libdbm-deep-perl 2.0011-1 / usr / share / perl5 / DBM / Deep / Engine / File.pm

package DBM::Deep::Engine::File;

use 5.008_004;

use strict;
use warnings FATAL => 'all';
no warnings 'recursion';

use base qw( DBM::Deep::Engine );

use Scalar::Util ();

use DBM::Deep::Null ();
use DBM::Deep::Sector::File ();
use DBM::Deep::Storage::File ();

sub sector_type { 'DBM::Deep::Sector::File' }
sub iterator_class { 'DBM::Deep::Iterator::File' }

my $STALE_SIZE = 2;

# Setup file and tag signatures.  These should never change.
sub SIG_FILE     () { 'DPDB' }
sub SIG_HEADER   () { 'h'    }
sub SIG_NULL     () { 'N'    }
sub SIG_DATA     () { 'D'    }
sub SIG_UNIDATA  () { 'U'    }
sub SIG_INDEX    () { 'I'    }
sub SIG_BLIST    () { 'B'    }
sub SIG_FREE     () { 'F'    }
sub SIG_SIZE     () {  1     }
# SIG_HASH and SIG_ARRAY are defined in DBM::Deep::Engine

# Please refer to the pack() documentation for further information
my %StP = (
    1 => 'C', # Unsigned char value (no order needed as it's just one byte)
    2 => 'n', # Unsigned short in "network" (big-endian) order
    4 => 'N', # Unsigned long in "network" (big-endian) order
    8 => 'Q', # Usigned quad (no order specified, presumably machine-dependent)
);

=head1 NAME

DBM::Deep::Engine::File - engine for use with DBM::Deep::Storage::File

=head1 PURPOSE

This is the engine for use with L<DBM::Deep::Storage::File>.

=head1 EXTERNAL METHODS

=head2 new()

This takes a set of args. These args are described in the documentation for
L<DBM::Deep/new>.

=cut

sub new {
    my $class = shift;
    my ($args) = @_;

    $args->{storage} = DBM::Deep::Storage::File->new( $args )
        unless exists $args->{storage};

    my $self = bless {
        byte_size   => 4,

        digest      => undef,
        hash_size   => 16,  # In bytes
        hash_chars  => 256, # Number of chars the algorithm uses per byte
        max_buckets => 16,
        num_txns    => 1,   # The HEAD
        trans_id    => 0,   # Default to the HEAD

        data_sector_size => 64, # Size in bytes of each data sector

        entries => {}, # This is the list of entries for transactions
        storage => undef,

        external_refs => undef,
    }, $class;

    # Never allow byte_size to be set directly.
    delete $args->{byte_size};
    if ( defined $args->{pack_size} ) {
        if ( lc $args->{pack_size} eq 'small' ) {
            $args->{byte_size} = 2;
        }
        elsif ( lc $args->{pack_size} eq 'medium' ) {
            $args->{byte_size} = 4;
        }
        elsif ( lc $args->{pack_size} eq 'large' ) {
            $args->{byte_size} = 8;
        }
        else {
            DBM::Deep->_throw_error( "Unknown pack_size value: '$args->{pack_size}'" );
        }
    }

    # Grab the parameters we want to use
    foreach my $param ( keys %$self ) {
        next unless exists $args->{$param};
        $self->{$param} = $args->{$param};
    }

    my %validations = (
        max_buckets      => { floor => 16, ceil => 256 },
        num_txns         => { floor => 1,  ceil => 255 },
        data_sector_size => { floor => 32, ceil => 256 },
    );

    while ( my ($attr, $c) = each %validations ) {
        if (   !defined $self->{$attr}
            || !length $self->{$attr}
            || $self->{$attr} =~ /\D/
            || $self->{$attr} < $c->{floor}
        ) {
            $self->{$attr} = '(undef)' if !defined $self->{$attr};
            warn "Floor of $attr is $c->{floor}. Setting it to $c->{floor} from '$self->{$attr}'\n";
            $self->{$attr} = $c->{floor};
        }
        elsif ( $self->{$attr} > $c->{ceil} ) {
            warn "Ceiling of $attr is $c->{ceil}. Setting it to $c->{ceil} from '$self->{$attr}'\n";
            $self->{$attr} = $c->{ceil};
        }
    }

    if ( !$self->{digest} ) {
        require Digest::MD5;
        $self->{digest} = \&Digest::MD5::md5;
    }

    return $self;
}

sub read_value {
    my $self = shift;
    my ($obj, $key) = @_;

    # This will be a Reference sector
    my $sector = $self->load_sector( $obj->_base_offset )
        or return;

    if ( $sector->staleness != $obj->_staleness ) {
        return;
    }

    my $key_md5 = $self->_apply_digest( $key );

    my $value_sector = $sector->get_data_for({
        key_md5    => $key_md5,
        allow_head => 1,
    });

    unless ( $value_sector ) {
        return undef
    }

    return $value_sector->data;
}

sub get_classname {
    my $self = shift;
    my ($obj) = @_;

    # This will be a Reference sector
    my $sector = $self->load_sector( $obj->_base_offset )
        or DBM::Deep->_throw_error( "How did get_classname fail (no sector for '$obj')?!" );

    if ( $sector->staleness != $obj->_staleness ) {
        return;
    }

    return $sector->get_classname;
}

sub make_reference {
    my $self = shift;
    my ($obj, $old_key, $new_key) = @_;

    # This will be a Reference sector
    my $sector = $self->load_sector( $obj->_base_offset )
        or DBM::Deep->_throw_error( "How did make_reference fail (no sector for '$obj')?!" );

    if ( $sector->staleness != $obj->_staleness ) {
        return;
    }

    my $old_md5 = $self->_apply_digest( $old_key );

    my $value_sector = $sector->get_data_for({
        key_md5    => $old_md5,
        allow_head => 1,
    });

    unless ( $value_sector ) {
        $value_sector = DBM::Deep::Sector::File::Null->new({
            engine => $self,
            data   => undef,
        });

        $sector->write_data({
            key_md5 => $old_md5,
            key     => $old_key,
            value   => $value_sector,
        });
    }

    if ( $value_sector->isa( 'DBM::Deep::Sector::File::Reference' ) ) {
        $sector->write_data({
            key     => $new_key,
            key_md5 => $self->_apply_digest( $new_key ),
            value   => $value_sector,
        });
        $value_sector->increment_refcount;
    }
    else {
        $sector->write_data({
            key     => $new_key,
            key_md5 => $self->_apply_digest( $new_key ),
            value   => $value_sector->clone,
        });
    }

    return;
}

# exists returns '', not undefined.
sub key_exists {
    my $self = shift;
    my ($obj, $key) = @_;

    # This will be a Reference sector
    my $sector = $self->load_sector( $obj->_base_offset )
        or return '';

    if ( $sector->staleness != $obj->_staleness ) {
        return '';
    }

    my $data = $sector->get_data_for({
        key_md5    => $self->_apply_digest( $key ),
        allow_head => 1,
    });

    # exists() returns 1 or '' for true/false.
    return $data ? 1 : '';
}

sub delete_key {
    my $self = shift;
    my ($obj, $key) = @_;

    my $sector = $self->load_sector( $obj->_base_offset )
        or return;

    if ( $sector->staleness != $obj->_staleness ) {
        return;
    }

    return $sector->delete_key({
        key_md5    => $self->_apply_digest( $key ),
        allow_head => 0,
    });
}

sub write_value {
    my $self = shift;
    my ($obj, $key, $value) = @_;

    my $r = Scalar::Util::reftype( $value ) || '';
    {
        last if $r eq '';
        last if $r eq 'HASH';
        last if $r eq 'ARRAY';

        DBM::Deep->_throw_error(
            "Storage of references of type '$r' is not supported."
        );
    }

    # This will be a Reference sector
    my $sector = $self->load_sector( $obj->_base_offset )
        or DBM::Deep->_throw_error( "Cannot write to a deleted spot in DBM::Deep." );

    if ( $sector->staleness != $obj->_staleness ) {
        DBM::Deep->_throw_error( "Cannot write to a deleted spot in DBM::Deep." );
    }

    my ($class, $type);
    if ( !defined $value ) {
        $class = 'DBM::Deep::Sector::File::Null';
    }
    elsif ( ref $value eq 'DBM::Deep::Null' ) {
        DBM::Deep::_warnif(
             'uninitialized', 'Assignment of stale reference'
        );
        $class = 'DBM::Deep::Sector::File::Null';
        $value = undef;
    }
    elsif ( $r eq 'ARRAY' || $r eq 'HASH' ) {
        my $tmpvar;
        if ( $r eq 'ARRAY' ) {
            $tmpvar = tied @$value;
        } elsif ( $r eq 'HASH' ) {
            $tmpvar = tied %$value;
        }

        if ( $tmpvar ) {
            my $is_dbm_deep = eval { local $SIG{'__DIE__'}; $tmpvar->isa( 'DBM::Deep' ); };

            unless ( $is_dbm_deep ) {
                DBM::Deep->_throw_error( "Cannot store something that is tied." );
            }

            unless ( $tmpvar->_engine->storage == $self->storage ) {
                DBM::Deep->_throw_error( "Cannot store values across DBM::Deep files. Please use export() instead." );
            }

            # First, verify if we're storing the same thing to this spot. If we
            # are, then this should be a no-op. -EJS, 2008-05-19
            my $loc = $sector->get_data_location_for({
                key_md5 => $self->_apply_digest( $key ),
                allow_head => 1,
            });

            if ( defined($loc) && $loc == $tmpvar->_base_offset ) {
                return 1;
            }

            #XXX Can this use $loc?
            my $value_sector = $self->load_sector( $tmpvar->_base_offset );
            $sector->write_data({
                key     => $key,
                key_md5 => $self->_apply_digest( $key ),
                value   => $value_sector,
            });
            $value_sector->increment_refcount;

            return 1;
        }

        $class = 'DBM::Deep::Sector::File::Reference';
        $type = substr( $r, 0, 1 );
    }
    else {
        if ( tied($value) ) {
            DBM::Deep->_throw_error( "Cannot store something that is tied." );
        }
        $class = 'DBM::Deep::Sector::File::Scalar';
    }

    # Create this after loading the reference sector in case something bad
    # happens. This way, we won't allocate value sector(s) needlessly.
    my $value_sector = $class->new({
        engine => $self,
        data   => $value,
        type   => $type,
    });

    $sector->write_data({
        key     => $key,
        key_md5 => $self->_apply_digest( $key ),
        value   => $value_sector,
    });

    $self->_descend( $value, $value_sector );

    return 1;
}

sub setup {
    my $self = shift;
    my ($obj) = @_;

    # We're opening the file.
    unless ( $obj->_base_offset ) {
        my $bytes_read = $self->_read_file_header;

        # Creating a new file
        unless ( $bytes_read ) {
            $self->_write_file_header;

            # 1) Create Array/Hash entry
            my $initial_reference = DBM::Deep::Sector::File::Reference->new({
                engine => $self,
                type   => $obj->_type,
            });
            $obj->{base_offset} = $initial_reference->offset;
            $obj->{staleness} = $initial_reference->staleness;

            $self->storage->flush;
        }
        # Reading from an existing file
        else {
            $obj->{base_offset} = $bytes_read;
            my $initial_reference = DBM::Deep::Sector::File::Reference->new({
                engine => $self,
                offset => $obj->_base_offset,
            });
            unless ( $initial_reference ) {
                DBM::Deep->_throw_error("Corrupted file, no master index record");
            }

            unless ($obj->_type eq $initial_reference->type) {
                DBM::Deep->_throw_error("File type mismatch");
            }

            $obj->{staleness} = $initial_reference->staleness;
        }
    }

    $self->storage->set_inode;

    return 1;
}

sub begin_work {
    my $self = shift;
    my ($obj) = @_;

    if ( $self->trans_id ) {
        DBM::Deep->_throw_error( "Cannot begin_work within an active transaction" );
    }

    my @slots = $self->read_txn_slots;
    my $found;
    for my $i ( 0 .. $self->num_txns-2 ) {
        next if $slots[$i];

        $slots[$i] = 1;
        $self->set_trans_id( $i + 1 );
        $found = 1;
        last;
    }
    unless ( $found ) {
        DBM::Deep->_throw_error( "Cannot allocate transaction ID" );
    }
    $self->write_txn_slots( @slots );

    if ( !$self->trans_id ) {
        DBM::Deep->_throw_error( "Cannot begin_work - no available transactions" );
    }

    return;
}

sub rollback {
    my $self = shift;
    my ($obj) = @_;

    if ( !$self->trans_id ) {
        DBM::Deep->_throw_error( "Cannot rollback without an active transaction" );
    }

    # Each entry is the file location for a bucket that has a modification for
    # this transaction. The entries need to be expunged.
    foreach my $entry (@{ $self->get_entries } ) {
        # Remove the entry here
        my $read_loc = $entry
          + $self->hash_size
          + $self->byte_size
          + $self->byte_size
          + ($self->trans_id - 1) * ( $self->byte_size + $STALE_SIZE );

        my $data_loc = $self->storage->read_at( $read_loc, $self->byte_size );
        $data_loc = unpack( $StP{$self->byte_size}, $data_loc );
        $self->storage->print_at( $read_loc, pack( $StP{$self->byte_size}, 0 ) );

        if ( $data_loc > 1 ) {
            $self->load_sector( $data_loc )->free;
        }
    }

    $self->clear_entries;

    my @slots = $self->read_txn_slots;
    $slots[$self->trans_id-1] = 0;
    $self->write_txn_slots( @slots );
    $self->inc_txn_staleness_counter( $self->trans_id );
    $self->set_trans_id( 0 );

    return 1;
}

sub commit {
    my $self = shift;
    my ($obj) = @_;

    if ( !$self->trans_id ) {
        DBM::Deep->_throw_error( "Cannot commit without an active transaction" );
    }

    foreach my $entry (@{ $self->get_entries } ) {
        # Overwrite the entry in head with the entry in trans_id
        my $base = $entry
          + $self->hash_size
          + $self->byte_size;

        my $head_loc = $self->storage->read_at( $base, $self->byte_size );
        $head_loc = unpack( $StP{$self->byte_size}, $head_loc );

        my $spot = $base + $self->byte_size + ($self->trans_id - 1) * ( $self->byte_size + $STALE_SIZE );
        my $trans_loc = $self->storage->read_at(
            $spot, $self->byte_size,
        );

        $self->storage->print_at( $base, $trans_loc );
        $self->storage->print_at(
            $spot,
            pack( $StP{$self->byte_size} . ' ' . $StP{$STALE_SIZE}, (0) x 2 ),
        );

        if ( $head_loc > 1 ) {
            $self->load_sector( $head_loc )->free;
        }
    }

    $self->clear_entries;

    my @slots = $self->read_txn_slots;
    $slots[$self->trans_id-1] = 0;
    $self->write_txn_slots( @slots );
    $self->inc_txn_staleness_counter( $self->trans_id );
    $self->set_trans_id( 0 );

    return 1;
}

=head1 INTERNAL METHODS

The following methods are internal-use-only to DBM::Deep::Engine::File.

=cut

=head2 read_txn_slots()

This takes no arguments.

This will return an array with a 1 or 0 in each slot. Each spot represents one
available transaction. If the slot is 1, that transaction is taken. If it is 0,
the transaction is available.

=cut

sub read_txn_slots {
    my $self = shift;
    my $bl = $self->txn_bitfield_len;
    my $num_bits = $bl * 8;
    return split '', unpack( 'b'.$num_bits,
        $self->storage->read_at(
            $self->trans_loc, $bl,
        )
    );
}

=head2 write_txn_slots( @slots )

This takes an array of 1's and 0's. This array represents the transaction slots
returned by L</read_txn_slots()>. In other words, the following is true:

  @x = read_txn_slots( write_txn_slots( @x ) );

(With the obviously missing object referents added back in.)

=cut

sub write_txn_slots {
    my $self = shift;
    my $num_bits = $self->txn_bitfield_len * 8;
    $self->storage->print_at( $self->trans_loc,
        pack( 'b'.$num_bits, join('', @_) ),
    );
}

=head2 get_running_txn_ids()

This takes no arguments.

This will return an array of taken transaction IDs. This wraps L</read_txn_slots()>.

=cut

sub get_running_txn_ids {
    my $self = shift;
    my @transactions = $self->read_txn_slots;
    my @trans_ids = map { $_+1 } grep { $transactions[$_] } 0 .. $#transactions;
}

=head2 get_txn_staleness_counter( $trans_id )

This will return the staleness counter for the given transaction ID. Please see
L<DBM::Deep::Engine/STALENESS> for more information.

=cut

sub get_txn_staleness_counter {
    my $self = shift;
    my ($trans_id) = @_;

    # Hardcode staleness of 0 for the HEAD
    return 0 unless $trans_id;

    return unpack( $StP{$STALE_SIZE},
        $self->storage->read_at(
            $self->trans_loc + $self->txn_bitfield_len + $STALE_SIZE * ($trans_id - 1),
            $STALE_SIZE,
        )
    );
}

=head2 inc_txn_staleness_counter( $trans_id )

This will increment the staleness counter for the given transaction ID. Please see
L<DBM::Deep::Engine/STALENESS> for more information.

=cut

sub inc_txn_staleness_counter {
    my $self = shift;
    my ($trans_id) = @_;

    # Hardcode staleness of 0 for the HEAD
    return 0 unless $trans_id;

    $self->storage->print_at(
        $self->trans_loc + $self->txn_bitfield_len + $STALE_SIZE * ($trans_id - 1),
        pack( $StP{$STALE_SIZE}, $self->get_txn_staleness_counter( $trans_id ) + 1 ),
    );
}

=head2 get_entries()

This takes no arguments.

This returns a list of all the sectors that have been modified by this transaction.

=cut

sub get_entries {
    my $self = shift;
    return [ keys %{ $self->{entries}{$self->trans_id} ||= {} } ];
}

=head2 add_entry( $trans_id, $location )

This takes a transaction ID and a file location and marks the sector at that
location as having been modified by the transaction identified by $trans_id.

This returns nothing.

B<NOTE>: Unlike all the other _entries() methods, there are several cases where
C<< $trans_id != $self->trans_id >> for this method.

=cut

sub add_entry {
    my $self = shift;
    my ($trans_id, $loc) = @_;

    $self->{entries}{$trans_id} ||= {};
    $self->{entries}{$trans_id}{$loc} = undef;
}

=head2 reindex_entry( $old_loc, $new_loc )

This takes two locations (old and new, respectively). If a location that has
been modified by this transaction is subsequently reindexed due to a bucketlist
overflowing, then the entries hash needs to be made aware of this change.

This returns nothing.

=cut

sub reindex_entry {
    my $self = shift;
    my ($old_loc, $new_loc) = @_;

    TRANS:
    while ( my ($trans_id, $locs) = each %{ $self->{entries} } ) {
        if ( exists $locs->{$old_loc} ) {
            delete $locs->{$old_loc};
            $locs->{$new_loc} = undef;
            next TRANS;
        }
    }
}

=head2 clear_entries()

This takes no arguments. It will clear the entries list for the running
transaction.

This returns nothing.

=cut

sub clear_entries {
    my $self = shift;
    delete $self->{entries}{$self->trans_id};
}

=head2 _write_file_header()

This writes the file header for a new file. This will write the various settings
that set how the file is interpreted.

=head2 _read_file_header()

This reads the file header from an existing file. This will read the various
settings that set how the file is interpreted.

=cut

{
    my $header_fixed = length( __PACKAGE__->SIG_FILE ) + 1 + 4 + 4;
    my $this_file_version = 4;
    my $min_file_version = 3;

    sub _write_file_header {
        my $self = shift;

        my $nt = $self->num_txns;
        my $bl = $self->txn_bitfield_len;

        my $header_var = 1 + 1 + 1 + 1 + $bl + $STALE_SIZE * ($nt - 1) + 3 * $self->byte_size;

        my $loc = $self->storage->request_space( $header_fixed + $header_var );

        $self->storage->print_at( $loc,
            $self->SIG_FILE,
            $self->SIG_HEADER,
            pack('N', $this_file_version), # At this point, we're at 9 bytes
            pack('N', $header_var),        # header size
            # --- Above is $header_fixed. Below is $header_var
            pack('C', $self->byte_size),

            # These shenanigans are to allow a 256 within a C
            pack('C', $self->max_buckets - 1),
            pack('C', $self->data_sector_size - 1),

            pack('C', $nt),
            pack('C' . $bl, 0 ),                           # Transaction activeness bitfield
            pack($StP{$STALE_SIZE}.($nt-1), 0 x ($nt-1) ), # Transaction staleness counters
            pack($StP{$self->byte_size}, 0), # Start of free chain (blist size)
            pack($StP{$self->byte_size}, 0), # Start of free chain (data size)
            pack($StP{$self->byte_size}, 0), # Start of free chain (index size)
        );

        #XXX Set these less fragilely
        $self->set_trans_loc( $header_fixed + 4 );
        $self->set_chains_loc( $header_fixed + 4 + $bl + $STALE_SIZE * ($nt-1) );

        $self->{v} = $this_file_version;

        return;
    }

    sub _read_file_header {
        my $self = shift;

        my $buffer = $self->storage->read_at( 0, $header_fixed );
        return unless length($buffer);

        my ($file_signature, $sig_header, $file_version, $size) = unpack(
            'A4 A N N', $buffer
        );

        unless ( $file_signature eq $self->SIG_FILE ) {
            $self->storage->close;
            DBM::Deep->_throw_error( "Signature not found -- file is not a Deep DB" );
        }

        unless ( $sig_header eq $self->SIG_HEADER ) {
            $self->storage->close;
            DBM::Deep->_throw_error( "Pre-1.00 file version found" );
        }

        if ( $file_version < $min_file_version ) {
            $self->storage->close;
            DBM::Deep->_throw_error(
                "This file version is too old - "
              . _guess_version($file_version) .
                " - expected " . _guess_version($min_file_version)
              . " to " . _guess_version($this_file_version)
            );
        }
        if ( $file_version > $this_file_version ) {
            $self->storage->close;
            DBM::Deep->_throw_error(
                "This file version is too new - probably "
              . _guess_version($file_version) .
                " - expected " . _guess_version($min_file_version)
              . " to " . _guess_version($this_file_version)
            );
        }
        $self->{v} = $file_version;

        my $buffer2 = $self->storage->read_at( undef, $size );
        my @values = unpack( 'C C C C', $buffer2 );

        if ( @values != 4 || grep { !defined } @values ) {
            $self->storage->close;
            DBM::Deep->_throw_error("Corrupted file - bad header");
        }

        #XXX Add warnings if values weren't set right
        @{$self}{qw(byte_size max_buckets data_sector_size num_txns)} = @values;

        # These shenanigans are to allow a 256 within a C
        $self->{max_buckets} += 1;
        $self->{data_sector_size} += 1;

        my $bl = $self->txn_bitfield_len;

        my $header_var = scalar(@values) + $bl + $STALE_SIZE * ($self->num_txns - 1) + 3 * $self->byte_size;
        unless ( $size == $header_var ) {
            $self->storage->close;
            DBM::Deep->_throw_error( "Unexpected size found ($size <-> $header_var)." );
        }

        $self->set_trans_loc( $header_fixed + scalar(@values) );
        $self->set_chains_loc( $header_fixed + scalar(@values) + $bl + $STALE_SIZE * ($self->num_txns - 1) );

        return length($buffer) + length($buffer2);
    }

    sub _guess_version {
        $_[0] == 4 and return  2;
        $_[0] == 3 and return '1.0003';
        $_[0] == 2 and return '1.00';
        $_[0] == 1 and return '0.99';
        $_[0] == 0 and return '0.91';

        return $_[0]-2;
    }
}

=head2 _apply_digest( @stuff )

This will apply the digest method (default to Digest::MD5::md5) to the arguments
passed in and return the result.

=cut

sub _apply_digest {
    my $self = shift;
    my $victim = shift;
    utf8::encode $victim if $self->{v} >= 4;
    return $self->{digest}->($victim);
}

=head2 _add_free_blist_sector( $offset, $size )

=head2 _add_free_data_sector( $offset, $size )

=head2 _add_free_index_sector( $offset, $size )

These methods are all wrappers around _add_free_sector(), providing the proper
chain offset ($multiple) for the sector type.

=cut

sub _add_free_blist_sector { shift->_add_free_sector( 0, @_ ) }
sub _add_free_data_sector { shift->_add_free_sector( 1, @_ ) }
sub _add_free_index_sector { shift->_add_free_sector( 2, @_ ) }

=head2 _add_free_sector( $multiple, $offset, $size )

_add_free_sector() takes the offset into the chains location, the offset of the
sector, and the size of that sector. It will mark the sector as a free sector
and put it into the list of sectors that are free of this type for use later.

This returns nothing.

B<NOTE>: $size is unused?

=cut

sub _add_free_sector {
    my $self = shift;
    my ($multiple, $offset, $size) = @_;

    my $chains_offset = $multiple * $self->byte_size;

    my $storage = $self->storage;

    # Increment staleness.
    # XXX Can this increment+modulo be done by "&= 0x1" ?
    my $staleness = unpack( $StP{$STALE_SIZE}, $storage->read_at( $offset + $self->SIG_SIZE, $STALE_SIZE ) );
    $staleness = ($staleness + 1 ) % ( 2 ** ( 8 * $STALE_SIZE ) );
    $storage->print_at( $offset + $self->SIG_SIZE, pack( $StP{$STALE_SIZE}, $staleness ) );

    my $old_head = $storage->read_at( $self->chains_loc + $chains_offset, $self->byte_size );

    $storage->print_at( $self->chains_loc + $chains_offset,
        pack( $StP{$self->byte_size}, $offset ),
    );

    # Record the old head in the new sector after the signature and staleness counter
    $storage->print_at( $offset + $self->SIG_SIZE + $STALE_SIZE, $old_head );
}

=head2 _request_blist_sector( $size )

=head2 _request_data_sector( $size )

=head2 _request_index_sector( $size )

These methods are all wrappers around _request_sector(), providing the proper
chain offset ($multiple) for the sector type.

=cut

sub _request_blist_sector { shift->_request_sector( 0, @_ ) }
sub _request_data_sector { shift->_request_sector( 1, @_ ) }
sub _request_index_sector { shift->_request_sector( 2, @_ ) }

=head2 _request_sector( $multiple $size )

This takes the offset into the chains location and the size of that sector.

This returns the object with the sector. If there is an available free sector of
that type, then it will be reused. If there isn't one, then a new one will be
allocated.

=cut

sub _request_sector {
    my $self = shift;
    my ($multiple, $size) = @_;

    my $chains_offset = $multiple * $self->byte_size;

    my $old_head = $self->storage->read_at( $self->chains_loc + $chains_offset, $self->byte_size );
    my $loc = unpack( $StP{$self->byte_size}, $old_head );

    # We don't have any free sectors of the right size, so allocate a new one.
    unless ( $loc ) {
        my $offset = $self->storage->request_space( $size );

        # Zero out the new sector. This also guarantees correct increases
        # in the filesize.
        $self->storage->print_at( $offset, chr(0) x $size );

        return $offset;
    }

    # Read the new head after the signature and the staleness counter
    my $new_head = $self->storage->read_at( $loc + $self->SIG_SIZE + $STALE_SIZE, $self->byte_size );
    $self->storage->print_at( $self->chains_loc + $chains_offset, $new_head );
    $self->storage->print_at(
        $loc + $self->SIG_SIZE + $STALE_SIZE,
        pack( $StP{$self->byte_size}, 0 ),
    );

    return $loc;
}

=head2 ACCESSORS

The following are readonly attributes.

=over 4

=item * byte_size

=item * hash_size

=item * hash_chars

=item * num_txns

=item * max_buckets

=item * blank_md5

=item * data_sector_size

=item * txn_bitfield_len

=back

=cut

sub byte_size   { $_[0]{byte_size} }
sub hash_size   { $_[0]{hash_size} }
sub hash_chars  { $_[0]{hash_chars} }
sub num_txns    { $_[0]{num_txns} }
sub max_buckets { $_[0]{max_buckets} }
sub blank_md5   { chr(0) x $_[0]->hash_size }
sub data_sector_size { $_[0]{data_sector_size} }

# This is a calculated value
sub txn_bitfield_len {
    my $self = shift;
    unless ( exists $self->{txn_bitfield_len} ) {
        my $temp = ($self->num_txns) / 8;
        if ( $temp > int( $temp ) ) {
            $temp = int( $temp ) + 1;
        }
        $self->{txn_bitfield_len} = $temp;
    }
    return $self->{txn_bitfield_len};
}

=pod

The following are read/write attributes. 

=over 4

=item * trans_id / set_trans_id( $new_id )

=item * trans_loc / set_trans_loc( $new_loc )

=item * chains_loc / set_chains_loc( $new_loc )

=back

=cut

sub trans_id     { $_[0]{trans_id} }
sub set_trans_id { $_[0]{trans_id} = $_[1] }

sub trans_loc     { $_[0]{trans_loc} }
sub set_trans_loc { $_[0]{trans_loc} = $_[1] }

sub chains_loc     { $_[0]{chains_loc} }
sub set_chains_loc { $_[0]{chains_loc} = $_[1] }

sub supports {
    shift;
    my ($feature) = @_;

    return 1 if $feature =~ /^(?:(?:transacti|singlet)ons|unicode)\z/;
    return;
}

sub db_version {
    return $_[0]{v} == 3 ? '1.0003' : 2;
}

sub clear {
    my $self = shift;
    my $obj = shift;

    my $sector = $self->load_sector( $obj->_base_offset )
        or return;

    return unless $sector->staleness == $obj->_staleness;

    $sector->clear;

    return;
}

=head2 _dump_file()

This method takes no arguments. It's used to print out a textual representation
of the DBM::Deep DB file. It assumes the file is not-corrupted.

=cut

sub _dump_file {
    my $self = shift;

    # Read the header
    my $spot = $self->_read_file_header();

    my %types = (
        0 => 'B',
        1 => 'D',
        2 => 'I',
    );

    my %sizes = (
        'D' => $self->data_sector_size,
        'B' => DBM::Deep::Sector::File::BucketList->new({engine=>$self,offset=>1})->size,
        'I' => DBM::Deep::Sector::File::Index->new({engine=>$self,offset=>1})->size,
    );

    my $return = "";

    # Header values
    $return .= "NumTxns: " . $self->num_txns . $/;

    # Read the free sector chains
    my %sectors;
    foreach my $multiple ( 0 .. 2 ) {
        $return .= "Chains($types{$multiple}):";
        my $old_loc = $self->chains_loc + $multiple * $self->byte_size;
        while ( 1 ) {
            my $loc = unpack(
                $StP{$self->byte_size},
                $self->storage->read_at( $old_loc, $self->byte_size ),
            );

            # We're now out of free sectors of this kind.
            unless ( $loc ) {
                last;
            }

            $sectors{ $types{$multiple} }{ $loc } = undef;
            $old_loc = $loc + $self->SIG_SIZE + $STALE_SIZE;
            $return .= " $loc";
        }
        $return .= $/;
    }

    SECTOR:
    while ( $spot < $self->storage->{end} ) {
        # Read each sector in order.
        my $sector = $self->load_sector( $spot );
        if ( !$sector ) {
            # Find it in the free-sectors that were found already
            foreach my $type ( keys %sectors ) {
                if ( exists $sectors{$type}{$spot} ) {
                    my $size = $sizes{$type};
                    $return .= sprintf "%08d: %s %04d\n", $spot, 'F' . $type, $size;
                    $spot += $size;
                    next SECTOR;
                }
            }

            die "********\n$return\nDidn't find free sector for $spot in chains\n********\n";
        }
        else {
            $return .= sprintf "%08d: %s  %04d", $spot, $sector->type, $sector->size;
            if ( $sector->type =~ /^[DU]\z/ ) {
                $return .= ' ' . $sector->data;
            }
            elsif ( $sector->type eq 'A' || $sector->type eq 'H' ) {
                $return .= ' REF: ' . $sector->get_refcount;
            }
            elsif ( $sector->type eq 'B' ) {
                foreach my $bucket ( $sector->chopped_up ) {
                    $return .= "\n    ";
                    $return .= sprintf "%08d", unpack($StP{$self->byte_size},
                        substr( $bucket->[-1], $self->hash_size, $self->byte_size),
                    );
                    my $l = unpack( $StP{$self->byte_size},
                        substr( $bucket->[-1],
                            $self->hash_size + $self->byte_size,
                            $self->byte_size,
                        ),
                    );
                    $return .= sprintf " %08d", $l;
                    foreach my $txn ( 0 .. $self->num_txns - 2 ) {
                        my $l = unpack( $StP{$self->byte_size},
                            substr( $bucket->[-1],
                                $self->hash_size + 2 * $self->byte_size + $txn * ($self->byte_size + $STALE_SIZE),
                                $self->byte_size,
                            ),
                        );
                        $return .= sprintf " %08d", $l;
                    }
                }
            }
            $return .= $/;

            $spot += $sector->size;
        }
    }

    return $return;
}

1;
__END__