Code Snippets

Many error conditions result in an exception (or signal -- same thing) which is raised in order to tell the operating system that a condition has arisen which needs more urgent attention than can be given by other means. One of the most familiar ways of raising a signal is to hit Ctrl-C on your terminal's keyboard. The signal interrupts the processor. In the case of Ctrl-C the INT signal is generated and the interrupt is usually trapped by a default signal handler supplied by OS, which causes the operating system to stop the process currently attached to the terminal.

Under mod_perl, a Perl runtime error causes an exception. By default this exception is trapped by default mod_perl handler. The handler logs information about the error (such as the date and time that the error occurred) to error_log. If you want to redirect this information to the client instead of to error_log you have to take the responsibility yourself, by writing your own exception handler to implement this behaviour. See the section ``Exception Handling for mod_perl'' for more information.

The code examples below can be useful with your own exception handlers as well as with the default handlers.

META: Integrate the 2 sections

The CGI::Carp package implements handlers for signals. To trap (almost) all Perl run-time errors and send the output to the client instead of to Apache's error_log add this line to your script:

use CGI::Carp qw(fatalsToBrowser);

Refer to the CGI::Carp man page for more detailed information.

You can trap individual exceptions: for example you can write custom __DIE__ and __WARN__ signal handlers. The special %SIG hash contains references to signal handlers. The signal handler is just a subroutine, in the example below it is called ``mydie''. To install the handler we assign a reference to our handler to the appropriate element of the %SIG hash. This causes the signal handler to call mydie(error_message) whenever the die() sub is called as a result of something which happened when our script was executing.

Do not forget the local keyword! If you do, then after the signal handler has been loaded it will be called whenever die() is called by any script executed by the same process. Probably that's not what you want. If it is, you can put the assignment statement in any module, as long as it will be executed at the right time.

Here is an example of a handler which I wrote because I wanted users to know that there was an error, without displaying the error message, but instead email it to me. If the error is caused by user (e.g. uploading image whose size is bigger than the limit I had set) I want to tell them about it. I wrote this handler for the mod_perl environment, but it works correctly when called from the shell. The code shown below is a stripped-down version with additional comments.

The following code must be added to the script:

# Using the local() keyword restricts the scope of the directive to # the block in which it is found, so this line must be added at the # right place in the right script. It will not affect other blocks # unless the local() keyword is removed. Usually you will want the # directive to affect the entire script, so you just place it near # the beginning of the file, where the innermost enclosing block is # the file itself. local $SIG{__DIE__} = \&mydie;

# The line above assumes that the subroutine "mydie" is in the same script. # Alternatively you can create a separate module for the error handler. # If you put the signal handler in a separate module, e.g. Error.pm, # you must explicitly give the package name to set the handler in your # script, using a line like this instead of the one above: local $SIG{__DIE__} = \&Error::mydie; # again within the script! # Do not forget the C<local()>, unless you want this signal handler to # be invoked every time any scripts dies (including events where this # treatment may be undesirable).

my $max_image_size = 100*1024; # 100k my $admin_email = 'foo@example.com';

# and the handler itself # Here is the handler itself: # The handler is called with a text message in a scalar argument sub mydie{ my $why = shift; chomp $why; my $orig_why = $why; # an ASCII copy for email report # handle the shell execution case (so we will not get all the HTML) print("Error: $why\n"), exit unless $ENV{MOD_PERL}; my $should_email = 0; my $message = ''; $why =~ s/[<&>]/"&#".ord($&).";"/ge; # entity escape # Now we need to trap various kinds of errors that come from CGI.pm # We don't want these errors to be emailed to us, since # they aren't programmatical errors if ($orig_why =~ /Client attempted to POST (\d+) bytes/o) { $message = qq{ You cannot POST messages bigger than @{[1024*$max_image_size]} bytes.<BR> You have tried to post $1 bytes<BR> If you are trying to upload an image, make sure its size is no bigger than @{[1024*$max_image_size]} bytes.<P> Thank you! }; } elsif ($orig_why =~ /Malformed multipart POST/o) { $message = qq{ Have you tried to upload an image in the wrong way?<P> To sucessfully upload an image you must use a browser that supports image upload and use the 'Browse' button to select that image. DO NOT type the path to the image into the upload field.<P> Thank you! }; } elsif ($orig_why =~ /closed socket during multipart read/o) { $message = qq{ Have you pressed a 'STOP' button?<BR> Please try again!<P> Thank you! };

} else { $message = qq{ <B>You need take no action since the error report has already been sent to the webmaster. <BR><P> <B>Thank you for your patience!</B> }; $should_email = 1; } print qq{Content-type: text/html <HTML><BODY BGCOLOR="white"> <B>Oops, Something went wrong.</B><P> $message </BODY></HTML>}; # send email report if appropriate if ($should_email){ # import sendmail subs use Mail (); # prepare the email error report: my $subject ="Error Report"; my $body = qq| An error has happened: $orig_why |;

# send error reports to admin send_mail($admin_email,$admin_email,$subject,$body); print STDERR "[".scalar localtime()."] [SIGDIE] Sending Error Email\n"; } # print to error_log so we will know there was an error print STDERR "[".scalar localtime()."] [SIGDIE] $orig_why \n"; exit 1; } # end of sub mydie

You may have noticed that I trap the CGI.pm's die() calls here, I don't see any reason why my users should see ugly error messages, but that's the way CGI.pm written. The workaround is to trap them yourself.

Please note that as of version 2.49, CGI.pm provides the cgi_error() method to print the errors and won't die() unless you want it to.

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What happens if you need to access the POSTed data more than once, say to reuse it on subsequent requests? POSTed data comes directly from the socket, and at the low level data can only be read from a socket once. So you have to store it to make it available for reuse.

There is an experimental option for Makefile.PL called PERL_STASH_POST_DATA. If you turn it on, you can get at it again with $r->subprocess_env("POST_DATA"). This is not enabled by default because it adds a processing overhead for each POST request.

But what do we do with large multipart file uploads? Because POST data is not all read in one clump, it's a problem that's not easy to solve in a general way. A transparent way to do this is to switch the request method from POST to GET, and store the POST data in the query string. This handler does exactly this:

Apache/POST2GET.pm ------------------ package Apache::POST2GET; use Apache::Constants qw(M_GET OK DECLINED); sub handler { my $r = shift; return DECLINED unless $r->method eq "POST"; $r->args(scalar $r->content); $r->method('GET'); $r->method_number(M_GET); $r->headers_in->unset('Content-length'); return OK; } 1; __END__

In httpd.conf add:

PerlInitHandler Apache::POST2GET

or even this:

<Limit POST> PerlInitHandler Apache::POST2GET </Limit>

To save a few more cycles, so the handler will be called only for POST requests.

Effectively, this trick turns the POST request into a GET request internally. Now when CGI.pm, Apache::Request or whatever module parses the client data, it can do so more than once since $r->args doesn't go away (unless you make it go away by resetting it).

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Under mod_cgi it's not easy to redirect POST requests to some other location. With mod_perl you can easily redirect POST requests. All you have to do is read in the content, set the method to GET, populate args() with the content to be forwarded and finally do the redirect:

use Apache::Constants qw(M_GET); my $r = shift; my $content = $r->content; $r->method("GET"); $r->method_number(M_GET); $r->headers_in->unset("Content-length"); $r->args($content); $r->internal_redirect_handler("/new/url");

Of course that last line can be any other kind of redirect.

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Let's say you have a module that sets some environment variables.

If you redirect, that's most likely telling the web browser to fetch the new page. This makes it a totally new request, so no environment variables are preserved.

However, if you're using internal_redirect(), you can make the environment variables seen in the sub-process via subprocess_env(). The only nuance is that the %ENV keys will be prefixed with REDIRECT_.

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If you want to terminate the child process serving the current request, upon completion of processing anywhere in the code call:

$r->child_terminate;

Apache won't actually terminate the child until everything it needs to do is done and the connection is closed.

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Many people use relative paths for require, use, etc., and when they open files in their scripts they make assumptions about the current directory. This will fail if you don't chdir() to the correct directory first (as could easily happen if you have another script which calls the first script by its full path).

For example:

/home/httpd/perl/test.pl: ------------------------- #!/usr/bin/perl open IN, "./foo.txt"; -------------------------

This snippet would work if we call the script like this:

% chdir /home/httpd/perl % ./test.pl

since foo.txt is located in the current directory. But when the current directory isn't /home/httpd/perl, if we call the script like this:

% /home/httpd/perl/test.pl

then the script will fail to find foo.txt. Think about crontabs!

Notice that you cannot use the FindBin.pm package, something that you'd do in the regular Perl scripts, because it relies on the BEGIN block it won't work under mod_perl. It's loaded and executed only for the first script executed inside the process, all the others will use the cached value, which would be probably incorrect if they reside in different directories.

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I wrote this script a long time ago, when I had to debug my CGI scripts but didn't have access to the error_log file. I asked the admin to install this script and have used it happily since then.

If your scripts are running on these 'Get-free-site' servers, and you cannot debug your script because you can't telnet to the server or can't see the error_log, you can ask your sysadmin to install this script.

Note, that it was written for plain Apache, and isn't prepared to handle the complex multiline error and warning messages generated by mod_perl. It also uses a system() call to do the main work with the tail() utility, probably a more efficient perl implementation is due (take a look at File::Tail module). You are welcome to fix it and contribute it back to mod_perl community. Thank you!

Here is the code:

# !/usr/bin/perl -Tw use strict; my $default = 10; my $error_log = "/usr/local/apache/logs/error_log"; use CGI; # untaint $ENV{PATH} $ENV{'PATH'} = '/bin:/usr/bin'; delete @ENV{'IFS', 'CDPATH', 'ENV', 'BASH_ENV'}; my $q = new CGI; my $counts = (defined $q->param('count') and $q->param('count')) ? $q->param('count') : $default; print $q->header, $q->start_html(-bgcolor => "white", -title => "Error logs"), $q->start_form, $q->center( $q->b('How many lines to fetch? '), $q->textfield('count',10,3,3), $q->submit('', 'Fetch'), $q->reset, ), $q->end_form, $q->hr; # untaint $counts $counts = ($counts =~ /(\d+)/) ? $1 : 0; print($q->b("$error_log doesn't exist!!!")),exit unless -e $error_log; open LOG, "tail -$counts $error_log|" or die "Can't tail $error_log :$!\n"; my @logs = <LOG>; close LOG; # format and colorize each line nicely foreach (@logs) { s{ \[(.*?)\]\s* # date \[(.*?)\]\s* # type of error \[(.*?)\]\s* # client part (.*) # the message } { "[$1] <BR> [". colorize($2,$2). "] <BR> [$3] <PRE>". colorize($2,$4). "</PRE>" }ex; print "<BR>$_<BR>"; } ############# sub colorize{ my ($type,$context) = @_; my %colors = ( error => 'red', crit => 'black', notice => 'green', warn => 'brown', ); return exists $colors{$type} ? qq{<B><FONT COLOR="$colors{$type}">$context</FONT></B>} : $context; }

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Sometimes you want to access variables from the caller's package. One way is to do something like this:

{ no strict 'vars' ; my $caller = caller; print qq[$caller --- ${"${caller}::var"}]; }

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Unless you use some well known module like CGI::Cookie or Apache::Cookie, you need to handle cookies yourself.

Cookies come in the $ENV{HTTP_COOKIE} variable. You can print the raw cookie string as $ENV{HTTP_COOKIE}.

Here is a fairly well-known bit of code to take cookie values and put them into a hash:

sub get_cookies { # cookies are seperated by a semicolon and a space, this will # split them and return a hash of cookies local(@rawCookies) = split (/; /,$ENV{'HTTP_COOKIE'}); local(%cookies); foreach(@rawCookies){ ($key, $val) = split (/=/,$_); $cookies{$key} = $val; } return %cookies; }

Or a slimmer version:

sub get_cookies { map { split /=/, $_, 2 } split /; /, $ENV{'HTTP_COOKIE'} ; }

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Given that you have prepared your cookies in @cookies, the following code will submit all the cookies:

for (@cookies){ $r->headers_out->add( 'Set-Cookie' => $_ ); }

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You should use err_headers_out() and not headers_out() when you want to send cookies in the REDIRECT response.

use Apache::Constants qw(REDIRECT OK); my $r = shift; # prepare the cookie in $cookie $r->err_headers_out->add('Set-Cookie' => $cookie); $r->headers_out(Location => $location); $r->status(REDIRECT); $r->send_http_header; return OK;

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Let's say that you wrote a few handlers to process a request, and they all need to share some custom Perl data structure. The pnotes() method comes to your rescue.

# a handler that gets executed first my %my_data = (foo => 'mod_perl', bar => 'rules'); $r->pnotes('my_data' => \%my_data);

The handler prepares the data in hash %my_data and calls pnotes() method to store the data internally for other handlers to re-use. All the subsequently called handlers can retrieve the stored data in this way:

my $info = $r->pnotes('my_data'); print $info->{foo};

prints:

mod_perl

The stored information will be destroyed at the end of the request.

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The notes() method can be used to make various Apache modules talk to each other. In the following snippet the PHP module talks to the mod_perl code (PHP code):

if (isset($user) && substr($user,0,1) == "+") { apache_note("user", substr($user,1)); virtual("/internal/getquota"); $quota = apache_note("quota"); $usage_pp = apache_note("usage_pp"); $percent_pp = apache_note("percent_pp"); if ($quota) $message .= " | Using $percent_pp% of $quota_pp limit"; }

The PHP code sets the user and the username pair using the notes mechanism. Then issuing a sub-request to a perl handler:

use Apache::Constants qw(REDIRECT OK); my $r = shift; my $notes = $r->main->notes(); my ($quota,usage_pp,percent_pp) = getquota($notes->{user}||''); $r->notes('quota',$quota); $r->notes('usage_pp',$usage_pp); $r->notes('percent_pp',$percent_pp); return OK;

which retrieves the username from the notes (using $r->main->notes), uses some getquota() function to get the quota related data and then sets the acquired data in the notes for the PHP code. Now the PHP code reads the data from the notes and proceeds with setting $message if $quota is set.

So any Apache modules can communicate with each other over the Apache notes() mechanism.

You can use notes along with the sub-request methods lookup_uri() and lookup_filename() too. To make it work, you need to set a note in the sub-request. For example if you want to call a php sub-request from within mod_perl and pass it a note, you can do it in the following way:

my $subr = $r->lookup_uri('wizard.php3'); $subr->notes('answer' => 42); $subr->run;

As of the time of this writing you cannot access the parent request tables from a PHP handler, therefore you must set this note for the sub-request. Whereas if the sub-request is running in the mod_perl domain, you can always keep the notes in the parent request notes table and access them via the method main():

$r->main->notes('answer');

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This is a simple example of passing environment variables between handlers:

Having a configuration:

PerlAccessHandler My::Access PerlLogHandler My::Log

and in startup.pl:

sub My::Access::handler { my $r = shift; $r->subprocess_env(TICKET => $$); $r->notes(TICKET => $$); } sub My::Log::handler { my $r = shift; my $env = $r->subprocess_env('TICKET'); my $note = $r->notes('TICKET'); warn "env=$env, note=$note\n"; }

Adding %{TICKET}e and %{TICKET}n to the LogFormat for access_log works fine too.

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You can retrieve the request parameters in a similar to CGI::params way using this technique:

my $r = shift; # or $r = Apache->request my %params = $r->method eq 'POST' ? $r->content : $r->args;

assuming that all your variables are single key-value pairs.

Also take a look at Apache::Request which has the same API as CGI.pm for extracting and setting request parameters.

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To subclass a package you simply modify @ISA, for example:

package My::TestAPR; use strict; use vars qw/@ISA/; @ISA = qw/Apache::Request/; sub new { my ($proto, $apr) = @_; my $class = ref($proto) || $proto; bless { _r => $apr }, $class; } sub param { my ($self, $key) = @_; my $apr = $self->{_r}; # Here we are calling the Apache::Request object's param method $apr->param($key); } sub sum { my ($self, $key) = @_; my $apr = $self->{_r}; my @values = $apr->param($key); my $sum = 0; for (@values) { $sum += $_; } $sum; } 1; __END__

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There is nothing special about sending email from mod_perl, it's just that we do it a lot. There are a few important issues. The most widely used approach is starting a sendmail process and piping the headers and the body to it. The problem is that sendmail is a very heavy process and it makes mod_perl processes less efficient.

If you don't want your process to wait until delivery is complete, you can tell sendmail not to deliver the email straight away, but either do it in the background or just queue the job until the next queue run. This can significantly reduce the delay for the mod_perl process which would otherwise have to wait for the sendmail process to complete. This can be specified for all deliveries in sendmail.cf or on each invocation on the sendmail command line:

• -odb (deliver in the background)

• -odq (queue-only) or

• -odd (queue, and also defer the DNS/NIS lookups).

The trend is to move away from sendmail(1) and switch to using lighter mail delivery programs like qmail(1) or postfix(1). You should check the manpage of your favorite mailer application for equivalent configuration presented for sendmail(1).

The most efficient approach is to talk directly to the SMTP server. Luckily Net::SMTP modules makes this very easy. The only problem is when <Net::SMTP> fails to deliver the mail, because the destination peer server is temporarily down. But from the other side Net::SMTP allows you to send email much much faster, since you don't have to invoke a dedicated process. Here is an example of a subroutine that sends email.

use Net::SMTP (); use Carp qw(carp verbose); # # Sends email by using the SMTP Server # # The SMTP server as defined in Net::Config # Alternatively you can hardcode it here, look for $smtp_server below # sub send_mail{ my ($from, $to, $subject, $body) = @_;

carp "From missing" unless defined $from ; # Prefer to exit early if errors carp "To missing" unless defined $to ; my $mail_message = <<__END_OF_MAIL__; To: $to From: $from Subject: $subject $body __END_OF_MAIL__ # Set this parameter if you don't have a valid Net/Config.pm # entry for SMTP host and uncomment it in the Net::SMTP->new # call # my $smtp_server = 'localhost'; # init the server my $smtp = Net::SMTP->new( # $smtp_server, Timeout => 60, Debug => 0, ); $smtp->mail($from) or carp ("Failed to specify a sender [$from]\n"); $smtp->to($to) or carp ("Failed to specify a recipient [$to]\n"); $smtp->data([$mail_message]) or carp ("Failed to send a message\n"); $smtp->quit or carp ("Failed to quit\n"); } # end of sub send_mail

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The code:

package MyEnv; use Apache; use Apache::Constants; sub handler{ my $r = shift; print $r->send_http_header("text/plain"); print map {"$_ => $ENV{$_}\n"} keys %ENV; return OK; } 1;

The configuration:

PerlModule MyEnv <Location /env> SetHandler perl-script PerlHandler MyEnv </Location>

The invocation:

http://localhost/env

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We can easily implement everything mod_rewrite does in Perl. We do this with help of PerlTransHandler, which is invoked at the beginning of request processing. For example consider that we need to perform a redirect based on query string and URI, the following handler does that.

package Apache::MyRedirect; use Apache::Constants qw(OK REDIRECT); use constant DEFAULT_URI => 'http://www.example.org'; sub handler { my $r = shift; my %args = $r->args; my $path = $r->uri; my $uri = (($args{'uri'}) ? $args{'uri'} : DEFAULT_URI) . $path; $r->header_out(Location => $uri); $r->status(REDIRECT); $r->send_http_header; return OK; }

Set it up in httpd.conf as:

PerlTransHandler Apache::MyRedirect

The code consists of three parts: request data retrieval, deciding what to do based on this data and finally setting the headers and the status and issuing redirect.

So if a client submits a request of this kind:

http://www.example.com/news/?uri=http://www.example-2.com/

$uri will hold http://www.example-2.com/news/ and that's where the request will be redirected.

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Suppose that before a content handler is invoked you want to make this translation:

/articles/10/index.html => /articles/index.html?id=10

This TransHandler will do that for you:

My/Trans.pm ----------- package My::Trans; use Apache::Constants qw(:common); sub handler { my $r = shift; my $uri = $r->uri; my ($id) = ($uri =~ m|^/articles/(.*?)/|); $r->uri("/articles/index.html"); $r->args("id=$id"); return DECLINED; } 1;

and in httpd.conf:

PerlModule My::Trans PerlTransHandler My::Trans

The handler code retrieves the request object and the URI. Then it retrieves the id using the regular expression. Finally it sets the new value of the URI and the arguments string. The handler returns DECLINED so the default Apache transhandler will take care of URI to filename remapping.

Notice the technique to set the arguments. By the time the Apache-request object has been created, arguments are handled in a separate slot, so you cannot just push them into the original URI. Therefore the args() method should be used.

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It's very easy to implement a dispatching module based on the MIME type of request. So a different content handler will be called for a different MIME type. This is an example of such a dispatcher:

package My::MimeTypeDispatch; use Apache::Constants qw(DECLINED); my %mime_types = ( 'text/html' => \&HTML::Template::handler, 'text/plain' => \&My::Text::handler, ); sub handler { my $r = shift; if (my $h = $mime_types{$r->content_type}) { $r->push_handlers(PerlHandler => $h); $r->handler('perl-script'); } return DECLINED; } 1; __END__

And in httpd.conf we add:

PerlFixupHandler My::MimeTypeDispatch

After declaring the package name and importing constants, we set a translation table of MIME types and corresponding handlers to be called. Then comes the handler, where the request object is retrieved and if its MIME type is found in our translation table we set the handler that should handle this request. Otherwise we do nothing. At the end we return DECLINED so some other fixup handler could take over.

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This handler lets you use both SSI and Embperl in the same request:

Use it in a <FilesMatch> Section or similar:

PerlModule Apache::EmbperlFilter Apache::SSI <FilesMatch "\.epl"> PerlSetVar Filter On PerlHandler Apache::EmbperlFilter Apache::SSI </FilesMatch>

package Apache::EmbperlFilter; use Apache::Util qw(parsedate); use HTML::Embperl; use Apache::SSI (); use Apache::Constants; use strict; use vars qw($VERSION); $VERSION = '0.03'; my ($r, %param, $input, $output); sub handler { $r = shift; my ($fh, $status) = $r->filter_input(); unless ($status == OK) { return $status } local $/ = undef; $input = scalar(<$fh>); %param = (); $param{input} = \$input; $param{req_rec} = $r; $param{output} = \$output; $param{mtime} = mtime(); $param{inputfile} = $r->filename(); HTML::Embperl::ScanEnvironement(\%param); HTML::Embperl::Execute(\%param); print $output; return OK; } sub mtime { my $mtime = undef; if (my $last_modified = $r->headers_out->{'Last-Modified'}) { $mtime = parsedate $last_modified; } $mtime; } 1; __END__

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Assume that you have more than one front-end server, and you want to dynamically figure out the front-end server name in the back-end server. mod_proxy and mod_rewrite provide the solution.

Compile apache with both mod_proxy and mod_rewrite, then use a directive something like this:

RewriteEngine On RewriteLog /somewhere/rewrite.log RewriteLogLevel 3 RewriteRule ^/foo/bar(.*)$ \ http://example.com:8080/foo/bar/$1?IP=%{REMOTE_HOST} [QSA,P]

This will have all the urls starting with /some/url proxied off to the other server at the same url. It will append the REMOTE_HOST header as a query string argument. (QSA = Query String Append, P = Proxy). There is probably a way to remap it as an X-Header of some sort, but if query string is good enough for you, then this should work really nicely.

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Getting the authenticated username: $r->connection->user(), or $ENV{REMOTE_USER} if you're in a CGI emulation.

Example:

my $r = shift; my ($res, $sent_pwd) = $r->get_basic_auth_pw; return $res if $res; #decline if not Basic my $user = $r->connection->user;

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You can provide your own mechanism to authenticate users, instead of the standard one. If you want to make Apache think that the user was authenticated by the standard mechanism, set the username with:

$r->connection->user('username');

Now you can use this information for example during the logging, so that you can have your ``username'' passed as if it was transmitted to Apache through HTTP authentication.

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META: should be annotated at some point. (an example was posted to the mod_perl list)

use strict; use DBI; use Apache::Session::DBI; use CGI; # [...] # Initiate a session ID my $session = (); my $opts = { autocommit => 0, lifetime => 3600 }; # 3600 is one hour # Read in the cookie if this is an old session my $r = Apache->request; my $no_cookie = ''; my $cookie = $r->header_in('Cookie'); { # eliminate logging from Apache::Session::DBI's use of `warn' local $^W = 0; if (defined($cookie) && $cookie ne '') { $cookie =~ s/SESSION_ID=(\w*)/$1/; $session = Apache::Session::DBI->open($cookie, $opts); $no_cookie = 'Y' unless defined($session); } # Could have been obsolete - get a new one $session = Apache::Session::DBI->new($opts) unless defined($session); } # Might be a new session, so let's give them a cookie back if (! defined($cookie) || $no_cookie) { local $^W = 0; my $session_cookie = "SESSION_ID=$session->{'_ID'}"; $r->header_out("Set-Cookie" => $session_cookie); }

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Well, as always with Perl -- TMTOWTDI (There's More Than One Way To Do It), one of the readers is using DESTROY to finalize output, and as a cheap means of buffering.

package buffer; use Apache; sub new { my $class = shift; my $self = bless { 'r' => shift, 'message' => "" }, $class; $self->{apr} = Apache::Request->new($self->{r}, POST_MAX=>(32*1024)); $self->content_type('text/plain'); $self->{r}->no_cache(1); } sub message { my $self = shift; $self->{message} .= join("\n", @_); } sub DESTROY { my $self = shift; $self->{apr}->send_http_header; $self->{apr}->print($self->{message}); } 1;

Now you can have perl scripts like:

use buffer; my $b = new buffer(shift); $b->message(p("Hello World")); # end

and save a bunch of duplicate code across otherwise inconvenient gaggles of small scripts.

But suppose you also want to redirect the client under some circumstances, and send the HTTP status code 302. You might try this:

sub redir { my $self = shift; $self->{redirect} = shift; exit; }

and re-code DESTROY as:

sub DESTROY { my $self = shift; if ($self->{redirect}) { $self->{apr}->status{REDIRECT}; $self->{apr}->header_out("Location", $self->{redirect}); $self->{apr}->send_http_header; $self->{apr}->print($self->{redirect}); } else { $self->{apr}->send_http_header; $self->{apr}->print($self->{message}); } }

But you'll find that while the browser redirects itself, mod_perl logs the result code as 200. It turns out that status() only touches the Apache response, and the log message is determined by the Apache return code.

Aha! So we'll change the exit() in redir() to exit(REDIRECT). This fixes the log code, but causes a bogus "[error] 302" line in the error_log. That comes from Apache::Registry:

my $errsv = ""; if($@) { $errsv = $@; $@ = ''; #XXX fix me, if we don't do this Apache::exit() breaks $@{$uri} = $errsv; } if($errsv) { $r->log_error($errsv); return SERVER_ERROR unless $Debug && $Debug & 2; return Apache::Debug::dump($r, SERVER_ERROR); }

So you see that any time the return code causes $@ to return true, we'll get an error line. Not wanting this, what can we do?

We can hope that a future version of mod_perl will allow us to set the HTTP result code independent from the handler return code (perhaps a log_status() method? or at least an Apache::LOG_HANDLER_RESULT config variable?).

In the meantime, there's Apache::RedirectLogFix.

Put this in your httpd.conf

PerlLogHandler Apache::RedirectLogFix

and take a look at the source code below. Note that it requires us to return the HTTP status code 200.

package Apache::RedirectLogFix; use Apache::Constants qw(OK DECLINED REDIRECT); sub handler { my $r = shift; return DECLINED unless $r->handler && ($r->handler eq "perl-script"); if(my $loc = $r->header_out("Location")) { if($r->status == 200 and substr($loc, 0, 1) ne "/") { $r->status(REDIRECT); return OK } } return DECLINED; } 1;

Now, if we wanted to do the same sort of thing for an error 500 handler, we could write another PerlLogHandler (call it ServerErrorLogFix). But we'll leave that as an exercise for the reader, and hope that it won't be needed in the next mod_perl release. After all, it's a little awkward to need a LogHandler to clean up after ourselves....

[ TOC ]

Perl uses sh() for its system() and open() calls. So if you want to set a temporary variable when you call a script from your CGI you do something like this:

open UTIL, "USER=stas ; script.pl | " or die "...: $!\n";

or

system "USER=stas ; script.pl";

This is useful, for example, if you need to invoke a script that uses CGI.pm from within a mod_perl script. We are tricking the Perl script into thinking it's a simple CGI, which is not running under mod_perl.

open(PUBLISH, "GATEWAY_INTERFACE=CGI/1.1 ; script.cgi \"param1=value1&param2=value2\" |") or die "...: $!\n";

Make sure that the parameters you pass are shell safe -- all ``unsafe'' characters like single-quote and back-tick should be properly escaped.

Unfortunately mod_perl uses fork() to run the script, so you have probably thrown out the window most of the performance gained from using mod_perl. To avoid the fork, change script.cgi to a module containing a subroutine which you can then call directly from your mod_perl script.

[ TOC ]

This is somewhat off-topic, but since many of us use mysql or some other RDBMS in their work with mod_perl driven sites, it's good to know how to backup and restore the databases in case of database corruption.

First we should tell mysql to log all the clauses that modify the databases (we don't care about SELECT queries for database backups). Modify the safe_mysql script by adding the --log-update options to the mysql server startup parameters and restart the server. From now on all the non-select queries will be logged to the /var/lib/mysql/www.bar.com logfile. Your hostname will show up instead of www.bar.com.

Now create a dump directory under /var/lib/mysql/. That's where the backups will be stored (you can name the directory as you wish of course).

Prepare the backup script and store it in a file, e.g: /usr/local/sbin/mysql/mysql.backup.pl

mysql-3.22.29_backup.pl -- This is the original code

mysql-3.22.30+_backup.pl -- This is the code modified to work with mysql-3.22.30+

You might need to change the executable paths according to your system. List the names of the databases you want to backup using the db_names array.

Now make the script executable and arrange the crontab entry to run the backup script nightly. Note that the disk space used by the backups will grow without bound and you should remove the old backups. Here is a sample crontab entry to run the script at 4am every day:

0 4 * * * /usr/local/sbin/mysql/mysql.backup.pl > /dev/null 2>&1

So now at any moment we have the dump of the databases from the last execution of the backup script and the log file of all the clauses that have updated the databases since then. If the database gets corrupted we have all the information to restore it to the state it was in at our last backup. We restore it with the following script, which I put in: /usr/local/sbin/mysql/mysql.restore.pl

mysql-3.22.29_restore.pl -- This is the original code

mysql-3.22.30+_restore.pl -- This is the code modified to work with mysql-3.22.30+

These are kinda dirty scripts, but they work... if you come up with cleaner scripts, please contribute them... thanks

Update: there is now a ``mysqlhotcopy'' utility distributed with MySQL that can make an atomic snapshot of a database. (by Tim Bunce) So you may consider using it instead. [ TOC ]

Your corrections of the technical and grammatical errors are very welcome. You are encouraged to help me improve this guide. If you have something to contribute please send it directly to me.

The Writing Apache Modules with Perl and C book can be purchased online from O'Reilly and Amazon.com.

[ TOC ]

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Written by Stas Bekman. Last Modified at 03/21/2001

Use of the Camel for Perl is a trademark of O'Reilly & Associates, and is used by permission.