1package DBM_Filter ; 2 3use strict; 4use warnings; 5our $VERSION = '0.06'; 6 7package Tie::Hash ; 8 9use strict; 10use warnings; 11 12use Carp; 13 14 15our %LayerStack = (); 16our %origDESTROY = (); 17 18our %Filters = map { $_, undef } qw( 19 Fetch_Key 20 Fetch_Value 21 Store_Key 22 Store_Value 23 ); 24 25our %Options = map { $_, 1 } qw( 26 fetch 27 store 28 ); 29 30#sub Filter_Enable 31#{ 32#} 33# 34#sub Filter_Disable 35#{ 36#} 37 38sub Filtered 39{ 40 my $this = shift; 41 return defined $LayerStack{$this} ; 42} 43 44sub Filter_Pop 45{ 46 my $this = shift; 47 my $stack = $LayerStack{$this} || return undef ; 48 my $filter = pop @{ $stack }; 49 50 # remove the filter hooks if this is the last filter to pop 51 if ( @{ $stack } == 0 ) { 52 $this->filter_store_key ( undef ); 53 $this->filter_store_value( undef ); 54 $this->filter_fetch_key ( undef ); 55 $this->filter_fetch_value( undef ); 56 delete $LayerStack{$this}; 57 } 58 59 return $filter; 60} 61 62sub Filter_Key_Push 63{ 64 &_do_Filter_Push; 65} 66 67sub Filter_Value_Push 68{ 69 &_do_Filter_Push; 70} 71 72 73sub Filter_Push 74{ 75 &_do_Filter_Push; 76} 77 78sub _do_Filter_Push 79{ 80 my $this = shift; 81 my %callbacks = (); 82 my $caller = (caller(1))[3]; 83 $caller =~ s/^.*:://; 84 85 croak "$caller: no parameters present" unless @_ ; 86 87 if ( ! $Options{lc $_[0]} ) { 88 my $class = shift; 89 my @params = @_; 90 91 # if $class already contains "::", don't prefix "DBM_Filter::" 92 $class = "DBM_Filter::$class" unless $class =~ /::/; 93 94 no strict 'refs'; 95 # does the "DBM_Filter::$class" exist? 96 if ( ! %{ "${class}::"} ) { 97 # Nope, so try to load it. 98 eval " require $class ; " ; 99 croak "$caller: Cannot Load DBM Filter '$class': $@" if $@; 100 } 101 102 my $fetch = *{ "${class}::Fetch" }{CODE}; 103 my $store = *{ "${class}::Store" }{CODE}; 104 my $filter = *{ "${class}::Filter" }{CODE}; 105 use strict 'refs'; 106 107 my $count = defined($filter) + defined($store) + defined($fetch) ; 108 109 if ( $count == 0 ) 110 { croak "$caller: No methods (Filter, Fetch or Store) found in class '$class'" } 111 elsif ( $count == 1 && ! defined $filter) { 112 my $need = defined($fetch) ? 'Store' : 'Fetch'; 113 croak "$caller: Missing method '$need' in class '$class'" ; 114 } 115 elsif ( $count >= 2 && defined $filter) 116 { croak "$caller: Can't mix Filter with Store and Fetch in class '$class'" } 117 118 if (defined $filter) { 119 my $callbacks = &{ $filter }(@params); 120 croak "$caller: '${class}::Filter' did not return a hash reference" 121 unless ref $callbacks && ref $callbacks eq 'HASH'; 122 %callbacks = %{ $callbacks } ; 123 } 124 else { 125 $callbacks{Fetch} = $fetch; 126 $callbacks{Store} = $store; 127 } 128 } 129 else { 130 croak "$caller: not even params" unless @_ % 2 == 0; 131 %callbacks = @_; 132 } 133 134 my %filters = %Filters ; 135 my @got = (); 136 while (my ($k, $v) = each %callbacks ) 137 { 138 my $key = $k; 139 $k = lc $k; 140 if ($k eq 'fetch') { 141 push @got, 'Fetch'; 142 if ($caller eq 'Filter_Push') 143 { $filters{Fetch_Key} = $filters{Fetch_Value} = $v } 144 elsif ($caller eq 'Filter_Key_Push') 145 { $filters{Fetch_Key} = $v } 146 elsif ($caller eq 'Filter_Value_Push') 147 { $filters{Fetch_Value} = $v } 148 } 149 elsif ($k eq 'store') { 150 push @got, 'Store'; 151 if ($caller eq 'Filter_Push') 152 { $filters{Store_Key} = $filters{Store_Value} = $v } 153 elsif ($caller eq 'Filter_Key_Push') 154 { $filters{Store_Key} = $v } 155 elsif ($caller eq 'Filter_Value_Push') 156 { $filters{Store_Value} = $v } 157 } 158 else 159 { croak "$caller: Unknown key '$key'" } 160 161 croak "$caller: value associated with key '$key' is not a code reference" 162 unless ref $v && ref $v eq 'CODE'; 163 } 164 165 if ( @got != 2 ) { 166 push @got, 'neither' if @got == 0 ; 167 croak "$caller: expected both Store & Fetch - got @got"; 168 } 169 170 # remember the class 171 push @{ $LayerStack{$this} }, \%filters ; 172 173 my $str_this = "$this" ; # Avoid a closure with $this in the subs below 174 175 $this->filter_store_key ( sub { store_hook($str_this, 'Store_Key') }); 176 $this->filter_store_value( sub { store_hook($str_this, 'Store_Value') }); 177 $this->filter_fetch_key ( sub { fetch_hook($str_this, 'Fetch_Key') }); 178 $this->filter_fetch_value( sub { fetch_hook($str_this, 'Fetch_Value') }); 179 180 # Hijack the callers DESTROY method 181 $this =~ /^(.*)=/; 182 my $type = $1 ; 183 no strict 'refs'; 184 if ( *{ "${type}::DESTROY" }{CODE} ne \&MyDESTROY ) 185 { 186 $origDESTROY{$type} = *{ "${type}::DESTROY" }{CODE}; 187 no warnings 'redefine'; 188 *{ "${type}::DESTROY" } = \&MyDESTROY ; 189 } 190} 191 192sub store_hook 193{ 194 my $this = shift ; 195 my $type = shift ; 196 foreach my $layer (@{ $LayerStack{$this} }) 197 { 198 &{ $layer->{$type} }() if defined $layer->{$type} ; 199 } 200} 201 202sub fetch_hook 203{ 204 my $this = shift ; 205 my $type = shift ; 206 foreach my $layer (reverse @{ $LayerStack{$this} }) 207 { 208 &{ $layer->{$type} }() if defined $layer->{$type} ; 209 } 210} 211 212sub MyDESTROY 213{ 214 my $this = shift ; 215 delete $LayerStack{$this} ; 216 217 # call real DESTROY 218 $this =~ /^(.*)=/; 219 &{ $origDESTROY{$1} }($this); 220} 221 2221; 223 224__END__ 225 226=head1 NAME 227 228DBM_Filter -- Filter DBM keys/values 229 230=head1 SYNOPSIS 231 232 use DBM_Filter ; 233 use SDBM_File; # or DB_File, GDBM_File, NDBM_File, or ODBM_File 234 235 $db = tie %hash, ... 236 237 $db->Filter_Push(Fetch => sub {...}, 238 Store => sub {...}); 239 240 $db->Filter_Push('my_filter1'); 241 $db->Filter_Push('my_filter2', params...); 242 243 $db->Filter_Key_Push(...) ; 244 $db->Filter_Value_Push(...) ; 245 246 $db->Filter_Pop(); 247 $db->Filtered(); 248 249 package DBM_Filter::my_filter1; 250 251 sub Store { ... } 252 sub Fetch { ... } 253 254 1; 255 256 package DBM_Filter::my_filter2; 257 258 sub Filter 259 { 260 my @opts = @_; 261 ... 262 return ( 263 sub Store { ... }, 264 sub Fetch { ... } ); 265 } 266 267 1; 268 269=head1 DESCRIPTION 270 271This module provides an interface that allows filters to be applied 272to tied Hashes associated with DBM files. It builds on the DBM Filter 273hooks that are present in all the *DB*_File modules included with the 274standard Perl source distribution from version 5.6.1 onwards. In addition 275to the *DB*_File modules distributed with Perl, the BerkeleyDB module, 276available on CPAN, supports the DBM Filter hooks. See L<perldbmfilter> 277for more details on the DBM Filter hooks. 278 279=head1 What is a DBM Filter? 280 281A DBM Filter allows the keys and/or values in a tied hash to be modified 282by some user-defined code just before it is written to the DBM file and 283just after it is read back from the DBM file. For example, this snippet 284of code 285 286 $some_hash{"abc"} = 42; 287 288could potentially trigger two filters, one for the writing of the key 289"abc" and another for writing the value 42. Similarly, this snippet 290 291 my ($key, $value) = each %some_hash 292 293will trigger two filters, one for the reading of the key and one for 294the reading of the value. 295 296Like the existing DBM Filter functionality, this module arranges for the 297C<$_> variable to be populated with the key or value that a filter will 298check. This usually means that most DBM filters tend to be very short. 299 300=head2 So what's new? 301 302The main enhancements over the standard DBM Filter hooks are: 303 304=over 4 305 306=item * 307 308A cleaner interface. 309 310=item * 311 312The ability to easily apply multiple filters to a single DBM file. 313 314=item * 315 316The ability to create "canned" filters. These allow commonly used filters 317to be packaged into a stand-alone module. 318 319=back 320 321=head1 METHODS 322 323This module will arrange for the following methods to be available via 324the object returned from the C<tie> call. 325 326=head2 $db->Filter_Push() / $db->Filter_Key_Push() / $db->Filter_Value_Push() 327 328Add a filter to filter stack for the database, C<$db>. The three formats 329vary only in whether they apply to the DBM key, the DBM value or both. 330 331=over 5 332 333=item Filter_Push 334 335The filter is applied to I<both> keys and values. 336 337=item Filter_Key_Push 338 339The filter is applied to the key I<only>. 340 341=item Filter_Value_Push 342 343The filter is applied to the value I<only>. 344 345=back 346 347 348=head2 $db->Filter_Pop() 349 350Removes the last filter that was applied to the DBM file associated with 351C<$db>, if present. 352 353=head2 $db->Filtered() 354 355Returns TRUE if there are any filters applied to the DBM associated 356with C<$db>. Otherwise returns FALSE. 357 358 359 360=head1 Writing a Filter 361 362Filters can be created in two main ways 363 364=head2 Immediate Filters 365 366An immediate filter allows you to specify the filter code to be used 367at the point where the filter is applied to a dbm. In this mode the 368Filter_*_Push methods expects to receive exactly two parameters. 369 370 my $db = tie %hash, 'SDBM_File', ... 371 $db->Filter_Push( Store => sub { }, 372 Fetch => sub { }); 373 374The code reference associated with C<Store> will be called before any 375key/value is written to the database and the code reference associated 376with C<Fetch> will be called after any key/value is read from the 377database. 378 379For example, here is a sample filter that adds a trailing NULL character 380to all strings before they are written to the DBM file, and removes the 381trailing NULL when they are read from the DBM file 382 383 my $db = tie %hash, 'SDBM_File', ... 384 $db->Filter_Push( Store => sub { $_ .= "\x00" ; }, 385 Fetch => sub { s/\x00$// ; }); 386 387 388Points to note: 389 390=over 5 391 392=item 1. 393 394Both the Store and Fetch filters manipulate C<$_>. 395 396=back 397 398=head2 Canned Filters 399 400Immediate filters are useful for one-off situations. For more generic 401problems it can be useful to package the filter up in its own module. 402 403The usage is for a canned filter is: 404 405 $db->Filter_Push("name", params) 406 407where 408 409=over 5 410 411=item "name" 412 413is the name of the module to load. If the string specified does not 414contain the package separator characters "::", it is assumed to refer to 415the full module name "DBM_Filter::name". This means that the full names 416for canned filters, "null" and "utf8", included with this module are: 417 418 DBM_Filter::null 419 DBM_Filter::utf8 420 421=item params 422 423any optional parameters that need to be sent to the filter. See the 424encode filter for an example of a module that uses parameters. 425 426=back 427 428The module that implements the canned filter can take one of two 429forms. Here is a template for the first 430 431 package DBM_Filter::null ; 432 433 use strict; 434 use warnings; 435 436 sub Store 437 { 438 # store code here 439 } 440 441 sub Fetch 442 { 443 # fetch code here 444 } 445 446 1; 447 448 449Notes: 450 451=over 5 452 453=item 1. 454 455The package name uses the C<DBM_Filter::> prefix. 456 457=item 2. 458 459The module I<must> have both a Store and a Fetch method. If only one is 460present, or neither are present, a fatal error will be thrown. 461 462=back 463 464The second form allows the filter to hold state information using a 465closure, thus: 466 467 package DBM_Filter::encoding ; 468 469 use strict; 470 use warnings; 471 472 sub Filter 473 { 474 my @params = @_ ; 475 476 ... 477 return { 478 Store => sub { $_ = $encoding->encode($_) }, 479 Fetch => sub { $_ = $encoding->decode($_) } 480 } ; 481 } 482 483 1; 484 485 486In this instance the "Store" and "Fetch" methods are encapsulated inside a 487"Filter" method. 488 489 490=head1 Filters Included 491 492A number of canned filers are provided with this module. They cover a 493number of the main areas that filters are needed when interfacing with 494DBM files. They also act as templates for your own filters. 495 496The filter included are: 497 498=over 5 499 500=item * utf8 501 502This module will ensure that all data written to the DBM will be encoded 503in UTF-8. 504 505This module needs the Encode module. 506 507=item * encode 508 509Allows you to choose the character encoding will be store in the DBM file. 510 511=item * compress 512 513This filter will compress all data before it is written to the database 514and uncompressed it on reading. 515 516This module needs Compress::Zlib. 517 518=item * int32 519 520This module is used when interoperating with a C/C++ application that 521uses a C int as either the key and/or value in the DBM file. 522 523=item * null 524 525This module ensures that all data written to the DBM file is null 526terminated. This is useful when you have a perl script that needs 527to interoperate with a DBM file that a C program also uses. A fairly 528common issue is for the C application to include the terminating null 529in a string when it writes to the DBM file. This filter will ensure that 530all data written to the DBM file can be read by the C application. 531 532=back 533 534=head1 NOTES 535 536=head2 Maintain Round Trip Integrity 537 538When writing a DBM filter it is I<very> important to ensure that it is 539possible to retrieve all data that you have written when the DBM filter 540is in place. In practice, this means that whatever transformation is 541applied to the data in the Store method, the I<exact> inverse operation 542should be applied in the Fetch method. 543 544If you don't provide an exact inverse transformation, you will find that 545code like this will not behave as you expect. 546 547 while (my ($k, $v) = each %hash) 548 { 549 ... 550 } 551 552Depending on the transformation, you will find that one or more of the 553following will happen 554 555=over 5 556 557=item 1 558 559The loop will never terminate. 560 561=item 2 562 563Too few records will be retrieved. 564 565=item 3 566 567Too many will be retrieved. 568 569=item 4 570 571The loop will do the right thing for a while, but it will unexpectedly fail. 572 573=back 574 575=head2 Don't mix filtered & non-filtered data in the same database file. 576 577This is just a restatement of the previous section. Unless you are 578completely certain you know what you are doing, avoid mixing filtered & 579non-filtered data. 580 581=head1 EXAMPLE 582 583Say you need to interoperate with a legacy C application that stores 584keys as C ints and the values and null terminated UTF-8 strings. Here 585is how you would set that up 586 587 my $db = tie %hash, 'SDBM_File', ... 588 589 $db->Filter_Key_Push('int32') ; 590 591 $db->Filter_Value_Push('utf8'); 592 $db->Filter_Value_Push('null'); 593 594=head1 SEE ALSO 595 596<DB_File>, L<GDBM_File>, L<NDBM_File>, L<ODBM_File>, L<SDBM_File>, L<perldbmfilter> 597 598=head1 AUTHOR 599 600Paul Marquess <pmqs@cpan.org> 601 602