanks.)

METHODS
   new()

       my $trace = Debug::FaultAutoBT->new(
           dir            => "$tmp_dir",
           verbose        => 1,
           exec_path      => '/home/stas/perl/bin/perl',
           core_path_base => catfile($tmp_dir, "mycore"),
           command_path   => catfile($tmp_dir, "my-gdb-command"),
           debugger       => "gdb",
       );

     Attributes:

     *dir*
         a writable by the process directory.

         This is a required attribute.

     *verbose*
         Whether to be silent (0) or verbose (1).

         This is an optional attribute. The default is 0.

         Currently it's always a non-verbose, with just a few traces printed
         out. Will work in the future.

     *exec_path*
         "gdb" needs to know the path to the executable in order to 
attach to
         the process (though gdb 5.2 and higher needs only pid to do that).
         This module is trying to automatically figure out the executable
         path, using several methods in the following order:

           $^X, /proc/self/exe, $Config{perlpath}

         If all these methods fail the module will die(), unless you
         explicitly set the *exec_path* attribute. Notice I named it
         *exec_path* because the executable doesn't have to be perl, when
         Perl is embedded, which is the case with mod_perl, which sets "$^X"
         to the path to the Apache httpd server.

     *core_path_base*
         The base path of the core file. e.g. if *core_path_base* is set to
         */tmp/mycore* and the pid of the process that has segfaulted is
         12345, the generated core is written to the file 
*/tmp/mycore12345*.

         This is an optional attribute.

         By default *core_path_base* is a concatenation of the *dir*
         attribute and the string *core.*.

     *command_path*
         The path to the file with debugger commands. If this attribute is
         set the file should already include the commands. Notice that the
         commands should include 'quit' as the last command, so the debugger
         will quit.

         This is an optional attribute.

         By default *command_path* is a concatenation of the *dir* attribute
         and the string *gdb-command*, which is getting populated with the
         following commands:

           bt
           quit

     *debugger*
         Curently not used. In the future could be used to specify which
         debugger to use (when more than one debugger is supported). For the
         future compatibility "gdb" is going to be the default.

   ready()

       $trace->ready();

     This method sets the SIGSEGV sighandler. Only after this method is
     called the extract of the trace will be attempted on the event of
     SegFault.

     Notice that it sets the handler to be called only once. If another
     segfault happens during the processing of the handler, the SIGSEGV
     handler that was previously set will get invoked. If none was 
previously
     set the default SIGSEGV handler will attempt to dump the core file if
     the environment is configured to allow one (via shell's "limit" command
     and possibly other system-dependent manipulations).

   RELATED NOTES

     When you want to get a useful backtrace the debugger must be able to
     resolve symbols. Therefore the object in question must have its symbols
     preserved and not stripped. This is usually accomplished by compiling
     the C code with "-g". Since this code gets called from Perl, which in
     turn may be embedded into some other application (e.g., mod_perl 
enabled
     Apache), you probably want to have *libperl.so* and the application 
it's
     embedded to, to be compiled with the debug symbols non-stripped.

     For example to build a Perl package which includes XS/C objects, add:

       WriteMakefile(
           ...
           DEFINE            => '-g',
           ...
       );

     To build Perl in debug mode:

       ./Configure ... -Doptimize='-g' ...

     To build Apache 1.3 without stripping the symbols:

       ./configure ... --without-execstrip

     To build Apache 2.0 in the debug mode:

       ./configure ... --enable-maintainer-mode ...

BUGS
     *   For some reason gdb invoked from sighandler doesn't see the last
         frame the actual fault happened at, rendering the tool less useful
         as it could be. If you know how to cure that, please let me know.

     *   When you run the handler you might get things like:

           /tmp/Debug-FaultAutoBT-0.01/24043: No such file or directory.

         This is a bug in older versions of gdb, simply ignore it.

     *   It probably won't compile on Win32. If you know how please submit
         patches.

EXPORT
     None.

TODO
     * the code is not thread-safe (so it's not running under mod_perl 2.0
     with worker mpm :(. The question is how to pass data to the SIGSEGV
     signal handler, without using static variables.

     * clean the backtrace from extra frames added by this module

     * how do we pass the test suite if we exit(2)? currently used fork() to
     workaround it, but it's not very portable.

     * how do we clean-up the autogenerated gdb-command file if we exit(2)?

     * support other debuggers than gdb. Need your input/patches.

     Currently this module works only on systems with gdb installed.

     I'm not sure how portable is my C code, but should probable work on any
     POSIX-complient system.

     If you know how to make the code more portable, or support other
     debuggers on other OSes please send patches.

ACKNOWLEDGEMENTS
     The idea has been borrowed from the GNOME's gnome-crash project, which
     is used to automatically extract a backtrace when reporting a bug.

     Parts of the C non-blocking-read implementation were borrowed from Matt
     Sergeant's PPerl project.

AUTHOR
     Stas Bekman <st...@stason.org>

SEE ALSO
     perl(3), "Debug::DumpCore(3)".



__________________________________________________________________
Stas Bekman            JAm_pH ------> Just Another mod_perl Hacker
http://stason.org/     mod_perl Guide ---> http://perl.apache.org
mailto:stas@stason.org http://use.perl.org http://apacheweek.com
http://modperlbook.org http://apache.org   http://ticketmaster.com