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 <[email protected]>

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

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