attempt again to const vtables in threads-shared

[perl5.git] / dist / threads-shared / lib / threads / shared.pm

package threads::shared;

use 5.008;

use strict;
use warnings;

use Scalar::Util qw(reftype refaddr blessed);

our $VERSION = '1.48'; # Please update the pod, too.
my $XS_VERSION = $VERSION;
$VERSION = eval $VERSION;

# Declare that we have been loaded
$threads::shared::threads_shared = 1;

# Method of complaint about things we can't clone
$threads::shared::clone_warn = undef;

# Load the XS code, if applicable
if ($threads::threads) {
    require XSLoader;
    XSLoader::load('threads::shared', $XS_VERSION);

    *is_shared = \&_id;

} else {
    # String eval is generally evil, but we don't want these subs to
    # exist at all if 'threads' is not loaded successfully.
    # Vivifying them conditionally this way saves on average about 4K
    # of memory per thread.
    eval <<'_MARKER_';
        sub share          (\[$@%])         { return $_[0] }
        sub is_shared      (\[$@%])         { undef }
        sub cond_wait      (\[$@%];\[$@%])  { undef }
        sub cond_timedwait (\[$@%]$;\[$@%]) { undef }
        sub cond_signal    (\[$@%])         { undef }
        sub cond_broadcast (\[$@%])         { undef }
_MARKER_
}


### Export ###

sub import
{
    # Exported subroutines
    my @EXPORT = qw(share is_shared cond_wait cond_timedwait
                    cond_signal cond_broadcast shared_clone);
    if ($threads::threads) {
        push(@EXPORT, 'bless');
    }

    # Export subroutine names
    my $caller = caller();
    foreach my $sym (@EXPORT) {
        no strict 'refs';
        *{$caller.'::'.$sym} = \&{$sym};
    }
}


# Predeclarations for internal functions
my ($make_shared);


### Methods, etc. ###

sub threads::shared::tie::SPLICE
{
    require Carp;
    Carp::croak('Splice not implemented for shared arrays');
}


# Create a thread-shared clone of a complex data structure or object
sub shared_clone
{
    if (@_ != 1) {
        require Carp;
        Carp::croak('Usage: shared_clone(REF)');
    }

    return $make_shared->(shift, {});
}


### Internal Functions ###

# Used by shared_clone() to recursively clone
#   a complex data structure or object
$make_shared = sub {
    my ($item, $cloned) = @_;

    # Just return the item if:
    # 1. Not a ref;
    # 2. Already shared; or
    # 3. Not running 'threads'.
    return $item if (! ref($item) || is_shared($item) || ! $threads::threads);

    # Check for previously cloned references
    #   (this takes care of circular refs as well)
    my $addr = refaddr($item);
    if (exists($cloned->{$addr})) {
        # Return the already existing clone
        return $cloned->{$addr};
    }

    # Make copies of array, hash and scalar refs and refs of refs
    my $copy;
    my $ref_type = reftype($item);

    # Copy an array ref
    if ($ref_type eq 'ARRAY') {
        # Make empty shared array ref
        $copy = &share([]);
        # Add to clone checking hash
        $cloned->{$addr} = $copy;
        # Recursively copy and add contents
        push(@$copy, map { $make_shared->($_, $cloned) } @$item);
    }

    # Copy a hash ref
    elsif ($ref_type eq 'HASH') {
        # Make empty shared hash ref
        $copy = &share({});
        # Add to clone checking hash
        $cloned->{$addr} = $copy;
        # Recursively copy and add contents
        foreach my $key (keys(%{$item})) {
            $copy->{$key} = $make_shared->($item->{$key}, $cloned);
        }
    }

    # Copy a scalar ref
    elsif ($ref_type eq 'SCALAR') {
        $copy = \do{ my $scalar = $$item; };
        share($copy);
        # Add to clone checking hash
        $cloned->{$addr} = $copy;
    }

    # Copy of a ref of a ref
    elsif ($ref_type eq 'REF') {
        # Special handling for $x = \$x
        if ($addr == refaddr($$item)) {
            $copy = \$copy;
            share($copy);
            $cloned->{$addr} = $copy;
        } else {
            my $tmp;
            $copy = \$tmp;
            share($copy);
            # Add to clone checking hash
            $cloned->{$addr} = $copy;
            # Recursively copy and add contents
            $tmp = $make_shared->($$item, $cloned);
        }

    } else {
        require Carp;
        if (! defined($threads::shared::clone_warn)) {
            Carp::croak("Unsupported ref type: ", $ref_type);
        } elsif ($threads::shared::clone_warn) {
            Carp::carp("Unsupported ref type: ", $ref_type);
        }
        return undef;
    }

    # If input item is an object, then bless the copy into the same class
    if (my $class = blessed($item)) {
        bless($copy, $class);
    }

    # Clone READONLY flag
    if ($ref_type eq 'SCALAR') {
        if (Internals::SvREADONLY($$item)) {
            Internals::SvREADONLY($$copy, 1) if ($] >= 5.008003);
        }
    }
    if (Internals::SvREADONLY($item)) {
        Internals::SvREADONLY($copy, 1) if ($] >= 5.008003);
    }

    return $copy;
};

1;

__END__

=head1 NAME

threads::shared - Perl extension for sharing data structures between threads

=head1 VERSION

This document describes threads::shared version 1.48

=head1 SYNOPSIS

  use threads;
  use threads::shared;

  my $var :shared;
  my %hsh :shared;
  my @ary :shared;

  my ($scalar, @array, %hash);
  share($scalar);
  share(@array);
  share(%hash);

  $var = $scalar_value;
  $var = $shared_ref_value;
  $var = shared_clone($non_shared_ref_value);
  $var = shared_clone({'foo' => [qw/foo bar baz/]});

  $hsh{'foo'} = $scalar_value;
  $hsh{'bar'} = $shared_ref_value;
  $hsh{'baz'} = shared_clone($non_shared_ref_value);
  $hsh{'quz'} = shared_clone([1..3]);

  $ary[0] = $scalar_value;
  $ary[1] = $shared_ref_value;
  $ary[2] = shared_clone($non_shared_ref_value);
  $ary[3] = shared_clone([ {}, [] ]);

  { lock(%hash); ...  }

  cond_wait($scalar);
  cond_timedwait($scalar, time() + 30);
  cond_broadcast(@array);
  cond_signal(%hash);

  my $lockvar :shared;
  # condition var != lock var
  cond_wait($var, $lockvar);
  cond_timedwait($var, time()+30, $lockvar);

=head1 DESCRIPTION

By default, variables are private to each thread, and each newly created
thread gets a private copy of each existing variable.  This module allows you
to share variables across different threads (and pseudo-forks on Win32).  It
is used together with the L<threads> module.

This module supports the sharing of the following data types only:  scalars
and scalar refs, arrays and array refs, and hashes and hash refs.

=head1 EXPORT

The following functions are exported by this module: C<share>,
C<shared_clone>, C<is_shared>, C<cond_wait>, C<cond_timedwait>, C<cond_signal>
and C<cond_broadcast>

Note that if this module is imported when L<threads> has not yet been loaded,
then these functions all become no-ops.  This makes it possible to write
modules that will work in both threaded and non-threaded environments.

=head1 FUNCTIONS

=over 4

=item share VARIABLE

C<share> takes a variable and marks it as shared:

  my ($scalar, @array, %hash);
  share($scalar);
  share(@array);
  share(%hash);

C<share> will return the shared rvalue, but always as a reference.

Variables can also be marked as shared at compile time by using the
C<:shared> attribute:

  my ($var, %hash, @array) :shared;

Shared variables can only store scalars, refs of shared variables, or
refs of shared data (discussed in next section):

  my ($var, %hash, @array) :shared;
  my $bork;

  # Storing scalars
  $var = 1;
  $hash{'foo'} = 'bar';
  $array[0] = 1.5;

  # Storing shared refs
  $var = \%hash;
  $hash{'ary'} = \@array;
  $array[1] = \$var;

  # The following are errors:
  #   $var = \$bork;                    # ref of non-shared variable
  #   $hash{'bork'} = [];               # non-shared array ref
  #   push(@array, { 'x' => 1 });       # non-shared hash ref

=item shared_clone REF

C<shared_clone> takes a reference, and returns a shared version of its
argument, performing a deep copy on any non-shared elements.  Any shared
elements in the argument are used as is (i.e., they are not cloned).

  my $cpy = shared_clone({'foo' => [qw/foo bar baz/]});

Object status (i.e., the class an object is blessed into) is also cloned.

  my $obj = {'foo' => [qw/foo bar baz/]};
  bless($obj, 'Foo');
  my $cpy = shared_clone($obj);
  print(ref($cpy), "\n");         # Outputs 'Foo'

For cloning empty array or hash refs, the following may also be used:

  $var = &share([]);   # Same as $var = shared_clone([]);
  $var = &share({});   # Same as $var = shared_clone({});

Not all Perl data types can be cloned (e.g., globs, code refs).  By default,
C<shared_clone> will L<croak|Carp> if it encounters such items.  To change
this behaviour to a warning, then set the following:

  $threads::shared::clone_warn = 1;

In this case, C<undef> will be substituted for the item to be cloned.  If
set to zero:

  $threads::shared::clone_warn = 0;

then the C<undef> substitution will be performed silently.

=item is_shared VARIABLE

C<is_shared> checks if the specified variable is shared or not.  If shared,
returns the variable's internal ID (similar to
C<refaddr()> (see L<Scalar::Util>).  Otherwise, returns C<undef>.

  if (is_shared($var)) {
      print("\$var is shared\n");
  } else {
      print("\$var is not shared\n");
  }

When used on an element of an array or hash, C<is_shared> checks if the
specified element belongs to a shared array or hash.  (It does not check
the contents of that element.)

  my %hash :shared;
  if (is_shared(%hash)) {
      print("\%hash is shared\n");
  }

  $hash{'elem'} = 1;
  if (is_shared($hash{'elem'})) {
      print("\$hash{'elem'} is in a shared hash\n");
  }

=item lock VARIABLE

C<lock> places a B<advisory> lock on a variable until the lock goes out of
scope.  If the variable is locked by another thread, the C<lock> call will
block until it's available.  Multiple calls to C<lock> by the same thread from
within dynamically nested scopes are safe -- the variable will remain locked
until the outermost lock on the variable goes out of scope.

C<lock> follows references exactly I<one> level:

  my %hash :shared;
  my $ref = \%hash;
  lock($ref);           # This is equivalent to lock(%hash)

Note that you cannot explicitly unlock a variable; you can only wait for the
lock to go out of scope.  This is most easily accomplished by locking the
variable inside a block.

  my $var :shared;
  {
      lock($var);
      # $var is locked from here to the end of the block
      ...
  }
  # $var is now unlocked

As locks are advisory, they do not prevent data access or modification by
another thread that does not itself attempt to obtain a lock on the variable.

You cannot lock the individual elements of a container variable:

  my %hash :shared;
  $hash{'foo'} = 'bar';
  #lock($hash{'foo'});          # Error
  lock(%hash);                  # Works

If you need more fine-grained control over shared variable access, see
L<Thread::Semaphore>.

=item cond_wait VARIABLE

=item cond_wait CONDVAR, LOCKVAR

The C<cond_wait> function takes a B<locked> variable as a parameter, unlocks
the variable, and blocks until another thread does a C<cond_signal> or
C<cond_broadcast> for that same locked variable.  The variable that
C<cond_wait> blocked on is re-locked after the C<cond_wait> is satisfied.  If
there are multiple threads C<cond_wait>ing on the same variable, all but one
will re-block waiting to reacquire the
lock on the variable.  (So if you're only
using C<cond_wait> for synchronization, give up the lock as soon as possible).
The two actions of unlocking the variable and entering the blocked wait state
are atomic, the two actions of exiting from the blocked wait state and
re-locking the variable are not.

In its second form, C<cond_wait> takes a shared, B<unlocked> variable followed
by a shared, B<locked> variable.  The second variable is unlocked and thread
execution suspended until another thread signals the first variable.

It is important to note that the variable can be notified even if no thread
C<cond_signal> or C<cond_broadcast> on the variable.  It is therefore
important to check the value of the variable and go back to waiting if the
requirement is not fulfilled.  For example, to pause until a shared counter
drops to zero:

  { lock($counter); cond_wait($counter) until $counter == 0; }

=item cond_timedwait VARIABLE, ABS_TIMEOUT

=item cond_timedwait CONDVAR, ABS_TIMEOUT, LOCKVAR

In its two-argument form, C<cond_timedwait> takes a B<locked> variable and an
absolute timeout in I<epoch> seconds (see L<time() in perlfunc|perlfunc/time>
for more) as parameters, unlocks the variable, and blocks until the
timeout is reached or another thread signals the variable.  A false value is
returned if the timeout is reached, and a true value otherwise.  In either
case, the variable is re-locked upon return.

Like C<cond_wait>, this function may take a shared, B<locked> variable as an
additional parameter; in this case the first parameter is an B<unlocked>
condition variable protected by a distinct lock variable.

Again like C<cond_wait>, waking up and reacquiring the lock are not atomic,
and you should always check your desired condition after this function
returns.  Since the timeout is an absolute value, however, it does not have to
be recalculated with each pass:

  lock($var);
  my $abs = time() + 15;
  until ($ok = desired_condition($var)) {
      last if !cond_timedwait($var, $abs);
  }
  # we got it if $ok, otherwise we timed out!

=item cond_signal VARIABLE

The C<cond_signal> function takes a B<locked> variable as a parameter and
unblocks one thread that's C<cond_wait>ing
on that variable.  If more than one
thread is blocked in a C<cond_wait> on that variable, only one (and which one
is indeterminate) will be unblocked.

If there are no threads blocked in a C<cond_wait> on the variable, the signal
is discarded.  By always locking before
signaling, you can (with care), avoid
signaling before another thread has entered cond_wait().

C<cond_signal> will normally generate a warning if you attempt to use it on an
unlocked variable.  On the rare occasions
where doing this may be sensible, you
can suppress the warning with:

  { no warnings 'threads'; cond_signal($foo); }

=item cond_broadcast VARIABLE

The C<cond_broadcast> function works similarly to C<cond_signal>.
C<cond_broadcast>, though, will unblock B<all> the threads that are blocked in
a C<cond_wait> on the locked variable, rather than only one.

=back

=head1 OBJECTS

L<threads::shared> exports a version of L<bless()|perlfunc/"bless REF"> that
works on shared objects such that I<blessings> propagate across threads.

  # Create a shared 'Foo' object
  my $foo :shared = shared_clone({});
  bless($foo, 'Foo');

  # Create a shared 'Bar' object
  my $bar :shared = shared_clone({});
  bless($bar, 'Bar');

  # Put 'bar' inside 'foo'
  $foo->{'bar'} = $bar;

  # Rebless the objects via a thread
  threads->create(sub {
      # Rebless the outer object
      bless($foo, 'Yin');

      # Cannot directly rebless the inner object
      #bless($foo->{'bar'}, 'Yang');

      # Retrieve and rebless the inner object
      my $obj = $foo->{'bar'};
      bless($obj, 'Yang');
      $foo->{'bar'} = $obj;

  })->join();

  print(ref($foo),          "\n");    # Prints 'Yin'
  print(ref($foo->{'bar'}), "\n");    # Prints 'Yang'
  print(ref($bar),          "\n");    # Also prints 'Yang'

=head1 NOTES

L<threads::shared> is designed to disable itself silently if threads are not
available.  This allows you to write modules and packages that can be used
in both threaded and non-threaded applications.

If you want access to threads, you must C<use threads> before you
C<use threads::shared>.  L<threads> will emit a warning if you use it after
L<threads::shared>.

=head1 WARNINGS

=over 4

=item cond_broadcast() called on unlocked variable

=item cond_signal() called on unlocked variable

See L</"cond_signal VARIABLE">, above.

=back

=head1 BUGS AND LIMITATIONS

When C<share> is used on arrays, hashes, array refs or hash refs, any data
they contain will be lost.

  my @arr = qw(foo bar baz);
  share(@arr);
  # @arr is now empty (i.e., == ());

  # Create a 'foo' object
  my $foo = { 'data' => 99 };
  bless($foo, 'foo');

  # Share the object
  share($foo);        # Contents are now wiped out
  print("ERROR: \$foo is empty\n")
      if (! exists($foo->{'data'}));

Therefore, populate such variables B<after> declaring them as shared.  (Scalar
and scalar refs are not affected by this problem.)

It is often not wise to share an object unless the class itself has been
written to support sharing.  For example, an object's destructor may get
called multiple times, once for each thread's scope exit.  Another danger is
that the contents of hash-based objects will be lost due to the above
mentioned limitation.  See F<examples/class.pl> (in the CPAN distribution of
this module) for how to create a class that supports object sharing.

Destructors may not be called on objects if those objects still exist at
global destruction time.  If the destructors must be called, make sure
there are no circular references and that nothing is referencing the
objects, before the program ends.

Does not support C<splice> on arrays.  Does not support explicitly changing
array lengths via $#array -- use C<push> and C<pop> instead.

Taking references to the elements of shared arrays and hashes does not
autovivify the elements, and neither does slicing a shared array/hash over
non-existent indices/keys autovivify the elements.

C<share()> allows you to C<< share($hashref->{key}) >> and
C<< share($arrayref->[idx]) >> without giving any error message.  But the
C<< $hashref->{key} >> or C<< $arrayref->[idx] >> is B<not> shared, causing
the error "lock can only be used on shared values" to occur when you attempt
to C<< lock($hashref->{key}) >> or C<< lock($arrayref->[idx]) >> in another
thread.

Using C<refaddr()> is unreliable for testing
whether or not two shared references are equivalent (e.g., when testing for
circular references).  Use L<is_shared()|/"is_shared VARIABLE">, instead:

    use threads;
    use threads::shared;
    use Scalar::Util qw(refaddr);

    # If ref is shared, use threads::shared's internal ID.
    # Otherwise, use refaddr().
    my $addr1 = is_shared($ref1) || refaddr($ref1);
    my $addr2 = is_shared($ref2) || refaddr($ref2);

    if ($addr1 == $addr2) {
        # The refs are equivalent
    }

L<each()|perlfunc/"each HASH"> does not work properly on shared references
embedded in shared structures.  For example:

    my %foo :shared;
    $foo{'bar'} = shared_clone({'a'=>'x', 'b'=>'y', 'c'=>'z'});

    while (my ($key, $val) = each(%{$foo{'bar'}})) {
        ...
    }

Either of the following will work instead:

    my $ref = $foo{'bar'};
    while (my ($key, $val) = each(%{$ref})) {
        ...
    }

    foreach my $key (keys(%{$foo{'bar'}})) {
        my $val = $foo{'bar'}{$key};
        ...
    }

This module supports dual-valued variables created using C<dualvar()> from
L<Scalar::Util>.  However, while C<$!> acts
like a dualvar, it is implemented as a tied SV.  To propagate its value, use
the follow construct, if needed:

    my $errno :shared = dualvar($!,$!);

View existing bug reports at, and submit any new bugs, problems, patches, etc.
to: L<http://rt.cpan.org/Public/Dist/Display.html?Name=threads-shared>

=head1 SEE ALSO

L<threads::shared> Discussion Forum on CPAN:
L<http://www.cpanforum.com/dist/threads-shared>

L<threads>, L<perlthrtut>

L<http://www.perl.com/pub/a/2002/06/11/threads.html> and
L<http://www.perl.com/pub/a/2002/09/04/threads.html>

Perl threads mailing list:
L<http://lists.perl.org/list/ithreads.html>

=head1 AUTHOR

Artur Bergman E<lt>sky AT crucially DOT netE<gt>

Documentation borrowed from the old Thread.pm.

CPAN version produced by Jerry D. Hedden E<lt>jdhedden AT cpan DOT orgE<gt>.

=head1 LICENSE

threads::shared is released under the same license as Perl.

=cut