Apache2::Filter - Perl API for Apache 2.0 Filtering
use Apache2::Filter ();
# filter attributes
my $c = $f->c;
my $r = $f->r;
my $frec = $f->frec();
my $next_f = $f->next;
my $ctx = $f->ctx;
$f->ctx($ctx);
# bucket brigade filtering API
$rc = $f->next->get_brigade($bb, $mode, $block, $readbytes);
$rc = $f->next->pass_brigade($bb);
$rc = $f->fflush($bb);
# streaming filtering API
while ($filter->read(my $buffer, $wanted)) {
# transform $buffer here
$filter->print($buffer);
}
if ($f->seen_eos) {
$filter->print("filter signature");
}
# filter manipulations
$r->add_input_filter(\&callback);
$c->add_input_filter(\&callback);
$r->add_output_filter(\&callback);
$c->add_output_filter(\&callback);
$f->remove;
Apache2::Filter provides Perl API for Apache 2.0 filtering
framework.
Make sure to read the Filtering
tutorial|docs::2.0::user::handlers::filters.
The following methods can be called from any filter handler:
c
Get the current connection object from a connection or a request filter:
$c = $f->c;
$f
( Apache2::Filter object )
$c
( Apache2::Connection object )
ctx
Get/set the filter context data.
$ctx = $f->ctx;
$f->ctx($ctx);
$f
( Apache2::Filter object )
$ctx ( SCALAR )
$ctx ( SCALAR )
A filter context is created before the filter is called for the first
time and it's destroyed at the end of the request. The context is
preserved between filter invocations of the same request. So if a
filter needs to store some data between invocations it should use the
filter context for that. The filter context is initialized with the
undef value.
The ctx method accepts a single SCALAR argument. Therefore if you
want to store any other perl datastructure you should use a reference
to it.
For example you can store a hash reference:
$f->ctx({ foo => 'bar' });
and then access it:
$foo = $f->ctx->{foo};
if you access the context more than once it's more efficient to copy it's value before using it:
my $ctx = $f->ctx;
$foo = $ctx->{foo};
to avoid redundant method calls. As of this writing $ctx is not a
tied variable, so if you modify it need to store it at the end:
$f->ctx($ctx);
META: later we might make it a TIEd-variable interface, so it'll be stored automatically.
Besides its primary purpose of storing context data across multiple filter invocations, this method is also useful when used as a flag. For example here is how to ensure that something happens only once during the filter's life:
unless ($f->ctx) {
do_something_once();
$f->ctx(1);
}
frec
Get/set the Apache2::FilterRec
(filter record) object.
$frec = $f->frec();
$f
( Apache2::Filter object )
$frec
( Apache2::FilterRec object )
For example you can call
$frec->name to get
filter's name.
next
Return the Apache2::Filter object of the next filter in chain.
$next_f = $f->next;
$f
( Apache2::Filter object )
$next_f
( Apache2::Filter object )
Since Apache inserts several core filters at the end of each chain,
normally this method always returns an object. However if it's not a
mod_perl filter handler, you can call only the following methods on
it: get_brigade,
pass_brigade, c, r,
frec and next. If you call other methods
the behavior is undefined.
The next filter can be a mod_perl one or not, it's easy to tell which
one is that by calling
$f->frec->name.
r
Inside an HTTP request filter retrieve the current request object:
$r = $f->r;
$f
( Apache2::Filter object )
$r
( Apache2::RequestRec object )
If a sub-request adds filters, then that sub-request object is associated with the filter.
remove
Remove the current filter from the filter chain (for the current request or connection).
$f->remove;
$f
( Apache2::Filter object )
Notice that you should either complete the current filter invocation
normally (by calling get_brigade or
pass_brigade depending on the filter kind) or
if nothing was done, return Apache2::Const::DECLINED and mod_perl will take
care of passing the current bucket brigade through unmodified to the
next filter in chain.
Note: calling remove() on the very top connection filter doesn't affect the filter chain due to a bug in Apache 2.0 (which may be fixed in 2.1). So don't use it with connection filters, till it gets fixed in Apache and then make sure to require the minimum Apache version if you rely on.
Remember that if the connection is
$c->keepalive
) and the connection filter is removed, it won't be added until the
connection is closed. Which may happen after many HTTP requests. You
may want to keep the filter in place and pass the data through
unmodified, by returning Apache2::Const::DECLINED. If you need to reset the
whole or parts of the filter context between requests, use the
technique based on $c->keepalives
counting.
This method works for native Apache (non-mod_perl) filters too.
The following methods can be called from any filter, directly manipulating bucket brigades:
fflush
Flush a bucket brigade down the filter stack.
$rc = $f->fflush($bb);
$f
( Apache2::Filter object )
$bb
( Apache2::Brigade object )
$rc ( APR::Const status
constant )
pass_brigade() entry.
APR::Error
get_brigade() entry for
details.
fflush is a shortcut method. So instead of doing:
my $b = APR::Bucket::flush_create($f->c->bucket_alloc); $bb->insert_tail($b); $f->pass_brigade($bb);
one can just write:
$f->fflush($bb);
get_brigade
This is a method to use in bucket brigade input filters. It acquires a bucket brigade from the upstream input filter.
$rc = $next_f->get_brigade($bb, $mode, $block, $readbytes); $rc = $next_f->get_brigade($bb, $mode, $block); $rc = $next_f->get_brigade($bb, $mode) $rc = $next_f->get_brigade($bb);
$next_f
( Apache2::Filter object )
The next filter in the filter chain.
Inside filter handlers it's
usually $f->next. Inside protocol
handlers:
$c->input_filters.
$bb
( APR::Brigade object )
The original bucket brigade passed to get_brigade(), which must be
empty.
Inside input filter handlers it's usually the second argument to the filter handler.
Otherwise it should be created:
my $bb = APR::Brigade->new($c->pool, $c->bucket_alloc);
On return it gets populated with the next bucket brigade. That brigade may contain nothing if there was no more data to read. The return status tells the outcome.
$mode ( Apache2::Const :input_mode
constant )
The filter mode in which the data should be read.
If inside the filter handler, you should normally pass the same mode that was passed to the filter handler (the third argument).
At the end of this section the available modes are presented.
If the argument $mode is not passed,
Apache2::Const::MODE_READBYTES
is used as a default value.
$block ( APR::Const :read_type
constant )
You may ask the reading operation to be blocking:
APR::Const::BLOCK_READ,
or nonblocking:
APR::Const::NONBLOCK_READ.
If inside the filter handler, you should normally pass the same blocking mode argument that was passed to the filter handler (the forth argument).
If the argument $block is not passed,
APR::Const::BLOCK_READ is
used as a default value.
$readbytes ( integer )
How many bytes to read from the next filter.
If inside the filter handler, you may want the same number of bytes, as the upstream filter, i.e. the argument that was passed to the filter handler (the fifth argument).
If the argument $block is not passed, 8192 is used as a default
value.
$rc ( APR::Const status
constant )
On success,
APR::Const::SUCCESS is
returned and $bb is populated (see the $bb entry).
In case of a failure -- a failure code is returned, in which case normally it should be returned to the caller.
If the bottom-most filter doesn't read from the network, then
Apache2::NOBODY_READ is returned (META: need to add this constant).
Inside protocol handlers the
return code can also be APR::Const::EOF, which is success as well.
APR::Error
You don't have to ask for the return value. If this function is called in the VOID context, e.g.:
$f->next->get_brigade($bb, $mode, $block, $readbytes);
mod_perl will do the error checking on your behalf, and if the return
code is not
APR::Const::SUCCESS, an
APR::Error exception will be thrown.
The only time you want to do the error checking yourself, is when
return codes besides
APR::Const::SUCCESS are
considered as successful and you want to manage them by yourself.
Available input filter modes (the optional second argument $mode)
are:
Apache2::Const::MODE_READBYTES
Apache2::Const::MODE_GETLINE
Apache2::Const::MODE_EATCRLF
Apache2::Const::MODE_SPECULATIVE
Apache2::Const::MODE_EXHAUSTIVE
Apache2::Const::MODE_INIT
Either compile all these constants with:
use Apache2::Const -compile => qw(:input_mode);
But it's a bit more efficient to compile only those constants that you need.
Example:
Here is a fragment of a filter handler, that receives a bucket brigade from the upstream filter:
use Apache2::Filter ();
use APR::Const -compile => qw(SUCCESS);
use Apache2::Const -compile => qw(OK);
sub filter {
my ($f, $bb, $mode, $block, $readbytes) = @_;
my $rc = $f->next->get_brigade($bb, $mode, $block, $readbytes);
return $rc unless $rc == APR::Const::SUCCESS;
# ... process $bb
return Apache2::Const::OK;
}
Usually arguments $mode, $block, $readbytes are the same as
passed to the filter itself.
You can see that in case of a failure, the handler returns immediately with that failure code, which gets propagated to the downstream filter.
If you decide not check the return code, you can write it as:
sub filter {
my ($f, $bb, $mode, $block, $readbytes) = @_;
$f->next->get_brigade($bb, $mode, $block, $readbytes);
# ... process $bb
return Apache2::Const::OK;
}
and the error checking will be done on your behalf.
You will find many more examples in the filter
handlers|docs::2.0::user::handlers::filters and
the protocol
handlers|docs::2.0::user::handlers::protocols tutorials.
pass_brigade
This is a method to use in bucket brigade output filters. It passes the current bucket brigade to the downstream output filter.
$rc = $next_f->pass_brigade($bb);
$next_f
( Apache2::Filter object )
The next filter in the filter chain.
Inside output filter handlers
it's usually $f->next. Inside protocol
handlers:
$c->output_filters.
$bb
( APR::Brigade object )
The bucket brigade to pass.
Inside output filter handlers it's usually the second argument to the filter handler (after potential manipulations).
$rc ( APR::Const status
constant )
On success,
APR::Const::SUCCESS is
returned.
In case of a failure -- a failure code is returned, in which case normally it should be returned to the caller.
If the bottom-most filter doesn't write to the network, then
Apache2::NOBODY_WROTE is returned (META: need to add this constant).
Also refer to the get_brigade() entry to see how
to avoid checking the errors explicitly.
APR::Error
get_brigade() entry for
details.
The caller relinquishes ownership of the brigade (i.e. it may get destroyed/overwritten/etc. by the callee).
Example:
Here is a fragment of a filter handler, that passes a bucket brigade to the downstream filter (after some potential processing of the buckets in the bucket brigade):
use Apache2::Filter ();
use APR::Const -compile => qw(SUCCESS);
use Apache2::Const -compile => qw(OK);
sub filter {
my ($f, $bb) = @_;
# ... process $bb
my $rc = $f->next->pass_brigade($bb);
return $rc unless $rc == APR::Const::SUCCESS;
return Apache2::Const::OK;
}
The following methods can be called from any filter, which uses the simplified streaming functionality:
print
Send the contents of $buffer to the next filter in chain (via
internal buffer).
$sent = $f->print($buffer);
$f
( Apache2::Filter object )
$buffer ( string )
$sent ( integer )
APR::Error
This method should be used only in streaming filters.
read
Read data from the filter
$read = $f->read($buffer, $wanted);
$f
( Apache2::Filter object )
$buffer ( SCALAR )
$wanted ( integer )
How many bytes to attempt to read.
If this optional argument is not specified -- the default 8192 will be used.
$read ( integer )
How many bytes were actually read.
$buffer gets populated with the string that is read. It will
contain an empty string if there was nothing to read.
APR::Error
Reads at most $wanted characters into $buffer. The returned
value $read tells exactly how many were read, making it easy to use
it in a while loop:
while ($filter->read(my $buffer, $wanted)) {
# transform $buffer here
$filter->print($buffer);
}
This is a streaming filter method, which acquires a single bucket brigade behind the scenes and reads data from all its buckets. Therefore it can only read from one bucket brigade per filter invocation.
If the EOS bucket is read, the seen_eos method will
return a true value.
seen_eos
This methods returns a true value when the EOS bucket is seen by the
read method.
$ok = $f->seen_eos;
$f
( Apache2::Filter object )
$ok ( boolean )
This method only works in streaming filters which exhaustively
$f->read all the incoming data in a while loop,
like so:
while ($f->read(my $buffer, $wanted)) {
# do something with $buffer
}
if ($f->seen_eos) {
# do something
}
The technique in this example is useful when a streaming filter wants to append something to the very end of data, or do something at the end of the last filter invocation. After the EOS bucket is read, the filter should expect not to be invoked again.
If an input streaming filter doesn't consume all data in the bucket brigade (or even in several bucket brigades), it has to generate the EOS event by itself. So when the filter is done it has to set the EOS flag:
$f->seen_eos(1);
when the filter handler returns, internally mod_perl will take care of creating and sending the EOS bucket to the upstream input filter.
A similar logic may apply for output filters.
In most other cases you shouldn't set this flag. When this flag is prematurely set (before the real EOS bucket has arrived) in the current filter invocation, instead of invoking the filter again, mod_perl will create and send the EOS bucket to the next filter, ignoring any other bucket brigades that may have left to consume. As mentioned earlier this special behavior is useful in writing special tests that test abnormal situations.
Other methods which affect filters, but called on
non-Apache2::Filter objects:
add_input_filter
Add &callback filter handler to input request filter chain.
$r->add_input_filter(\&callback);
Add &callback filter handler to input connection filter chain.
$c->add_input_filter(\&callback);
$c
( Apache2::Connection object ) or $r
( Apache2::RequestRec object )
&callback (CODE ref)
[META: It seems that you can't add a filter when another filter is called. I've tried to add an output connection filter from the input connection filter when it was called for the first time. It didn't have any affect for the first request (over keepalive connection). The only way I succeeded to do that is from that input connection filter's filter_init handler. In fact it does work if there is any filter additional filter of the same kind configured from httpd.conf or via filter_init. It looks like there is a bug in httpd, where it doesn't prepare the chain of 3rd party filter if none were inserted before the first filter was called.]
add_output_filter
Add &callback filter handler to output request filter chain.
$r->add_output_filter(\&callback);
Add &callback filter handler to output connection filter chain.
$c->add_output_filter(\&callback);
$c
( Apache2::Connection object ) or $r
( Apache2::RequestRec object )
&callback (CODE ref)
Packages using filter attributes have to subclass Apache2::Filter:
package MyApache2::FilterCool; use base qw(Apache2::Filter);
Attributes are parsed during the code compilation, by the function
MODIFY_CODE_ATTRIBUTES, inherited from the Apache2::Filter
package.
FilterRequestHandler
The FilterRequestHandler attribute tells mod_perl to insert the
filter into an HTTP request filter chain.
For example, to configure an output request filter handler, use the
FilterRequestHandler attribute in the handler subroutine's
declaration:
package MyApache2::FilterOutputReq;
sub handler : FilterRequestHandler { ... }
and add the configuration entry:
PerlOutputFilterHandler MyApache2::FilterOutputReq
This is the default mode. So if you are writing an HTTP request filter, you don't have to specify this attribute.
The section HTTP Request vs. Connection Filters delves into more details.
FilterConnectionHandler
The FilterConnectionHandler attribute tells mod_perl to insert this
filter into a connection filter chain.
For example, to configure an output connection filter handler, use the
FilterConnectionHandler attribute in the handler subroutine's
declaration:
package MyApache2::FilterOutputCon;
sub handler : FilterConnectionHandler { ... }
and add the configuration entry:
PerlOutputFilterHandler MyApache2::FilterOutputCon
The section HTTP Request vs. Connection Filters delves into more details.
FilterInitHandler
The attribute FilterInitHandler marks the function suitable to be
used as a filter initialization callback, which is called immediately
after a filter is inserted to the filter chain and before it's
actually called.
sub init : FilterInitHandler {
my $f = shift;
#...
return Apache2::Const::OK;
}
In order to hook this filter callback, the real filter has to assign
this callback using the
FilterHasInitHandler which accepts a
reference to the callback function.
For further discussion and examples refer to the Filter Initialization Phase tutorial section.
FilterHasInitHandler
If a filter wants to run an initialization callback it can register
such using the FilterHasInitHandler attribute. Similar to
push_handlers the callback reference is expected, rather than a
callback name. The used callback function has to have the
FilterInitHandler attribute. For example:
package MyApache2::FilterBar;
use base qw(Apache2::Filter);
sub init : FilterInitHandler { ... }
sub filter : FilterRequestHandler FilterHasInitHandler(\&init) {
my ($f, $bb) = @_;
# ...
return Apache2::Const::OK;
}
For further discussion and examples refer to the Filter Initialization Phase tutorial section.
mod_perl 2.0 filters configuration is explained in the filter handlers tutorial.
PerlInputFilterHandler
PerlOutputFilterHandler
PerlSetInputFilter
See
PerlSetInputFilter.
PerlSetOutputFilter
See
PerlSetInputFilter.
Apache2::Filter also implements a tied interface, so you can work
with the $f object as a hash reference.
The TIE interface is mostly unimplemented and might be implemented post 2.0 release.
TIEHANDLE
$ret = TIEHANDLE($stashsv, $sv);
$stashsv ( SCALAR )
$sv ( SCALAR )
$ret ( SCALAR )
PRINT
$ret = PRINT(...);
... (XXX)
$ret ( integer )
mod_perl 2.0 and its core modules are copyrighted under The Apache Software License, Version 2.0.