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NAME ^

Porting Apache:: Perl Modules from mod_perl 1.0 to 2.0

Description ^

This document describes the various options for porting a mod_perl 1.0 Apache module so that it runs on a Apache 2.0 / mod_perl 2.0 server. It's also helpful to those who start developing mod_perl 2.0 handlers.

Developers who need to port modules using XS code, should also read about porting Apache:: XS modules.

There is also: Porting CPAN modules to mod_perl 2.0 Status.

Introduction ^

In the vast majority of cases, a perl Apache module that runs under mod_perl 1.0 will not run under mod_perl 2.0 without at least some degree of modification.

Even a very simple module that does not in itself need any changes will at least need the mod_perl 2.0 Apache modules loaded, because in mod_perl 2.0 basic functionality, such as access to the request object and returning an HTTP status, is not found where, or implemented how it used to be in mod_perl 1.0.

Most real-life modules will in fact need to deal with the following changes:

Do not be alarmed! One way to deal with all of these issues is to load the Apache2::compat compatibility layer bundled with mod_perl 2.0. This magic spell will make almost any 1.0 module run under 2.0 without further changes. It is by no means the solution for every case, however, so please read carefully the following discussion of this and other options.

There are three basic options for porting. Let's take a quick look at each one and then discuss each in more detail.

1 Run the module on 2.0 under Apache2::compat with no further changes

As we have said mod_perl 2.0 ships with a module, Apache2::compat, that provides a complete drop-in compatibility layer for 1.0 modules. Apache2::compat does the following:

  • Loads all the mod_perl 2.0 Apache2:: modules
  • Adjusts method calls where the prototype has changed
  • Provides Perl implementation for methods that no longer exist in 2.0

The drawback to using Apache2::compat is the performance hit, which can be significant.

Authors of CPAN and other publicly distributed modules should not use Apache2::compat since this forces its use in environments where the administrator may have chosen to optimize memory use by making all code run natively under 2.0.

2 Modify the module to run only under 2.0

If you are not interested in providing backwards compatibility with mod_perl 1.0, or if you plan to leave your 1.0 module in place and develop a new version compatible with 2.0, you will need to make changes to your code. How significant or widespread the changes are depends largely of course on your existing code.

Several sections of this document provide detailed information on how to rewrite your code for mod_perl 2.0 Several tools are provided to help you, and it should be a relatively painless task and one that you only have to do once.

3 Modify the module so that it runs under both 1.0 and 2.0

You need to do this if you want to keep the same version number for your module, or if you distribute your module on CPAN and want to maintain and release just one codebase.

This is a relatively simple enhancement of option (2) above. The module tests to see which version of mod_perl is in use and then executes the appropriate method call.

The following sections provide more detailed information and instructions for each of these three porting strategies.

Using Apache2::porting ^

META: to be written. this is a new package which makes chunks of this doc simpler. for now see the Apache2::porting manpage.

Using the Apache2::compat Layer ^

The Apache2::compat module tries to hide the changes in API prototypes between version 1.0 and 2.0 of mod_perl, and implements "virtual methods" for the methods and functions that actually no longer exist.

Apache2::compat is extremely easy to use. Either add at the very beginning of startup.pl:

  use Apache2::compat;

or add to httpd.conf:

  PerlModule Apache2::compat

That's all there is to it. Now you can run your 1.0 module unchanged.

Remember, however, that using Apache2::compat will make your module run slower. It can create a larger memory footprint than you need and it implements functionality in pure Perl that is provided in much faster XS in mod_perl 1.0 as well as in 2.0. This module was really designed to assist in the transition from 1.0 to 2.0. Generally you will be better off if you port your code to use the mod_perl 2.0 API.

It's also especially important to repeat that CPAN module developers are requested not to use this module in their code, since this takes the control over performance away from users.

Porting a Perl Module to Run under mod_perl 2.0 ^

Note: API changes are listed in the mod_perl 1.0 backward compatibility document.

The following sections will guide you through the steps of porting your modules to mod_perl 2.0.

Using ModPerl::MethodLookup to Discover Which mod_perl 2.0 Modules Need to Be Loaded

It would certainly be nice to have our mod_perl 1.0 code run on the mod_perl 2.0 server unmodified. So first of all, try your luck and test the code.

It's almost certain that your code won't work when you try, however, because mod_perl 2.0 splits functionality across many more modules than version 1.0 did, and you have to load these modules before the methods that live in them can be used. So the first step is to figure out which these modules are and use() them.

The ModPerl::MethodLookup module provided with mod_perl 2.0 allows you to find out which module contains the functionality you are looking for. Simply provide it with the name of the mod_perl 1.0 method that has moved to a new module, and it will tell you what the module is.

For example, let's say we have a mod_perl 1.0 code snippet:

  $r->content_type('text/plain');
  $r->print("Hello cruel world!");

If we run this, mod_perl 2.0 will complain that the method content_type() can't be found. So we use ModPerl::MethodLookup to figure out which module provides this method. We can just run this from the command line:

  % perl -MModPerl::MethodLookup -e print_method content_type

This prints:

  to use method 'content_type' add:
           use Apache2::RequestRec ();

We do what it says and add this use() statement to our code, restart our server (unless we're using Apache2::Reload), and mod_perl will no longer complain about this particular method.

Since you may need to use this technique quite often you may want to define an alias. Once defined the last command line lookup can be accomplished with:

  % lookup content_type

ModPerl::MethodLookup also provides helper functions for finding which methods are defined in a given module, or which methods can be invoked on a given object.

Handling Methods Existing In More Than One Package

Some methods exists in several classes. For example this is the case with the print() method. We know the drill:

  % lookup print

This prints:

  There is more than one class with method 'print'
  try one of:
        use Apache2::RequestIO ();
        use Apache2::Filter ();

So there is more than one package that has this method. Since we know that we call the print() method with the $r object, it must be the Apache2::RequestIO module that we are after. Indeed, loading this module solves the problem.

Using ModPerl::MethodLookup Programmatically

The issue of picking the right module, when more than one matches, can be resolved when using ModPerl::MethodLookup programmatically -- lookup_method accepts an object as an optional second argument, which is used if there is more than one module that contains the method in question. ModPerl::MethodLookup knows that Apache2::RequestIO and and Apache2::Filter expect an object of type Apache2::RequestRec and type Apache2::Filter respectively. So in a program running under mod_perl we can call:

  ModPerl::MethodLookup::lookup_method('print', $r);

Now only one module will be matched.

This functionality can be used in AUTOLOAD, for example, although most users will not have a need for this robust of solution.

Pre-loading All mod_perl 2.0 Modules

Now if you use a wide range of methods and functions from the mod_perl 1.0 API, the process of finding all the modules that need to be loaded can be quite frustrating. In this case you may find the function preload_all_modules() to be the right tool for you. This function preloads all mod_perl 2.0 modules, implementing their API in XS.

While useful for testing and development, it is not recommended to use this function in production systems. Before going into production you should remove the call to this function and load only the modules that are used, in order to save memory.

CPAN module developers should not be tempted to call this function from their modules, because it prevents the user of their module from optimizing her system's memory usage.

Handling Missing and Modified mod_perl 1.0 Methods and Functions

The mod_perl 2.0 API is modeled even more closely upon the Apache API than was mod_perl version 1.0. Just as the Apache 2.0 API is substantially different from that of Apache 1.0, therefore, the mod_perl 2.0 API is quite different from that of mod_perl 1.0. Unfortunately, this means that certain method calls and functions that were present in mod_perl version 1.0 are missing or modified in mod_perl 2.0.

If mod_perl 2.0 tells you that some method is missing and it can't be found using ModPerl::MethodLookup, it's most likely because the method doesn't exist in the mod_perl 2.0 API. It's also possible that the method does still exist, but nevertheless it doesn't work, since its usage has changed (e.g. its prototype has changed, or it requires different arguments, etc.).

In either of these cases, refer to the backwards compatibility document for an exhaustive list of API calls that have been modified or removed.

Methods that No Longer Exist

Some methods that existed in mod_perl 1.0 simply do not exist anywhere in version 2.0 and you must therefore call a different method o methods to get the functionality you want.

For example, suppose we have a mod_perl 1.0 code snippet:

  $r->log_reason("Couldn't open the session file: $@");

If we try to run this under mod_perl 2.0 it will complain about the call to log_reason(). But when we use ModPerl::MethodLookup to see which module to load in order to call that method, nothing is found:

  % perl -MModPerl::MethodLookup -le \
    'print((ModPerl::MethodLookup::lookup_method(shift))[0])' \
    log_reason

This prints:

  don't know anything about method 'log_reason'

Looks like we are calling a non-existent method! Our next step is to refer to the backwards compatibility document, wherein we find that as we suspected, the method log_reason() no longer exists, and that instead we should use the other standard logging functions provided by the Apache2::Log module.

Methods Whose Usage Has Been Modified

Some methods still exist, but their usage has been modified, and your code must call them in the new fashion or it will generate an error. Most often the method call requires new or different arguments.

For example, say our mod_perl 1.0 code said:

  $parsed_uri = Apache2::URI->parse($r, $r->uri);

This code causes mod_perl 2.0 to complain first about not being able to load the method parse() via the package Apache2::URI. We use the tools described above to discover that the package containing our method has moved and change our code to load and use APR::URI:

  $parsed_uri = APR::URI->parse($r, $r->uri);

But we still get an error. It's a little cryptic, but it gets the point across:

  p is not of type APR::Pool at /path/to/OurModule.pm line 9.

What this is telling us is that the method parse requires an APR::Pool object as its first argument. (Some methods whose usage has changed emit more helpful error messages prefixed with "Usage: ...") So we change our code to:

  $parsed_uri = APR::URI->parse($r->pool, $r->uri);

and all is well in the world again.

Requiring a specific mod_perl version.

To require a module to run only under 2.0, simply add:

  use mod_perl 2.0;

META: In fact, before 2.0 is released you really have to say:

  use mod_perl 1.99;

And you can even require a specific version (for example when a certain API has been added only starting from that version). For example to require version 1.99_08, you can say:

  use mod_perl 1.9908;

Should the Module Name Be Changed?

If it is not possible to make your code run under both mod_perl versions (see below), you will have to maintain two separate versions of your own code. While you can change the name of the module for the new version, it's best to try to preserve the name and use some workarounds.

META: need to discuss this more.

Using Apache2::compat As a Tutorial

Even if you have followed the recommendation and eschewed use of the Apache2::compat module, you may find it useful to learn how the API has been changed and how to modify your own code. Simply look at the Apache2::compat source code and see how the functionality should be implemented in mod_perl 2.0.

For example, mod_perl 2.0 doesn't provide the Apache->gensym method. As we can see if we look at the Apache2/compat.pm source, the functionality is now available via the core Perl module Symbol and its gensym() function. (Since mod_perl 2.0 works only with Perl versions 5.6 and higher, and Symbol.pm is included in the core Perl distribution since version 5.6.0, there was no reason to keep providing Apache->gensym.)

So if the original code looked like:

  my $fh = Apache->gensym;
  open $fh, $file or die "Can't open $file: $!";

in order to port it mod_perl 2.0 we can write:

  my $fh = Symbol::gensym;
  open $fh, $file or die "Can't open $file: $!";

Or we can even skip loading Symbol.pm, since under Perl version 5.6 and higher we can just do:

  open my $fh, $file or die "Can't open $file: $!";

How Apache::MP3 was Ported to mod_perl 2.0

Apache::MP3 is an elaborate application that uses a lot of mod_perl API. After porting it, I have realized that if you go through the notes or even better try to do it by yourself, referring to the notes only when in trouble, you will most likely be able to port any other mod_perl 1.0 module to run under mod_perl 2.0. So here the log of what I have done while doing the porting.

Please notice that this tutorial should be considered as-is and I'm not claiming that I have got everything polished, so if you still find problems, that's absolutely OK. What's important is to try to learn from the process, so you can attack other modules on your own.

I've started to work with Apache::MP3 version 3.03 which you can retrieve from Lincoln's CPAN directory: http://search.cpan.org/CPAN/authors/id/L/LD/LDS/Apache-MP3-3.03.tar.gz Even though by the time you'll read this there will be newer versions available it's important that you use the same version as a starting point, since if you don't, the notes below won't make much sense.

Preparations

First of all, I scratched most of mine httpd.conf and startup.pl leaving the bare minimum to get mod_perl started. This is needed to ensure that once I've completed the porting, the module will work correct on other users systems. For example if my httpd.conf and startup.pl were loading some other modules, which in turn may load modules that a to-be-ported module may rely on, the ported module may work for me, but once released, it may not work for others. It's the best to create a new httpd.conf when doing the porting putting only the required bits of configuration into it.

httpd.conf

Next, I configure the Apache2::Reload module, so we don't have to constantly restart the server after we modify Apache::MP3. In order to do that add to httpd.conf:

  PerlModule Apache2::Reload
  PerlInitHandler Apache2::Reload
  PerlSetVar ReloadAll Off
  PerlSetVar ReloadModules "ModPerl::* Apache2::*"
  PerlSetVar ReloadConstantRedefineWarnings Off

You can refer to the Apache2::Reload manpage for more information if you aren't familiar with this module. The part:

  PerlSetVar ReloadAll Off
  PerlSetVar ReloadModules "ModPerl::* Apache::*"

tells Apache2::Reload to monitor only modules in the ModPerl:: and Apache:: namespaces. So Apache::MP3 will be monitored. If your module is named Foo::Bar, make sure to include the right pattern for the ReloadModules directive. Alternatively simply have:

  PerlSetVar ReloadAll On

which will monitor all modules in %INC, but will be a bit slower, as it'll have to stat(3) many more modules on each request.

Finally, Apache::MP3 uses constant subroutines. Because of that you will get lots of warnings every time the module is modified, which I wanted to avoid. I can safely shut those warnings off, since I'm not going to change those constants. Therefore I've used the setting

  PerlSetVar ReloadConstantRedefineWarnings Off

If you do change those constants, refer to the section on ReloadConstantRedefineWarnings .

Next I configured Apache::MP3. In my case I've followed the Apache::MP3 documentation, created a directory mp3/ under the server document root and added the corresponding directives to httpd.conf.

Now my httpd.conf looked like this:

  #file:httpd.conf
  #---------------
  Listen 127.0.0.1:8002
  #... standard Apache configuration bits omitted ...
  
  LoadModule perl_module modules/mod_perl.so
  
  PerlSwitches -wT
  
  PerlRequire "/home/httpd/2.0/perl/startup.pl"
  
  PerlModule Apache2::Reload
  PerlInitHandler Apache2::Reload
  PerlSetVar ReloadAll Off
  PerlSetVar ReloadModules "ModPerl::* Apache::*"
  PerlSetVar ReloadConstantRedefineWarnings Off
  
  AddType audio/mpeg     mp3 MP3
  AddType audio/playlist m3u M3U
  AddType audio/x-scpls  pls PLS
  AddType application/x-ogg ogg OGG
  <Location /mp3>
    SetHandler perl-script
    PerlResponseHandler Apache::MP3
    PerlSetVar PlaylistImage playlist.gif
    PerlSetVar StreamBase http://localhost:8002
    PerlSetVar BaseDir /mp3
  </Location>

startup.pl

Since chances are that no mod_perl 1.0 module will work out of box without at least preloading some modules, I've enabled the Apache2::compat module. Now my startup.pl looked like this:

  #file:startup.pl
  #---------------
  use lib qw(/home/httpd/2.0/perl);
  use Apache2::compat;

Apache/MP3.pm

Before I even started porting Apache::MP3, I've added the warnings pragma to Apache/MP3.pm (which wasn't there because mod_perl 1.0 had to work with Perl versions prior to 5.6.0, which is when the warnings pragma was added):

  #file:apache_mp3_prep.diff
  --- Apache/MP3.pm.orig 2003-06-03 18:44:21.000000000 +1000
  +++ Apache/MP3.pm      2003-06-03 18:44:47.000000000 +1000
  @@ -4,2 +4,5 @@
   use strict;
  +use warnings;
  +no warnings 'redefine';  # XXX: remove when done with porting
  +

From now on, I'm going to use unified diffs which you can apply using patch(1). Though you may have to refer to its manpage on your platform since the usage flags may vary. On linux I'd apply the above patch as:

  % cd ~/perl/blead-ithread/lib/site_perl/5.9.0/
  % patch -p0 < apache_mp3_prep.diff

(note: I've produced the above patch and one more below with diff -u1, to avoid the RCS Id tag geting into this document. Normally I produce diffs with diff -u which uses the default context of 3.)

assuming that Apache/MP3.pm is located in the directory ~/perl/blead-ithread/lib/site_perl/5.9.0/.

I've enabled the warnings pragma even though I did have warnings turned globally in httpd.conf with:

  PerlSwitches -wT

it's possible that some badly written module has done:

  $^W = 0;

without localizing the change, affecting other code. Also notice that the taint mode was enabled from httpd.conf, something that you shouldn't forget to do.

I have also told the warnings pragma not to complain about redefined subs via:

  no warnings 'redefine';  # XXX: remove when done with porting

I will remove that code, once porting is completed.

At this point I was ready to start the porting process and I have started the server.

  % hup2

I'm using the following aliases to save typing:

  alias err2      "tail -f ~/httpd/prefork/logs/error_log"
  alias acc2      "tail -f ~/httpd/prefork/logs/access_log"
  alias stop2     "~/httpd/prefork/bin/apachectl stop"
  alias start2    "~/httpd/prefork/bin/apachectl start"
  alias restart2  "~/httpd/prefork/bin/apachectl restart"
  alias graceful2 "~/httpd/prefork/bin/apachectl graceful"
  alias hup2 "stop2; sleep 3; start2; err2"

(I also have a similar set of aliases for mod_perl 1.0)

Porting with Apache2::compat

I have configured my server to listen on port 8002, so I issue a request http://localhost:8002/mp3/ in one console:

  % lynx --dump http://localhost:8002/mp3/

keeping the error_log open in the other:

  % err2

which expands to:

  % tail -f ~/httpd/prefork/logs/error_log

When the request is issued, the error_log file tells me:

  [Thu Jun 05 15:29:45 2003] [error] [client 127.0.0.1] 
  Usage: Apache2::RequestRec::new(classname, c, base_pool=NULL) 
  at .../Apache/MP3.pm line 60.

Looking at the code:

  58: sub handler ($$) {
  59:   my $class = shift;
  60:   my $obj = $class->new(@_) or die "Can't create object: $!";

The problem is that handler wasn't invoked as method, but had $r passed to it (we can tell because new() was invoked as Apache2::RequestRec::new(), whereas it should have been Apache::MP3::new(). Why Apache::MP3 wasn't passed as the first argument? I go to the mod_perl 1.0 backward compatibility document and find that method handlers are now marked using the method subroutine attribute. So I modify the code:

  --- Apache/MP3.pm.0     2003-06-05 15:29:19.000000000 +1000
  +++ Apache/MP3.pm       2003-06-05 15:38:41.000000000 +1000
  @@ -55,7 +55,7 @@
   my $NO  = '^(no|false)$';  # regular expression
   my $YES = '^(yes|true)$';  # regular expression
  
  -sub handler ($$) {
  +sub handler : method {
     my $class = shift;
     my $obj = $class->new(@_) or die "Can't create object: $!";
     return $obj->run();

and issue the request again (no server restart needed).

This time we get a bunch of looping redirect responses, due to a bug in mod_dir which kicks in to handle the existing dir and messing up with $r->path_info keeping it empty at all times. I thought I could work around this by not having the same directory and location setting, e.g. by moving the location to be /songs/ while keeping the physical directory with mp3 files as $DocumentRoot/mp3/, but Apache::MP3 won't let you do that. So a solution suggested by Justin Erenkrantz is to simply shortcut that piece of code with:

  --- Apache/MP3.pm.1     2003-06-06 14:50:59.000000000 +1000
  +++ Apache/MP3.pm       2003-06-06 14:51:11.000000000 +1000
  @@ -253,7 +253,7 @@
     my $self = shift;
     my $dir = shift;
  
  -  unless ($self->r->path_info){
  +  unless ($self->r->path_info eq ''){
       #Issue an external redirect if the dir isn't tailed with a '/'
       my $uri = $self->r->uri;
       my $query = $self->r->args;

which is equivalent to removing this code, until the bug is fixed (it was still there as of Apache 2.0.46). But the module still works without this code, because if you issue a request to /mp3 (w/o trailing slash) mod_dir, will do the redirect for you, replacing the code that we just removed. In any case this got me past this problem.

Since I have turned on the warnings pragma now I was getting loads of uninitialized value warnings from $r->dir_config() whose return value were used without checking whether they are defined or not. But you'd get them with mod_perl 1.0 as well, so they are just an example of not-so clean code, not really a relevant obstacle in my pursuit to port this module to mod_perl 2.0. Unfortunately they were cluttering the log file so I had to fix them. I've defined several convenience functions:

  sub get_config {
      my $val = shift->r->dir_config(shift);
      return defined $val ? $val : '';
  }
  
  sub config_yes { shift->get_config(shift) !~ /$YES/oi; }
  sub config_no  { shift->get_config(shift) !~ /$NO/oi; }

and replaced them as you can see in this patch: code/apache_mp3_2.diff, it was 194 lines long so I didn't inline it here, but it was quick to create with a few regexes search-n-replace manipulations in xemacs.

Now I have the browsing of the root /mp3/ directory and its sub-directories working. If I click on 'Fetch' of a particular song it works too. However if I try to 'Stream' a song, I get a 500 response with error_log telling me:

  [Fri Jun 06 15:33:33 2003] [error] [client 127.0.0.1] Bad arg length
  for Socket::unpack_sockaddr_in, length is 31, should be 16 at
  .../5.9.0/i686-linux-thread-multi/Socket.pm line 370.

It would be certainly nice for Socket.pm to use Carp::carp() instead of warn() so we will know where in the Apache::MP3 code this problem was triggered. However reading the Socket.pm manpage reveals that sockaddr_in() in the list context is the same as calling an explicit unpack_sockaddr_in(), and in the scalar context it's calling pack_sockaddr_in(). So I have found sockaddr_in was the only Socket.pm function used in Apache::MP3 and I have found this code in the function is_local():

  my $r = $self->r;
  my ($serverport,$serveraddr) = sockaddr_in($r->connection->local_addr);
  my ($remoteport,$remoteaddr) = sockaddr_in($r->connection->remote_addr);
  return $serveraddr eq $remoteaddr;

Since something is wrong with function calls $r->connection->local_addr and/or $r->connection->remote_addr and I referred to the mod_perl 1.0 backward compatibility document and found the relevant entry on these two functions. Indeed the API have changed. Instead of returning a packed SOCKADDR_IN string, Apache now returns an APR::SockAddr object, which I can query to get the bits of information I'm interested in. So I applied this patch:

  --- Apache/MP3.pm.3     2003-06-06 15:36:15.000000000 +1000
  +++ Apache/MP3.pm       2003-06-06 15:56:32.000000000 +1000
  @@ -1533,10 +1533,9 @@
   # allows the player to fast forward, pause, etc.
   sub is_local {
     my $self = shift;
  -  my $r = $self->r;
  -  my ($serverport,$serveraddr) = sockaddr_in($r->connection->local_addr);
  -  my ($remoteport,$remoteaddr) = sockaddr_in($r->connection->remote_addr);
  -  return $serveraddr eq $remoteaddr;
  +  my $c = $self->r->connection;
  +  require APR::SockAddr;
  +  return $c->local_addr->ip_get eq $c->remote_addr->ip_get;
   }
  
   # Check if the requesting client is on the local network, as defined by

And voila, the streaming option now works. I get a warning on 'Use of uninitialized value' on line 1516 though, but again this is unrelated to the porting issues, just a flow logic problem, which wasn't triggered without the warnings mode turned on. I have fixed it with:

  --- Apache/MP3.pm.4     2003-06-06 15:57:15.000000000 +1000
  +++ Apache/MP3.pm       2003-06-06 16:04:48.000
  @@ -1492,7 +1492,7 @@
     my $suppress_auth = shift;
     my $r = $self->r;
  
  -  my $auth_info;
  +  my $auth_info = '';
     # the check for auth_name() prevents an anno
     # the apache server log when authentication
     if ($r->auth_name && !$suppress_auth) {
  @@ -1509,10 +1509,9 @@
     }
  
     my $vhost = $r->hostname;
  -  unless ($vhost) {
  -    $vhost = $r->server->server_hostname;
  -    $vhost .= ':' . $r->get_server_port unless
  -  }
  +  $vhost = $r->server->server_hostname unless
  +  $vhost .= ':' . $r->get_server_port unless $
  +
     return "http://${auth_info}${vhost}";
   }

This completes the first part of the porting. I have tried to use all the visible functions of the interface and everything seemed to work and I haven't got any warnings logged. Certainly I may have missed some usage patterns which may be still problematic. But this is good enough for this tutorial.

Getting Rid of the Apache2::compat Dependency

The final stage is going to get rid of Apache2::compat since this is a CPAN module, which must not load Apache2::compat on its own. I'm going to make Apache::MP3 work with mod_perl 2.0 all by itself.

The first step is to comment out the loading of Apache2::compat in startup.pl:

  #file:startup.pl
  #---------------
  use lib qw(/home/httpd/2.0/perl);
  #use Apache2::compat ();

Ensuring that Apache2::compat is not loaded

The second step is to make sure that Apache2::compat doesn't get loaded indirectly, through some other module. So I've added this line of code to Apache/MP3.pm:

  --- Apache/MP3.pm.5     2003-06-06 16:17:50.000000000 +1000
  +++ Apache/MP3.pm       2003-06-06 16:21:14.000000000 +1000
  @@ -3,2 +3,6 @@
  
  +BEGIN {
  +    die "Apache2::compat is loaded loaded" if $INC{'Apache2/compat.pm'};
  +}
  +
   use strict;

and indeed, even though I've commented out the loading of Apache2::compat from startup.pl, this module was still getting loaded. I knew that because the request to /mp3 were failing with the error message:

  Apache2::compat is loaded loaded at ...

There are several ways to find the guilty party, you can grep(1) for it in the perl libraries, you can override CORE::GLOBAL::require() in startup.pl:

  BEGIN { 
    use Carp;
    *CORE::GLOBAL::require = sub { 
        Carp::cluck("Apache2::compat is loaded") if $_[0] =~ /compat/;
        CORE::require(@_);
    };
  }

or you can modify Apache2/compat.pm and make it print the calls trace when it gets compiled:

  --- Apache2/compat.pm.orig   2003-06-03 16:11:07.000000000 +1000
  +++ Apache2/compat.pm        2003-06-03 16:11:58.000000000 +1000
  @@ -1,5 +1,9 @@
   package Apache2::compat;
  
  +BEGIN {
  +    use Carp;
  +    Carp::cluck("Apache2::compat is loaded by");
  +}

I've used this last technique, since it's the safest one to use. Remember that Apache2::compat can also be loaded with:

    do "Apache2/compat.pm";

in which case, neither grep(1)'ping for Apache2::compat, nor overriding require() will do the job.

When I've restarted the server and tried to use Apache::MP3 (I wasn't preloading it at the server startup since I wanted the server to start normally and cope with problem when it's running), the error_log had an entry:

  Apache2::compat is loaded by at .../Apache2/compat.pm line 6
    Apache2::compat::BEGIN() called at .../Apache2/compat.pm line 8
    eval {...} called at .../Apache2/compat.pm line 8
    require Apache2/compat.pm called at .../5.9.0/CGI.pm line 169
    require CGI.pm called at .../site_perl/5.9.0/Apache/MP3.pm line 8
    Apache::MP3::BEGIN() called at .../Apache2/compat.pm line 8

(I've trimmed the whole paths of the libraries and the trace itself, to make it easier to understand.)

We could have used Carp::carp() which would have told us only the fact that Apache2::compat was loaded by CGI.pm, but by using Carp::cluck() we've obtained the whole stack backtrace so we also can learn which module has loaded CGI.pm.

Here I've learned that I had an old version of CGI.pm (2.89) which automatically loaded Apache2::compat (which should be never done by CPAN modules). Once I've upgraded CGI.pm to version 2.93 and restarted the server, Apache2::compat wasn't getting loaded any longer.

Installing the ModPerl::MethodLookup Helper

Now that Apache2::compat is not loaded, I need to deal with two issues: modules that need to be loaded and APIs that have changed.

For the second issue I'll have to refer to the the mod_perl 1.0 backward compatibility document.

But the first issue can be easily worked out using ModPerl::MethodLookup. As explained in the section Using ModPerl::MethodLookup Programmatically I've added the AUTOLOAD code to my startup.pl so it'll automatically lookup the packages that I need to load based on the request method and the object type.

So now my startup.pl looked like:

  #file:startup.pl
  #---------------
  use lib qw(/home/httpd/2.0/perl);
  
  {
    package ModPerl::MethodLookupAuto;
    use ModPerl::MethodLookup;
  
    use Carp;
    sub handler {
  
        # look inside mod_perl:: Apache2:: APR:: ModPerl:: excluding DESTROY
        my $skip = '^(?!DESTROY$';
        *UNIVERSAL::AUTOLOAD = sub {
            my $method = $AUTOLOAD;
            return if $method =~ /DESTROY/;
            my ($hint, @modules) =
                ModPerl::MethodLookup::lookup_method($method, @_);
            $hint ||= "Can't find method $AUTOLOAD";
            croak $hint;
        };
        return 0;
    }
  }
  1;

and I add to my httpd.conf:

  PerlChildInitHandler ModPerl::MethodLookupAuto

Adjusting the code to run under mod_perl 2

I restart the server and off I go to complete the second porting stage.

The first error that I've received was:

  [Fri Jun 06 16:28:32 2003] [error] failed to resolve handler `Apache::MP3'
  [Fri Jun 06 16:28:32 2003] [error] [client 127.0.0.1] Can't locate
  object method "boot" via package "mod_perl" at .../Apache/Constants.pm
  line 8. Compilation failed in require at .../Apache/MP3.pm line 12.

I go to line 12 and find the following code:

  use Apache::Constants qw(:common REDIRECT HTTP_NO_CONTENT
                           DIR_MAGIC_TYPE HTTP_NOT_MODIFIED);

Notice that I did have mod_perl 1.0 installed, so the Apache::Constant module from mod_perl 1.0 couldn't find the boot() method which doesn't exist in mod_perl 2.0. If you don't have mod_perl 1.0 installed the error would simply say, that it can't find Apache/Constants.pm in @INC. In any case, we are going to replace this code with mod_perl 2.0 equivalent:

  --- Apache/MP3.pm.6     2003-06-06 16:33:05.000000000 +1000
  +++ Apache/MP3.pm       2003-06-06 17:03:43.000000000 +1000
  @@ -9,7 +9,9 @@
   use warnings;
   no warnings 'redefine';  # XXX: remove when done with porting
  
  -use Apache::Constants qw(:common REDIRECT HTTP_NO_CONTENT DIR_MAGIC_TYPE HTTP_NOT_MODIFIED);
  +use Apache2::Const -compile => qw(:common REDIRECT HTTP_NO_CONTENT
  +                                 DIR_MAGIC_TYPE HTTP_NOT_MODIFIED);
  +
   use Apache::MP3::L10N;
   use IO::File;
   use Socket 'sockaddr_in';

and I also had to adjust the constants, since what used to be OK, now has to be Apache2::Const::OK, mainly because in mod_perl 2.0 there is an enormous amount of constants (coming from Apache and APR) and most of them are grouped in Apache2:: or APR:: namespaces. The Apache2::Const and APR::Const manpage provide more information on available constants.

This search and replace accomplished the job:

  % perl -pi -e 's/return\s(OK|DECLINED|FORBIDDEN| \
    REDIRECT|HTTP_NO_CONTENT|DIR_MAGIC_TYPE| \
    HTTP_NOT_MODIFIED)/return Apache2::$1/xg' Apache/MP3.pm

As you can see the regex explicitly lists all constants that were used in Apache::MP3. Your situation may vary. Here is the patch: code/apache_mp3_7.diff.

I had to manually fix the DIR_MAGIC_TYPE constant which didn't fit the regex pattern:

  --- Apache/MP3.pm.8     2003-06-06 17:24:33.000000000 +1000
  +++ Apache/MP3.pm       2003-06-06 17:26:29.000000000 +1000
  @@ -1055,7 +1055,7 @@
  
       my $mime = $self->r->lookup_file("$dir/$d")->content_type;
  
  -    push(@directories,$d) if !$seen{$d}++ && $mime eq DIR_MAGIC_TYPE;
  +    push(@directories,$d) if !$seen{$d}++ && $mime eq Apache2::Const::DIR_MAGIC_TYPE;
  
       # .m3u files should be configured as audio/playlist MIME types in your apache .conf file
       push(@playlists,$d) if $mime =~ m!^audio/(playlist|x-mpegurl|mpegurl|x-scpls)$!;

And I move on, the next error is:

  [Fri Jun 06 17:28:00 2003] [error] [client 127.0.0.1]
  Can't locate object method "header_in" via package 
  "Apache2::RequestRec" at .../Apache2/MP3.pm line 85.

The porting document quickly reveals me that header_in() and its brothers header_out() and err_header_out() are R.I.P. and that I have to use the corresponding functions headers_in(), headers_out() and err_headers_out() which are available in mod_perl 1.0 API as well.

So I adjust the code to use the new API:

  % perl -pi -e 's|header_in\((.*?)\)|headers_in->{$1}|g' Apache/MP3.pm
  % perl -pi -e 's|header_out\((.*?)\s*=>\s*(.*?)\);|headers_out->{$1} = $2;|g' Apache/MP3.pm

which results in this patch: code/apache_mp3_9.diff.

On the next error ModPerl::MethodLookup's AUTOLOAD kicks in. Instead of complaining:

  [Fri Jun 06 18:35:53 2003] [error] [client 127.0.0.1] 
  Can't locate object method "FETCH" via package "APR::Table" 
  at .../Apache/MP3.pm line 85.

I now get:

  [Fri Jun 06 18:36:35 2003] [error] [client 127.0.0.1] 
  to use method 'FETCH' add:
        use APR::Table ();
  at .../Apache/MP3.pm line 85

So I follow the suggestion and load APR::Table():

  --- Apache/MP3.pm.10    2003-06-06 17:57:54.000000000 +1000
  +++ Apache/MP3.pm       2003-06-06 18:37:33.000000000 +1000
  @@ -9,6 +9,8 @@
   use warnings;
   no warnings 'redefine';  # XXX: remove when done with porting
  
  +use APR::Table ();
  +
   use Apache2::Const -compile => qw(:common REDIRECT HTTP_NO_CONTENT
                                    DIR_MAGIC_TYPE HTTP_NOT_MODIFIED);

I continue issuing the request and adding the missing modules again and again till I get no more complaints. During this process I've added the following modules:

  --- Apache/MP3.pm.11    2003-06-06 18:38:47.000000000 +1000
  +++ Apache/MP3.pm       2003-06-06 18:39:10.000000000 +1000
  @@ -9,6 +9,14 @@
   use warnings;
   no warnings 'redefine';  # XXX: remove when done with porting
  
  +use Apache2::Connection ();
  +use Apache2::SubRequest ();
  +use Apache2::Access ();
  +use Apache2::RequestIO ();
  +use Apache2::RequestUtil ();
  +use Apache2::RequestRec ();
  +use Apache2::ServerUtil ();
  +use Apache2::Log;
   use APR::Table ();
  
   use Apache2::Const -compile => qw(:common REDIRECT HTTP_NO_CONTENT

The AUTOLOAD code helped me to trace the modules that contain the existing APIs, however I still have to deal with APIs that no longer exist. Rightfully the helper code says that it doesn't know which module defines the method: send_http_header() because it no longer exists in Apache 2.0 vocabulary:

  [Fri Jun 06 18:40:34 2003] [error] [client 127.0.0.1] 
  Don't know anything about method 'send_http_header'
  at .../Apache/MP3.pm line 498

So I go back to the porting document and find the relevant entry. In 2.0 lingo, we just need to set the content_type():

  --- Apache/MP3.pm.12    2003-06-06 18:43:42.000000000 +1000
  +++ Apache/MP3.pm       2003-06-06 18:51:23.000000000 +1000
  @@ -138,7 +138,7 @@
   sub help_screen {
     my $self = shift;
  
  -  $self->r->send_http_header( $self->html_content_type );
  +  $self->r->content_type( $self->html_content_type );
     return Apache2::Const::OK if $self->r->header_only;
  
     print start_html(
  @@ -336,7 +336,7 @@
     my $r = $self->r;
     my $base = $self->stream_base;
  
  -  $r->send_http_header('audio/mpegurl');
  +  $r->content_type('audio/mpegurl');
     return Apache2::Const::OK if $r->header_only;
  
     # local user
  @@ -495,7 +495,7 @@
     return Apache2::Const::DECLINED unless my ($directories,$mp3s,$playlists,$txtfiles)
       = $self->read_directory($dir);
  
  -  $self->r->send_http_header( $self->html_content_type );
  +  $self->r->content_type( $self->html_content_type );
     return Apache2::Const::OK if $self->r->header_only;
  
     $self->page_top($dir);

also I've noticed that there was this code:

  return Apache2::Const::OK if $self->r->header_only;

This technique is no longer needed in 2.0, since Apache 2.0 automatically discards the body if the request is of type HEAD -- the handler should still deliver the whole body, which helps to calculate the content-length if this is relevant to play nicer with proxies. So you may decide not to make a special case for HEAD requests.

At this point I was able to browse the directories and play files via most options without relying on Apache2::compat.

There were a few other APIs that I had to fix in the same way, while trying to use the application, looking at the error_log referring to the porting document and applying the suggested fixes. I'll make sure to send all these fixes to Lincoln Stein, so the new versions will work correctly with mod_perl 2.0. I also had to fix other Apache::MP3:: files, which come as a part of the Apache-MP3 distribution, pretty much using the same techniques explained here. A few extra fixes of interest in Apache::MP3 were:

send_fd()

As of this writing we don't have this function in the core, because Apache 2.0 doesn't have it (it's in Apache2::compat but implemented in a slow way). However we may provide one in the future. Currently one can use the function sendfile() which requires a filename as an argument and not the file descriptor. So I have fixed the code:

  -    if($r->request($r->uri)->content_type eq 'audio/x-scpls'){
  -      open(FILE,$r->filename) || return 404;
  -      $r->send_fd(\*FILE);
  -      close(FILE);
  +
  +    if($r->content_type eq 'audio/x-scpls'){
  +      $r->sendfile($r->filename) || return Apache2::Const::NOT_FOUND;
log_reason

log_reason is now log_error:

  -  $self->r->log_reason('Invalid parameters -- possible attempt to circumvent checks.');
  +  $r->log_error('Invalid parameters -- possible attempt to circumvent checks.')
;

I have found the porting process to be quite interesting, especially since I have found several bugs in Apache 2.0 and documented a few undocumented API changes. It was also fun, because I've got to listen to mp3 files when I did things right, and was getting silence in my headphones and a visual irritation in the form of error_log messages when I didn't ;)

Porting a Module to Run under both mod_perl 2.0 and mod_perl 1.0 ^

Sometimes code needs to work with both mod_perl versions. For example this is the case with CPAN module developers who wish to continue to maintain a single code base, rather than supplying two separate implementations.

Making Code Conditional on Running mod_perl Version

In this case you can test for which version of mod_perl your code is running under and act appropriately.

To continue our example above, let's say we want to support opening a filehandle in both mod_perl 2.0 and mod_perl 1.0. Our code can make use of the environment variable $ENV{MOD_PERL_API_VERSION}

  use mod_perl;
  use constant MP2 => ( exists $ENV{MOD_PERL_API_VERSION} and 
                        $ENV{MOD_PERL_API_VERSION} >= 2 ); 
  # ...
  require Symbol if MP2;
  # ...
  
  my $fh = MP2 ? Symbol::gensym : Apache->gensym;
  open $fh, $file or die "Can't open $file: $!";

Some modules, like CGI.pm may work under mod_perl and without it, and will want to use the mod_perl 1.0 API if that's available, or mod_perl 2.0 API otherwise. So the following idiom could be used for this purpose.

  use constant MP_GEN => $ENV{MOD_PERL}
      ? { ( exists $ENV{MOD_PERL_API_VERSION} and 
            $ENV{MOD_PERL_API_VERSION} >= 2 ) ? 2 : 1 }
      : 0;

It sets the constant MP_GEN to 0 if mod_perl is not available, to 1 if running under mod_perl 1.0 and 2 for mod_perl 2.0.

Here's another way to find out the mod_perl version. In the server configuration file you can use a special configuration "define" symbol MODPERL2, which is magically enabled internally, as if the server had been started with -DMODPERL2.

  # in httpd.conf
  <IfDefine MODPERL2>
      # 2.0 configuration
  </IfDefine>
  <IfDefine !MODPERL2>
      # else
  </IfDefine>

From within Perl code this can be tested with Apache2::exists_config_define(). For example, we can use this method to decide whether or not to call $r->send_http_header(), which no longer exists in mod_perl 2.0:

  sub handler {
      my $r = shift;
      $r->content_type('text/html');
      $r->send_http_header() unless Apache2::exists_config_define("MODPERL2");
      ...
  }

Relevant links to other places in the porting documents:

Method Handlers

Method handlers in mod_perl are declared using the 'method' attribute. However if you want to have the same code base for mod_perl 1.0 and 2.0 applications, whose handler has to be a method, you will need to do the following trick:

  sub handler_mp1 ($$)     { ... }
  sub handler_mp2 : method { ... }
  *handler = MP2 ? \&handler_mp2 : \&handler_mp1;

Note that this requires at least Perl 5.6.0, the :method attribute is not supported by older Perl versions, which will fail to compile such code.

Here are two complete examples. The first example implements MyApache2::Method which has a single method that works for both mod_perl generations:

The configuration:

  PerlModule MyApache2::Method
  <Location /method>
      SetHandler perl-script
      PerlHandler MyApache2::Method->handler
  </Location>

The code:

  #file:MyApache2/Method.pm
  package MyApache2::Method;
  
  # PerlModule MyApache2::Method
  # <Location /method>
  #      SetHandler perl-script
  #      PerlHandler MyApache2::Method->handler
  #  </Location>
  
  use strict;
  use warnings;
  
  use mod_perl;
  use constant MP2 => ( exists $ENV{MOD_PERL_API_VERSION} and 
                        $ENV{MOD_PERL_API_VERSION >= 2 ); 
  
  BEGIN {
      if (MP2) {
          require Apache2::RequestRec;
          require Apache2::RequestIO;
          require Apache2::Const;
          Apache2::Const->import(-compile => 'OK');
      }
      else {
          require Apache;
          require Apache::Constants;
          Apache::Constants->import('OK');
      }
  }
  
  sub handler_mp1 ($$)     { &run }
  sub handler_mp2 : method { &run }
  *handler = MP2 ? \&handler_mp2 : \&handler_mp1;
  
  sub run {
     my ($class, $r) = @_;
     MP2 ? $r->content_type('text/plain')
         : $r->send_http_header('text/plain');
     print "$class was called\n";
     return MP2 ? Apache2::Const::OK : Apache::Constants::OK;
  }

Here are two complete examples. The second example implements MyApache2::Method2, which is very similar to MyApache2::Method, but uses separate methods for mod_perl 1.0 and 2.0 servers.

The configuration is the same:

  PerlModule MyApache2::Method2
  <Location /method2>
      SetHandler perl-script
      PerlHandler MyApache2::Method2->handler
  </Location>

The code:

  #file:MyApache2/Method2.pm
  package MyApache2::Method2;
  
  # PerlModule MyApache2::Method
  # <Location /method>
  #      SetHandler perl-script
  #      PerlHandler MyApache2::Method->handler
  #  </Location>
  
  use strict;
  use warnings;
  
  use mod_perl;
  use constant MP2 => ( exists $ENV{MOD_PERL_API_VERSION} and 
                        $ENV{MOD_PERL_API_VERSION >= 2 ); 
  
  BEGIN {
      warn "running $ENV{MOD_PERL_API_VERSION}\n";
      if (MP2) {
          require Apache2::RequestRec;
          require Apache2::RequestIO;
          require Apache2::Const;
          Apache2::Const->import(-compile => 'OK');
      }
      else {
          require Apache;
          require Apache::Constants;
          Apache::Constants->import('OK');
      }
  }
  
  sub handler_mp1 ($$)     { &mp1 }
  sub handler_mp2 : method { &mp2 }
  
  *handler = MP2 ? \&handler_mp2 : \&handler_mp1;
  
  sub mp1 {
     my ($class, $r) = @_;
     $r->send_http_header('text/plain');
     $r->print("mp1: $class was called\n");
     return Apache::Constants::OK();
  }
  
  sub mp2 {
      my ($class, $r) = @_;
      $r->content_type('text/plain');
      $r->print("mp2: $class was called\n");
      return Apache2::Const::OK();
  }

Assuming that mod_perl 1.0 is listening on port 8001 and mod_perl 2.0 on 8002, we get the following results:

  % lynx --source http://localhost:8001/method
  MyApache2::Method was called

  % lynx --source http://localhost:8001/method2
  mp1: MyApache2::Method2 was called

  % lynx --source http://localhost:8002/method
  MyApache2::Method was called

  % lynx --source http://localhost:8002/method2
  mp2: MyApache2::Method2 was called

The Conflict of mp1 vs mp2 vs mp22 vs ... vs mpNN ^

META: should something be said here?

Distributors

Distributors should mark the different generations of mod_perl core as conflicting, so only one version can be installed using the binary package. Users requiring more than one installation should do a manual install.

In order to have any of the 3rd party modperl modules installed users need to have the correct modperl package installed. So there is no need to mark the 3rd party modules as conflicting, since their most important prerequisite (the modperl-core) is already handling that.

Of course packagers can decide to make the two generation packages as non-conflicting, by building all mp2 core and 3rd party modules into Apache2/ subdir, in which case the two will always co-exist. But this is not the most logical approach since 99% of users will want only one generation of mod_perl core and 3rd party modules.

Maintainers ^

Maintainer is the person(s) you should contact with updates, corrections and patches.

Stas Bekman [http://stason.org/]

Authors ^

Only the major authors are listed above. For contributors see the Changes file.

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