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Filename : Provider.pm
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#============================================================= -*-Perl-*-
#
# Template::Provider
#
# DESCRIPTION
#   This module implements a class which handles the loading, compiling
#   and caching of templates.  Multiple Template::Provider objects can
#   be stacked and queried in turn to effect a Chain-of-Command between
#   them.  A provider will attempt to return the requested template,
#   an error (STATUS_ERROR) or decline to provide the template
#   (STATUS_DECLINE), allowing subsequent providers to attempt to
#   deliver it.   See 'Design Patterns' for further details.
#
# AUTHORS
#   Andy Wardley <abw@wardley.org>
#
#   Refactored by Bill Moseley for v2.19 to add negative caching (i.e. 
#   tracking templates that are NOTFOUND so that we can decline quickly)
#   and to provide better support for subclassing the provider.
#
# COPYRIGHT
#   Copyright (C) 1996-2007 Andy Wardley.  All Rights Reserved.
#
#   This module is free software; you can redistribute it and/or
#   modify it under the same terms as Perl itself.
#
# WARNING:
#   This code is ugly and contorted and is being totally re-written for TT3.
#   In particular, we'll be throwing errors rather than messing around 
#   returning (value, status) pairs.  With the benefit of hindsight, that 
#   was a really bad design decision on my part. I deserve to be knocked
#   to the ground and kicked around a bit by hoards of angry TT developers
#   for that one.  Bill's refactoring has made the module easier to subclass, 
#   (so you can ease off the kicking now), but it really needs to be totally
#   redesigned and rebuilt from the ground up along with the bits of TT that
#   use it.                                           -- abw 2007/04/27
#============================================================================

package Template::Provider;

use strict;
use warnings;
use base 'Template::Base';
use Template::Config;
use Template::Constants;
use Template::Document;
use File::Basename;
use File::Spec;

use constant PREV   => 0;
use constant NAME   => 1;   # template name -- indexed by this name in LOOKUP
use constant DATA   => 2;   # Compiled template
use constant LOAD   => 3;   # mtime of template
use constant NEXT   => 4;   # link to next item in cache linked list
use constant STAT   => 5;   # Time last stat()ed

our $VERSION = 2.94;
our $DEBUG   = 0 unless defined $DEBUG;
our $ERROR   = '';

# name of document class
our $DOCUMENT = 'Template::Document' unless defined $DOCUMENT;

# maximum time between performing stat() on file to check staleness
our $STAT_TTL = 1 unless defined $STAT_TTL;

# maximum number of directories in an INCLUDE_PATH, to prevent runaways
our $MAX_DIRS = 64 unless defined $MAX_DIRS;

# UNICODE is supported in versions of Perl from 5.007 onwards
our $UNICODE = $] > 5.007 ? 1 : 0;

my $boms = [
    'UTF-8'    => "\x{ef}\x{bb}\x{bf}",
    'UTF-32BE' => "\x{0}\x{0}\x{fe}\x{ff}",
    'UTF-32LE' => "\x{ff}\x{fe}\x{0}\x{0}",
    'UTF-16BE' => "\x{fe}\x{ff}",
    'UTF-16LE' => "\x{ff}\x{fe}",
];

# regex to match relative paths
our $RELATIVE_PATH = qr[(?:^|/)\.+/];


# hack so that 'use bytes' will compile on versions of Perl earlier than
# 5.6, even though we never call _decode_unicode() on those systems
BEGIN {
    if ($] < 5.006) {
        package bytes;
        $INC{'bytes.pm'} = 1;
    }
}


#========================================================================
#                         -- PUBLIC METHODS --
#========================================================================

#------------------------------------------------------------------------
# fetch($name)
#
# Returns a compiled template for the name specified by parameter.
# The template is returned from the internal cache if it exists, or
# loaded and then subsequently cached.  The ABSOLUTE and RELATIVE
# configuration flags determine if absolute (e.g. '/something...')
# and/or relative (e.g. './something') paths should be honoured.  The
# INCLUDE_PATH is otherwise used to find the named file. $name may
# also be a reference to a text string containing the template text,
# or a file handle from which the content is read.  The compiled
# template is not cached in these latter cases given that there is no
# filename to cache under.  A subsequent call to store($name,
# $compiled) can be made to cache the compiled template for future
# fetch() calls, if necessary.
#
# Returns a compiled template or (undef, STATUS_DECLINED) if the
# template could not be found.  On error (e.g. the file was found
# but couldn't be read or parsed), the pair ($error, STATUS_ERROR)
# is returned.  The TOLERANT configuration option can be set to
# downgrade any errors to STATUS_DECLINE.
#------------------------------------------------------------------------

sub fetch {
    my ($self, $name) = @_;
    my ($data, $error);


    if (ref $name) {
        # $name can be a reference to a scalar, GLOB or file handle
        ($data, $error) = $self->_load($name);
        ($data, $error) = $self->_compile($data)
            unless $error;
        $data = $data->{ data }
            unless $error;
    }
    elsif (File::Spec->file_name_is_absolute($name)) {
        # absolute paths (starting '/') allowed if ABSOLUTE set
        ($data, $error) = $self->{ ABSOLUTE }
            ? $self->_fetch($name)
            : $self->{ TOLERANT }
                ? (undef, Template::Constants::STATUS_DECLINED)
            : ("$name: absolute paths are not allowed (set ABSOLUTE option)",
               Template::Constants::STATUS_ERROR);
    }
    elsif ($name =~ m/$RELATIVE_PATH/o) {
        # anything starting "./" is relative to cwd, allowed if RELATIVE set
        ($data, $error) = $self->{ RELATIVE }
            ? $self->_fetch($name)
            : $self->{ TOLERANT }
                ? (undef, Template::Constants::STATUS_DECLINED)
            : ("$name: relative paths are not allowed (set RELATIVE option)",
               Template::Constants::STATUS_ERROR);
    }
    else {
        # otherwise, it's a file name relative to INCLUDE_PATH
        ($data, $error) = $self->{ INCLUDE_PATH }
            ? $self->_fetch_path($name)
            : (undef, Template::Constants::STATUS_DECLINED);
    }

#    $self->_dump_cache()
#       if $DEBUG > 1;

    return ($data, $error);
}


#------------------------------------------------------------------------
# store($name, $data)
#
# Store a compiled template ($data) in the cached as $name.
# Returns compiled template
#------------------------------------------------------------------------

sub store {
    my ($self, $name, $data) = @_;
    $self->_store($name, {
        data => $data,
        load => 0,
    });
}


#------------------------------------------------------------------------
# load($name)
#
# Load a template without parsing/compiling it, suitable for use with
# the INSERT directive.  There's some duplication with fetch() and at
# some point this could be reworked to integrate them a little closer.
#------------------------------------------------------------------------

sub load {
    my ($self, $name) = @_;
    my ($data, $error);
    my $path = $name;

    if (File::Spec->file_name_is_absolute($name)) {
        # absolute paths (starting '/') allowed if ABSOLUTE set
        $error = "$name: absolute paths are not allowed (set ABSOLUTE option)"
            unless $self->{ ABSOLUTE };
    }
    elsif ($name =~ m[$RELATIVE_PATH]o) {
        # anything starting "./" is relative to cwd, allowed if RELATIVE set
        $error = "$name: relative paths are not allowed (set RELATIVE option)"
            unless $self->{ RELATIVE };
    }
    else {
      INCPATH: {
          # otherwise, it's a file name relative to INCLUDE_PATH
          my $paths = $self->paths()
              || return ($self->error(), Template::Constants::STATUS_ERROR);

          foreach my $dir (@$paths) {
              $path = File::Spec->catfile($dir, $name);
              last INCPATH
                  if $self->_template_modified($path);
          }
          undef $path;      # not found
      }
    }

    # Now fetch the content
    ($data, $error) = $self->_template_content($path)
        if defined $path && !$error;

    if ($error) {
        return $self->{ TOLERANT }
            ? (undef, Template::Constants::STATUS_DECLINED)
            : ($error, Template::Constants::STATUS_ERROR);
    }
    elsif (! defined $path) {
        return (undef, Template::Constants::STATUS_DECLINED);
    }
    else {
        return ($data, Template::Constants::STATUS_OK);
    }
}



#------------------------------------------------------------------------
# include_path(\@newpath)
#
# Accessor method for the INCLUDE_PATH setting.  If called with an
# argument, this method will replace the existing INCLUDE_PATH with
# the new value.
#------------------------------------------------------------------------

sub include_path {
     my ($self, $path) = @_;
     $self->{ INCLUDE_PATH } = $path if $path;
     return $self->{ INCLUDE_PATH };
}


#------------------------------------------------------------------------
# paths()
#
# Evaluates the INCLUDE_PATH list, ignoring any blank entries, and
# calling and subroutine or object references to return dynamically
# generated path lists.  Returns a reference to a new list of paths
# or undef on error.
#------------------------------------------------------------------------

sub paths {
    my $self   = shift;
    my @ipaths = @{ $self->{ INCLUDE_PATH } };
    my (@opaths, $dpaths, $dir);
    my $count = $MAX_DIRS;

    while (@ipaths && --$count) {
        $dir = shift @ipaths || next;

        # $dir can be a sub or object ref which returns a reference
        # to a dynamically generated list of search paths.

        if (ref $dir eq 'CODE') {
            eval { $dpaths = &$dir() };
            if ($@) {
                chomp $@;
                return $self->error($@);
            }
            unshift(@ipaths, @$dpaths);
            next;
        }
        elsif (ref($dir) && UNIVERSAL::can($dir, 'paths')) {
            $dpaths = $dir->paths()
                || return $self->error($dir->error());
            unshift(@ipaths, @$dpaths);
            next;
        }
        else {
            push(@opaths, $dir);
        }
    }
    return $self->error("INCLUDE_PATH exceeds $MAX_DIRS directories")
        if @ipaths;

    return \@opaths;
}


#------------------------------------------------------------------------
# DESTROY
#
# The provider cache is implemented as a doubly linked list which Perl
# cannot free by itself due to the circular references between NEXT <=>
# PREV items.  This cleanup method walks the list deleting all the NEXT/PREV
# references, allowing the proper cleanup to occur and memory to be
# repooled.
#------------------------------------------------------------------------

sub DESTROY {
    my $self = shift;
    my ($slot, $next);

    $slot = $self->{ HEAD };
    while ($slot) {
        $next = $slot->[ NEXT ];
        undef $slot->[ PREV ];
        undef $slot->[ NEXT ];
        $slot = $next;
    }
    undef $self->{ HEAD };
    undef $self->{ TAIL };
}




#========================================================================
#                        -- PRIVATE METHODS --
#========================================================================

#------------------------------------------------------------------------
# _init()
#
# Initialise the cache.
#------------------------------------------------------------------------

sub _init {
    my ($self, $params) = @_;
    my $size = $params->{ CACHE_SIZE   };
    my $path = $params->{ INCLUDE_PATH } || '.';
    my $cdir = $params->{ COMPILE_DIR  } || '';
    my $dlim = $params->{ DELIMITER    };
    my $debug;

    # tweak delim to ignore C:/
    unless (defined $dlim) {
        $dlim = ($^O eq 'MSWin32') ? ':(?!\\/)' : ':';
    }

    # coerce INCLUDE_PATH to an array ref, if not already so
    $path = [ split(/$dlim/, $path) ]
        unless ref $path eq 'ARRAY';

    # don't allow a CACHE_SIZE 1 because it breaks things and the
    # additional checking isn't worth it
    $size = 2
        if defined $size && ($size == 1 || $size < 0);

    if (defined ($debug = $params->{ DEBUG })) {
        $self->{ DEBUG } = $debug & ( Template::Constants::DEBUG_PROVIDER
                                    | Template::Constants::DEBUG_FLAGS );
    }
    else {
        $self->{ DEBUG } = $DEBUG;
    }

    if ($self->{ DEBUG }) {
        local $" = ', ';
        $self->debug("creating cache of ",
                     defined $size ? $size : 'unlimited',
                     " slots for [ @$path ]");
    }

    # create COMPILE_DIR and sub-directories representing each INCLUDE_PATH
    # element in which to store compiled files
    if ($cdir) {
        require File::Path;
        foreach my $dir (@$path) {
            next if ref $dir;
            my $wdir = $dir;
            $wdir =~ s[:][]g if $^O eq 'MSWin32';
            $wdir =~ /(.*)/;  # untaint
            $wdir = "$1";     # quotes work around bug in Strawberry Perl
            $wdir = File::Spec->catfile($cdir, $wdir);
            File::Path::mkpath($wdir) unless -d $wdir;
        }
    }

    $self->{ LOOKUP       } = { };
    $self->{ NOTFOUND     } = { };  # Tracks templates *not* found.
    $self->{ SLOTS        } = 0;
    $self->{ SIZE         } = $size;
    $self->{ INCLUDE_PATH } = $path;
    $self->{ DELIMITER    } = $dlim;
    $self->{ COMPILE_DIR  } = $cdir;
    $self->{ COMPILE_EXT  } = $params->{ COMPILE_EXT } || '';
    $self->{ ABSOLUTE     } = $params->{ ABSOLUTE } || 0;
    $self->{ RELATIVE     } = $params->{ RELATIVE } || 0;
    $self->{ TOLERANT     } = $params->{ TOLERANT } || 0;
    $self->{ DOCUMENT     } = $params->{ DOCUMENT } || $DOCUMENT;
    $self->{ PARSER       } = $params->{ PARSER   };
    $self->{ DEFAULT      } = $params->{ DEFAULT  };
    $self->{ ENCODING     } = $params->{ ENCODING };
#   $self->{ PREFIX       } = $params->{ PREFIX   };
    $self->{ STAT_TTL     } = $params->{ STAT_TTL } || $STAT_TTL;
    $self->{ PARAMS       } = $params;

    # look for user-provided UNICODE parameter or use default from package var
    $self->{ UNICODE      } = defined $params->{ UNICODE }
                                    ? $params->{ UNICODE } : $UNICODE;

    return $self;
}


#------------------------------------------------------------------------
# _fetch($name, $t_name)
#
# Fetch a file from cache or disk by specification of an absolute or
# relative filename.  No search of the INCLUDE_PATH is made.  If the
# file is found and loaded, it is compiled and cached.
# Call with:
#   $name       = path to search (possible prefixed by INCLUDE_PATH)
#   $t_name     = template name
#------------------------------------------------------------------------

sub _fetch {
    my ($self, $name, $t_name) = @_;
    my $stat_ttl = $self->{ STAT_TTL };

    $self->debug("_fetch($name)") if $self->{ DEBUG };

    # First see if the named template is in the memory cache
    if ((my $slot = $self->{ LOOKUP }->{ $name })) {
        # Test if cache is fresh, and reload/compile if not.
        my ($data, $error) = $self->_refresh($slot);

        return $error
            ? ( $data, $error )     # $data may contain error text
            : $slot->[ DATA ];      # returned document object
    }

    # Otherwise, see if we already know the template is not found
    if (my $last_stat_time = $self->{ NOTFOUND }->{ $name }) {
        my $expires_in = $last_stat_time + $stat_ttl - time;
        if ($expires_in > 0) {
            $self->debug(" file [$name] in negative cache.  Expires in $expires_in seconds")
                if $self->{ DEBUG };
            return (undef, Template::Constants::STATUS_DECLINED);
        }
        else {
            delete $self->{ NOTFOUND }->{ $name };
        }
    }

    # Is there an up-to-date compiled version on disk?
    if ($self->_compiled_is_current($name)) {
        # require() the compiled template.
        my $compiled_template = $self->_load_compiled( $self->_compiled_filename($name) );

        # Store and return the compiled template
        return $self->store( $name, $compiled_template ) if $compiled_template;

        # Problem loading compiled template:
        # warn and continue to fetch source template
        warn($self->error(), "\n");
    }

    # load template from source
    my ($template, $error) = $self->_load($name, $t_name);

    if ($error) {
        # Template could not be fetched.  Add to the negative/notfound cache.
        $self->{ NOTFOUND }->{ $name } = time;
        return ( $template, $error );
    }

    # compile template source
    ($template, $error) = $self->_compile($template, $self->_compiled_filename($name) );

    if ($error) {
        # return any compile time error
        return ($template, $error);
    }
    else {
        # Store compiled template and return it
        return $self->store($name, $template->{data}) ;
    }
}


#------------------------------------------------------------------------
# _fetch_path($name)
#
# Fetch a file from cache or disk by specification of an absolute cache
# name (e.g. 'header') or filename relative to one of the INCLUDE_PATH
# directories.  If the file isn't already cached and can be found and
# loaded, it is compiled and cached under the full filename.
#------------------------------------------------------------------------

sub _fetch_path {
    my ($self, $name) = @_;

    $self->debug("_fetch_path($name)") if $self->{ DEBUG };

    # the template may have been stored using a non-filename name
    # so look for the plain name in the cache first
    if ((my $slot = $self->{ LOOKUP }->{ $name })) {
        # cached entry exists, so refresh slot and extract data
        my ($data, $error) = $self->_refresh($slot);

        return $error
            ? ($data, $error)
            : ($slot->[ DATA ], $error );
    }

    my $paths = $self->paths
        || return ( $self->error, Template::Constants::STATUS_ERROR );

    # search the INCLUDE_PATH for the file, in cache or on disk
    foreach my $dir (@$paths) {
        my $path = File::Spec->catfile($dir, $name);

        $self->debug("searching path: $path\n") if $self->{ DEBUG };

        my ($data, $error) = $self->_fetch( $path, $name );

        # Return if no error or if a serious error.
        return ( $data, $error )
            if !$error || $error == Template::Constants::STATUS_ERROR;

    }

    # not found in INCLUDE_PATH, now try DEFAULT
    return $self->_fetch_path( $self->{DEFAULT} )
        if defined $self->{DEFAULT} && $name ne $self->{DEFAULT};

    # We could not handle this template name
    return (undef, Template::Constants::STATUS_DECLINED);
}

sub _compiled_filename {
    my ($self, $file) = @_;
    my ($compext, $compdir) = @$self{ qw( COMPILE_EXT COMPILE_DIR ) };
    my ($path, $compiled);

    return undef
        unless $compext || $compdir;

    $path = $file;
    $path =~ /^(.+)$/s or die "invalid filename: $path";
    $path =~ s[:][]g if $^O eq 'MSWin32';

    $compiled = "$path$compext";
    $compiled = File::Spec->catfile($compdir, $compiled) if length $compdir;

    return $compiled;
}

sub _load_compiled {
    my ($self, $file) = @_;
    my $compiled;

    # load compiled template via require();  we zap any
    # %INC entry to ensure it is reloaded (we don't
    # want 1 returned by require() to say it's in memory)
    delete $INC{ $file };
    eval { $compiled = require $file; };
    return $@
        ? $self->error("compiled template $compiled: $@")
        : $compiled;
}

#------------------------------------------------------------------------
# _load($name, $alias)
#
# Load template text from a string ($name = scalar ref), GLOB or file
# handle ($name = ref), or from an absolute filename ($name = scalar).
# Returns a hash array containing the following items:
#   name    filename or $alias, if provided, or 'input text', etc.
#   text    template text
#   time    modification time of file, or current time for handles/strings
#   load    time file was loaded (now!)
#
# On error, returns ($error, STATUS_ERROR), or (undef, STATUS_DECLINED)
# if TOLERANT is set.
#------------------------------------------------------------------------

sub _load {
    my ($self, $name, $alias) = @_;
    my ($data, $error);
    my $tolerant = $self->{ TOLERANT };
    my $now = time;

    $alias = $name unless defined $alias or ref $name;

    $self->debug("_load($name, ", defined $alias ? $alias : '<no alias>',
                 ')') if $self->{ DEBUG };

    # SCALAR ref is the template text
    if (ref $name eq 'SCALAR') {
        # $name can be a SCALAR reference to the input text...
        return {
            name => defined $alias ? $alias : 'input text',
            path => defined $alias ? $alias : 'input text',
            text => $$name,
            time => $now,
            load => 0,
        };
    }

    # Otherwise, assume GLOB as a file handle
    if (ref $name) {
        local $/;
        my $text = <$name>;
        $text = $self->_decode_unicode($text) if $self->{ UNICODE };
        return {
            name => defined $alias ? $alias : 'input file handle',
            path => defined $alias ? $alias : 'input file handle',
            text => $text,
            time => $now,
            load => 0,
        };
    }

    # Otherwise, it's the name of the template
    if ( $self->_template_modified( $name ) ) {  # does template exist?
        my ($text, $error, $mtime ) = $self->_template_content( $name );
        unless ( $error )  {
            $text = $self->_decode_unicode($text) if $self->{ UNICODE };
            return {
                name => $alias,
                path => $name,
                text => $text,
                time => $mtime,
                load => $now,
            };
        }

        return ( "$alias: $!", Template::Constants::STATUS_ERROR )
            unless $tolerant;
    }

    # Unable to process template, pass onto the next Provider.
    return (undef, Template::Constants::STATUS_DECLINED);
}


#------------------------------------------------------------------------
# _refresh(\@slot)
#
# Private method called to mark a cache slot as most recently used.
# A reference to the slot array should be passed by parameter.  The
# slot is relocated to the head of the linked list.  If the file from
# which the data was loaded has been upated since it was compiled, then
# it is re-loaded from disk and re-compiled.
#------------------------------------------------------------------------

sub _refresh {
    my ($self, $slot) = @_;
    my $stat_ttl = $self->{ STAT_TTL };
    my ($head, $file, $data, $error);

    $self->debug("_refresh([ ",
                 join(', ', map { defined $_ ? $_ : '<undef>' } @$slot),
                 '])') if $self->{ DEBUG };

    # if it's more than $STAT_TTL seconds since we last performed a
    # stat() on the file then we need to do it again and see if the file
    # time has changed
    my $now = time;
    my $expires_in_sec = $slot->[ STAT ] + $stat_ttl - $now;

    if ( $expires_in_sec <= 0 ) {  # Time to check!
        $slot->[ STAT ] = $now;

        # Grab mtime of template.
        # Seems like this should be abstracted to compare to
        # just ask for a newer compiled template (if it's newer)
        # and let that check for a newer template source.
        my $template_mtime = $self->_template_modified( $slot->[ NAME ] );
        if ( ! defined $template_mtime || ( $template_mtime != $slot->[ LOAD ] )) {
            $self->debug("refreshing cache file ", $slot->[ NAME ])
                if $self->{ DEBUG };

            ($data, $error) = $self->_load($slot->[ NAME ], $slot->[ DATA ]->{ name });
            ($data, $error) = $self->_compile($data)
                unless $error;

            if ($error) {
                # if the template failed to load/compile then we wipe out the
                # STAT entry.  This forces the provider to try and reload it
                # each time instead of using the previously cached version
                # until $STAT_TTL is next up
                $slot->[ STAT ] = 0;
            }
            else {
                $slot->[ DATA ] = $data->{ data };
                $slot->[ LOAD ] = $data->{ time };
            }
        }

    } elsif ( $self->{ DEBUG } ) {
        $self->debug( sprintf('STAT_TTL not met for file [%s].  Expires in %d seconds',
                        $slot->[ NAME ], $expires_in_sec ) );
    }

    # Move this slot to the head of the list
    unless( $self->{ HEAD } == $slot ) {
        # remove existing slot from usage chain...
        if ($slot->[ PREV ]) {
            $slot->[ PREV ]->[ NEXT ] = $slot->[ NEXT ];
        }
        else {
            $self->{ HEAD } = $slot->[ NEXT ];
        }
        if ($slot->[ NEXT ]) {
            $slot->[ NEXT ]->[ PREV ] = $slot->[ PREV ];
        }
        else {
            $self->{ TAIL } = $slot->[ PREV ];
        }

        # ..and add to start of list
        $head = $self->{ HEAD };
        $head->[ PREV ] = $slot if $head;
        $slot->[ PREV ] = undef;
        $slot->[ NEXT ] = $head;
        $self->{ HEAD } = $slot;
    }

    return ($data, $error);
}



#------------------------------------------------------------------------
# _store($name, $data)
#
# Private method called to add a data item to the cache.  If the cache
# size limit has been reached then the oldest entry at the tail of the
# list is removed and its slot relocated to the head of the list and
# reused for the new data item.  If the cache is under the size limit,
# or if no size limit is defined, then the item is added to the head
# of the list.
# Returns compiled template
#------------------------------------------------------------------------

sub _store {
    my ($self, $name, $data, $compfile) = @_;
    my $size = $self->{ SIZE };
    my ($slot, $head);

    # Return if memory cache disabled.  (overridding code should also check)
    # $$$ What's the expected behaviour of store()?  Can't tell from the
    # docs if you can call store() when SIZE = 0.
    return $data->{data} if defined $size and !$size;

    # extract the compiled template from the data hash
    $data = $data->{ data };
    $self->debug("_store($name, $data)") if $self->{ DEBUG };

    # check the modification time -- extra stat here
    my $load = $self->_modified($name);

    if (defined $size && $self->{ SLOTS } >= $size) {
        # cache has reached size limit, so reuse oldest entry
        $self->debug("reusing oldest cache entry (size limit reached: $size)\nslots: $self->{ SLOTS }") if $self->{ DEBUG };

        # remove entry from tail of list
        $slot = $self->{ TAIL };
        $slot->[ PREV ]->[ NEXT ] = undef;
        $self->{ TAIL } = $slot->[ PREV ];

        # remove name lookup for old node
        delete $self->{ LOOKUP }->{ $slot->[ NAME ] };

        # add modified node to head of list
        $head = $self->{ HEAD };
        $head->[ PREV ] = $slot if $head;
        @$slot = ( undef, $name, $data, $load, $head, time );
        $self->{ HEAD } = $slot;

        # add name lookup for new node
        $self->{ LOOKUP }->{ $name } = $slot;
    }
    else {
        # cache is under size limit, or none is defined

        $self->debug("adding new cache entry") if $self->{ DEBUG };

        # add new node to head of list
        $head = $self->{ HEAD };
        $slot = [ undef, $name, $data, $load, $head, time ];
        $head->[ PREV ] = $slot if $head;
        $self->{ HEAD } = $slot;
        $self->{ TAIL } = $slot unless $self->{ TAIL };

        # add lookup from name to slot and increment nslots
        $self->{ LOOKUP }->{ $name } = $slot;
        $self->{ SLOTS }++;
    }

    return $data;
}


#------------------------------------------------------------------------
# _compile($data)
#
# Private method called to parse the template text and compile it into
# a runtime form.  Creates and delegates a Template::Parser object to
# handle the compilation, or uses a reference passed in PARSER.  On
# success, the compiled template is stored in the 'data' item of the
# $data hash and returned.  On error, ($error, STATUS_ERROR) is returned,
# or (undef, STATUS_DECLINED) if the TOLERANT flag is set.
# The optional $compiled parameter may be passed to specify
# the name of a compiled template file to which the generated Perl
# code should be written.  Errors are (for now...) silently
# ignored, assuming that failures to open a file for writing are
# intentional (e.g directory write permission).
#------------------------------------------------------------------------

sub _compile {
    my ($self, $data, $compfile) = @_;
    my $text = $data->{ text };
    my ($parsedoc, $error);

    $self->debug("_compile($data, ",
                 defined $compfile ? $compfile : '<no compfile>', ')')
        if $self->{ DEBUG };

    my $parser = $self->{ PARSER }
        ||= Template::Config->parser($self->{ PARAMS })
        ||  return (Template::Config->error(), Template::Constants::STATUS_ERROR);

    # discard the template text - we don't need it any more
    delete $data->{ text };

    # call parser to compile template into Perl code
    if ($parsedoc = $parser->parse($text, $data)) {

        $parsedoc->{ METADATA } = {
            'name'    => $data->{ name },
            'modtime' => $data->{ time },
            %{ $parsedoc->{ METADATA } },
        };

        # write the Perl code to the file $compfile, if defined
        if ($compfile) {
            my $basedir = &File::Basename::dirname($compfile);
            $basedir =~ /(.*)/;
            $basedir = $1;

            unless (-d $basedir) {
                eval { File::Path::mkpath($basedir) };
                $error = "failed to create compiled templates directory: $basedir ($@)"
                    if ($@);
            }

            unless ($error) {
                my $docclass = $self->{ DOCUMENT };
                $error = 'cache failed to write '
                    . &File::Basename::basename($compfile)
                    . ': ' . $docclass->error()
                    unless $docclass->write_perl_file($compfile, $parsedoc);
            }

            # set atime and mtime of newly compiled file, don't bother
            # if time is undef
            if (!defined($error) && defined $data->{ time }) {
                my ($cfile) = $compfile =~ /^(.+)$/s or do {
                    return("invalid filename: $compfile",
                           Template::Constants::STATUS_ERROR);
                };

                my ($ctime) = $data->{ time } =~ /^(\d+)$/;
                unless ($ctime || $ctime eq 0) {
                    return("invalid time: $ctime",
                           Template::Constants::STATUS_ERROR);
                }
                utime($ctime, $ctime, $cfile);

                $self->debug(" cached compiled template to file [$compfile]")
                    if $self->{ DEBUG };
            }
        }

        unless ($error) {
            return $data                                        ## RETURN ##
                if $data->{ data } = $DOCUMENT->new($parsedoc);
            $error = $Template::Document::ERROR;
        }
    }
    else {
        $error = Template::Exception->new( 'parse', "$data->{ name } " .
                                           $parser->error() );
    }

    # return STATUS_ERROR, or STATUS_DECLINED if we're being tolerant
    return $self->{ TOLERANT }
        ? (undef, Template::Constants::STATUS_DECLINED)
        : ($error,  Template::Constants::STATUS_ERROR)
}

#------------------------------------------------------------------------
# _compiled_is_current( $template_name )
#
# Returns true if $template_name and its compiled name
# exist and they have the same mtime.
#------------------------------------------------------------------------

sub _compiled_is_current {
    my ( $self, $template_name ) = @_;
    my $compiled_name   = $self->_compiled_filename($template_name) || return;
    my $compiled_mtime  = (stat($compiled_name))[9] || return;
    my $template_mtime  = $self->_template_modified( $template_name ) || return;

    # This was >= in the 2.15, but meant that downgrading
    # a source template would not get picked up.
    return $compiled_mtime == $template_mtime;
}


#------------------------------------------------------------------------
# _template_modified($path)
#
# Returns the last modified time of the $path.
# Returns undef if the path does not exist.
# Override if templates are not on disk, for example
#------------------------------------------------------------------------

sub _template_modified {
    my $self = shift;
    my $template = shift || return;
    return (stat( $template ))[9];
}

#------------------------------------------------------------------------
# _template_content($path)
#
# Fetches content pointed to by $path.
# Returns the content in scalar context.
# Returns ($data, $error, $mtime) in list context where
#   $data       - content
#   $error      - error string if there was an error, otherwise undef
#   $mtime      - last modified time from calling stat() on the path
#------------------------------------------------------------------------

sub _template_content {
    my ($self, $path) = @_;

    return (undef, "No path specified to fetch content from ")
        unless $path;

    my $data;
    my $mod_date;
    my $error;

    local *FH;
    if (open(FH, "< $path")) {
        local $/;
        binmode(FH);
        $data = <FH>;
        $mod_date = (stat($path))[9];
        close(FH);
    }
    else {
        $error = "$path: $!";
    }

    return wantarray
        ? ( $data, $error, $mod_date )
        : $data;
}


#------------------------------------------------------------------------
# _modified($name)
# _modified($name, $time)
#
# When called with a single argument, it returns the modification time
# of the named template.  When called with a second argument it returns
# true if $name has been modified since $time.
#------------------------------------------------------------------------

sub _modified {
    my ($self, $name, $time) = @_;
    my $load = $self->_template_modified($name)
        || return $time ? 1 : 0;

    return $time
         ? $load > $time
         : $load;
}

#------------------------------------------------------------------------
# _dump()
#
# Debug method which returns a string representing the internal object
# state.
#------------------------------------------------------------------------

sub _dump {
    my $self = shift;
    my $size = $self->{ SIZE };
    my $parser = $self->{ PARSER };
    $parser = $parser ? $parser->_dump() : '<no parser>';
    $parser =~ s/\n/\n    /gm;
    $size = 'unlimited' unless defined $size;

    my $output = "[Template::Provider] {\n";
    my $format = "    %-16s => %s\n";
    my $key;

    $output .= sprintf($format, 'INCLUDE_PATH',
                       '[ ' . join(', ', @{ $self->{ INCLUDE_PATH } }) . ' ]');
    $output .= sprintf($format, 'CACHE_SIZE', $size);

    foreach $key (qw( ABSOLUTE RELATIVE TOLERANT DELIMITER
                      COMPILE_EXT COMPILE_DIR )) {
        $output .= sprintf($format, $key, $self->{ $key });
    }
    $output .= sprintf($format, 'PARSER', $parser);


    local $" = ', ';
    my $lookup = $self->{ LOOKUP };
    $lookup = join('', map {
        sprintf("    $format", $_, defined $lookup->{ $_ }
                ? ('[ ' . join(', ', map { defined $_ ? $_ : '<undef>' }
                               @{ $lookup->{ $_ } }) . ' ]') : '<undef>');
    } sort keys %$lookup);
    $lookup = "{\n$lookup    }";

    $output .= sprintf($format, LOOKUP => $lookup);

    $output .= '}';
    return $output;
}


#------------------------------------------------------------------------
# _dump_cache()
#
# Debug method which prints the current state of the cache to STDERR.
#------------------------------------------------------------------------

sub _dump_cache {
    my $self = shift;
    my ($node, $lut, $count);

    $count = 0;
    if ($node = $self->{ HEAD }) {
        while ($node) {
            $lut->{ $node } = $count++;
            $node = $node->[ NEXT ];
        }
        $node = $self->{ HEAD };
        print STDERR "CACHE STATE:\n";
        print STDERR "  HEAD: ", $self->{ HEAD }->[ NAME ], "\n";
        print STDERR "  TAIL: ", $self->{ TAIL }->[ NAME ], "\n";
        while ($node) {
            my ($prev, $name, $data, $load, $next) = @$node;
#           $name = '...' . substr($name, -10) if length $name > 10;
            $prev = $prev ? "#$lut->{ $prev }<-": '<undef>';
            $next = $next ? "->#$lut->{ $next }": '<undef>';
            print STDERR "   #$lut->{ $node } : [ $prev, $name, $data, $load, $next ]\n";
            $node = $node->[ NEXT ];
        }
    }
}

#------------------------------------------------------------------------
# _decode_unicode
#
# Decodes encoded unicode text that starts with a BOM and
# turns it into perl's internal representation
#------------------------------------------------------------------------

sub _decode_unicode {
    my $self   = shift;
    my $string = shift;
    return undef unless defined $string;

    use bytes;
    require Encode;

    return $string if Encode::is_utf8( $string );

    # try all the BOMs in order looking for one (order is important
    # 32bit BOMs look like 16bit BOMs)

    my $count  = 0;

    while ($count < @{ $boms }) {
        my $enc = $boms->[$count++];
        my $bom = $boms->[$count++];

        # does the string start with the bom?
        if ($bom eq substr($string, 0, length($bom))) {
            # decode it and hand it back
            return Encode::decode($enc, substr($string, length($bom)), 1);
        }
    }

    return $self->{ ENCODING }
        ? Encode::decode( $self->{ ENCODING }, $string )
        : $string;
}


1;

__END__

=head1 NAME

Template::Provider - Provider module for loading/compiling templates

=head1 SYNOPSIS

    $provider = Template::Provider->new(\%options);
    
    ($template, $error) = $provider->fetch($name);

=head1 DESCRIPTION

The L<Template::Provider> is used to load, parse, compile and cache template
documents. This object may be sub-classed to provide more specific facilities
for loading, or otherwise providing access to templates.

The L<Template::Context> objects maintain a list of L<Template::Provider>
objects which are polled in turn (via L<fetch()|Template::Context#fetch()>) to
return a requested template. Each may return a compiled template, raise an
error, or decline to serve the request, giving subsequent providers a chance
to do so.

The L<Template::Provider> can also be subclassed to provide templates from
a different source, e.g. a database. See L<SUBCLASSING> below.

This documentation needs work.

=head1 PUBLIC METHODS

=head2 new(\%options) 

Constructor method which instantiates and returns a new C<Template::Provider>
object.  A reference to a hash array of configuration options may be passed.

See L<CONFIGURATION OPTIONS> below for a summary of configuration options
and L<Template::Manual::Config> for full details.

=head2 fetch($name)

Returns a compiled template for the name specified. If the template cannot be
found then C<(undef, STATUS_DECLINED)> is returned. If an error occurs (e.g.
read error, parse error) then C<($error, STATUS_ERROR)> is returned, where
C<$error> is the error message generated. If the L<TOLERANT> option is set the
the method returns C<(undef, STATUS_DECLINED)> instead of returning an error.

=head2 load($name)

Loads a template without parsing or compiling it.  This is used by the 
the L<INSERT|Template::Manual::Directives#INSERT> directive.

=head2 store($name, $template)

Stores the compiled template, C<$template>, in the cache under the name, 
C<$name>.  Susbequent calls to C<fetch($name)> will return this template in
preference to any disk-based file.

=head2 include_path(\@newpath)

Accessor method for the C<INCLUDE_PATH> setting.  If called with an
argument, this method will replace the existing C<INCLUDE_PATH> with
the new value.

=head2 paths()

This method generates a copy of the C<INCLUDE_PATH> list.  Any elements in the
list which are dynamic generators (e.g. references to subroutines or objects
implementing a C<paths()> method) will be called and the list of directories 
returned merged into the output list.

It is possible to provide a generator which returns itself, thus sending
this method into an infinite loop.  To detect and prevent this from happening,
the C<$MAX_DIRS> package variable, set to C<64> by default, limits the maximum
number of paths that can be added to, or generated for the output list.  If
this number is exceeded then the method will immediately return an error 
reporting as much.

=head1 CONFIGURATION OPTIONS

The following list summarises the configuration options that can be provided
to the C<Template::Provider> L<new()> constructor. Please consult
L<Template::Manual::Config> for further details and examples of each
configuration option in use.

=head2 INCLUDE_PATH

The L<INCLUDE_PATH|Template::Manual::Config#INCLUDE_PATH> option is used to
specify one or more directories in which template files are located.

    # single path
    my $provider = Template::Provider->new({
        INCLUDE_PATH => '/usr/local/templates',
    });

    # multiple paths
    my $provider = Template::Provider->new({
        INCLUDE_PATH => [ '/usr/local/templates', 
                          '/tmp/my/templates' ],
    });

=head2 ABSOLUTE

The L<ABSOLUTE|Template::Manual::Config#ABSOLUTE> flag is used to indicate if
templates specified with absolute filenames (e.g. 'C</foo/bar>') should be
processed. It is disabled by default and any attempt to load a template by
such a name will cause a 'C<file>' exception to be raised.

    my $provider = Template::Provider->new({
        ABSOLUTE => 1,
    });

=head2 RELATIVE

The L<RELATIVE|Template::Manual::Config#RELATIVE> flag is used to indicate if
templates specified with filenames relative to the current directory (e.g.
C<./foo/bar> or C<../../some/where/else>) should be loaded. It is also disabled
by default, and will raise a C<file> error if such template names are
encountered.

    my $provider = Template::Provider->new({
        RELATIVE => 1,
    });

=head2 DEFAULT

The L<DEFAULT|Template::Manual::Config#DEFAULT> option can be used to specify
a default template which should be used whenever a specified template can't be
found in the L<INCLUDE_PATH>.

    my $provider = Template::Provider->new({
        DEFAULT => 'notfound.html',
    });

If a non-existant template is requested through the L<Template>
L<process()|Template#process()> method, or by an C<INCLUDE>, C<PROCESS> or
C<WRAPPER> directive, then the C<DEFAULT> template will instead be processed, if
defined. Note that the C<DEFAULT> template is not used when templates are
specified with absolute or relative filenames, or as a reference to a input
file handle or text string.

=head2 ENCODING

The Template Toolkit will automatically decode Unicode templates that
have a Byte Order Marker (BOM) at the start of the file.  This option
can be used to set the default encoding for templates that don't define
a BOM.

    my $provider = Template::Provider->new({
        ENCODING => 'utf8',
    });

See L<Encode> for further information.

=head2 CACHE_SIZE

The L<CACHE_SIZE|Template::Manual::Config#CACHE_SIZE> option can be used to
limit the number of compiled templates that the module should cache. By
default, the L<CACHE_SIZE|Template::Manual::Config#CACHE_SIZE> is undefined
and all compiled templates are cached.

    my $provider = Template::Provider->new({
        CACHE_SIZE => 64,   # only cache 64 compiled templates
    });


=head2 STAT_TTL

The L<STAT_TTL|Template::Manual::Config#STAT_TTL> value can be set to control
how long the C<Template::Provider> will keep a template cached in memory
before checking to see if the source template has changed.

    my $provider = Template::Provider->new({
        STAT_TTL => 60,  # one minute
    });

=head2 COMPILE_EXT

The L<COMPILE_EXT|Template::Manual::Config#COMPILE_EXT> option can be
provided to specify a filename extension for compiled template files.
It is undefined by default and no attempt will be made to read or write 
any compiled template files.

    my $provider = Template::Provider->new({
        COMPILE_EXT => '.ttc',
    });

=head2 COMPILE_DIR

The L<COMPILE_DIR|Template::Manual::Config#COMPILE_DIR> option is used to
specify an alternate directory root under which compiled template files should
be saved.

    my $provider = Template::Provider->new({
        COMPILE_DIR => '/tmp/ttc',
    });

=head2 TOLERANT

The L<TOLERANT|Template::Manual::Config#TOLERANT> flag can be set to indicate
that the C<Template::Provider> module should ignore any errors encountered while
loading a template and instead return C<STATUS_DECLINED>.

=head2 PARSER

The L<PARSER|Template::Manual::Config#PARSER> option can be used to define
a parser module other than the default of L<Template::Parser>.

    my $provider = Template::Provider->new({
        PARSER => MyOrg::Template::Parser->new({ ... }),
    });

=head2 DEBUG

The L<DEBUG|Template::Manual::Config#DEBUG> option can be used to enable
debugging messages from the L<Template::Provider> module by setting it to include
the C<DEBUG_PROVIDER> value.

    use Template::Constants qw( :debug );
    
    my $template = Template->new({
        DEBUG => DEBUG_PROVIDER,
    });

=head1 SUBCLASSING

The C<Template::Provider> module can be subclassed to provide templates from a 
different source (e.g. a database).  In most cases you'll just need to provide
custom implementations of the C<_template_modified()> and C<_template_content()>
methods.  If your provider requires and custom initialisation then you'll also
need to implement a new C<_init()> method.

Caching in memory and on disk will still be applied (if enabled)
when overriding these methods.

=head2 _template_modified($path)

Returns a timestamp of the C<$path> passed in by calling C<stat()>.
This can be overridden, for example, to return a last modified value from
a database.  The value returned should be a timestamp value (as returned by C<time()>,
although a sequence number should work as well.

=head2 _template_content($path)

This method returns the content of the template for all C<INCLUDE>, C<PROCESS>,
and C<INSERT> directives.

When called in scalar context, the method returns the content of the template
located at C<$path>, or C<undef> if C<$path> is not found.

When called in list context it returns C<($content, $error, $mtime)>,
where C<$content> is the template content, C<$error> is an error string
(e.g. "C<$path: File not found>"), and C<$mtime> is the template modification
time.

=head1 AUTHOR

Andy Wardley E<lt>abw@wardley.orgE<gt> L<http://wardley.org/>

=head1 COPYRIGHT

Copyright (C) 1996-2007 Andy Wardley.  All Rights Reserved.

This module is free software; you can redistribute it and/or
modify it under the same terms as Perl itself.

=head1 SEE ALSO

L<Template>, L<Template::Parser>, L<Template::Context>

=cut

# Local Variables:
# mode: perl
# perl-indent-level: 4
# indent-tabs-mode: nil
# End:
#
# vim: expandtab shiftwidth=4: