package System::Command;

use warnings;
use strict;
use 5.006;

use Carp;
use Cwd qw( cwd );
use IO::Handle;
use IPC::Open3 qw( open3 );
use List::Util qw( reduce );

use POSIX ":sys_wait_h";
use constant STATUS  => qw( exit signal core );

our $VERSION = '1.07';

our $QUIET = 0;

sub import {
    my ( $class, @args ) = @_;
    my %arg = ( quiet => sub { $QUIET = 1 } );
    for my $arg (@args) {
        $arg =~ s/^-//;    # allow dashed options
        croak "Unknown option '$arg' in 'use System::Command'"
            if !exists $arg{$arg};
        $arg{$arg}->();
    }
}

# a few simple accessors
for my $attr (qw( pid stdin stdout stderr exit signal core options )) {
    no strict 'refs';
    *$attr = sub { return $_[0]{$attr} };
}
for my $attr (qw( cmdline )) {
    no strict 'refs';
    *$attr = sub { return @{ $_[0]{$attr} } };
}

# a private sub-process spawning function
my $_seq   = 0;
my $_spawn = sub {
    my (@cmd) = @_;
    my ( $pid, $in, $out, $err );

    # setup filehandles
    {
        no strict 'refs';
        $in  = \do { local *{"IN$_seq"} };
        $out = \do { local *{"OUT$_seq"} };
        $err = \do { local *{"ERR$_seq"} };
        $_seq++;
    }

    # start the command
    $pid = eval { open3( $in, $out, $err, @cmd ); };

    return ( $pid, $in, $out, $err );
};

# module methods
sub new {
    my ( $class, @cmd ) = @_;

    # split the args
    my @o;
    @cmd = grep { !( ref eq 'HASH' ? push @o, $_ : 0 ) } @cmd;

    # merge the option hashes
    my $o = reduce {
        +{  %$a, %$b,
            exists $a->{env} && exists $b->{env}
            ? ( env => { %{ $a->{env} }, %{ $b->{env} } } )
            : ()
        };
    }
    @o;

    # chdir to the expected directory
    my $orig = cwd;
    my $dest = defined $o->{cwd} ? $o->{cwd} : undef;
    if ( defined $dest ) {
        chdir $dest or croak "Can't chdir to $dest: $!";
    }

    # keep changes to the environment local
    local %ENV = %ENV;

    # update the environment
    if ( exists $o->{env} ) {
        @ENV{ keys %{ $o->{env} } } = values %{ $o->{env} };
        delete $ENV{$_}
            for grep { !defined $o->{env}{$_} } keys %{ $o->{env} };
    }

    # start the command
    my ( $pid, $in, $out, $err ) = eval { $_spawn->(@cmd); };

    # FIXME - better check error conditions
    croak $@ if !defined $pid;

    # some input was provided
    if ( defined $o->{input} ) {
        local $SIG{PIPE}
            = sub { croak "Broken pipe when writing to: @cmd" };
        print {$in} $o->{input} if length $o->{input};
        $in->close;
    }

    # chdir back to origin
    if ( defined $dest ) {
        chdir $orig or croak "Can't chdir back to $orig: $!";
    }

    # create the object
    my $self = bless {
        cmdline => [ @cmd ],
        options => $o,
        pid     => $pid,
        stdin   => $in,
        stdout  => $out,
        stderr  => $err,
    }, $class;

    return $self;
}

sub spawn {
    my ( $class, @cmd ) = @_;
    return @{ $class->new(@cmd) }{qw( pid stdin stdout stderr )};
}

sub is_terminated {
    my ($self) = @_;
    my $pid = $self->{pid};

    # Zed's dead, baby. Zed's dead.
    return $pid if !kill 0, $pid and exists $self->{exit};

    # If that is a re-animated body, we're gonna have to kill it.
    return $self->_reap(WNOHANG);
}

sub _reap {
    my ( $self, @flags ) = @_;
    my $pid = $self->{pid};

    if ( my $reaped = waitpid( $pid, @flags ) and !exists $self->{exit} ) {
        my $zed = $reaped == $pid;
        carp "Child process already reaped, check for a SIGCHLD handler"
            if !$zed && !$QUIET;

        @{$self}{ STATUS() }
            = $zed
            ? ( $? >> 8, $? & 127, $? & 128 )
            : ( -1, -1, -1 );

        return $reaped;    # It's dead, Jim!
    }

    # Look! It's moving. It's alive. It's alive...
    return;
}

sub close {
    my ($self) = @_;

    # close all pipes
    my ( $in, $out, $err ) = @{$self}{qw( stdin stdout stderr )};
    $in  and $in->opened  and $in->close  || carp "error closing stdin: $!";
    $out and $out->opened and $out->close || carp "error closing stdout: $!";
    $err and $err->opened and $err->close || carp "error closing stderr: $!";

    # and wait for the child (if any)
    $self->_reap();

    return $self;
}

1;

__END__

=head1 NAME

System::Command - Object for running system commands

=head1 SYNOPSIS

    use System::Command;

    # invoke an external command, and return an object
    $cmd = System::Command->new( @cmd );

    # options can be passed as a hashref
    $cmd = System::Command->new( @cmd, \%option );

    # $cmd is basically a hash, with keys / accessors
    $cmd->stdin();     # filehandle to the process' stdin (write)
    $cmd->stdout();    # filehandle to the process' stdout (read)
    $cmd->stderr();    # filehandle to the process' stdout (read)
    $cmd->pid();       # pid of the child process

    # find out if the child process died
    if ( $cmd->is_terminated() ) {
        # the handles are not closed yet
        # but $cmd->exit() et al. are available
    }

    # done!
    $cmd->close();

    # exit information
    $cmd->exit();      # exit status
    $cmd->signal();    # signal
    $cmd->core();      # core dumped? (boolean)

    # cut to the chase
    my ( $pid, $in, $out, $err ) = System::Command->spawn(@cmd);

=head1 DESCRIPTION

C<System::Command> is a class that launches external system commands
and return an object representing them, allowing to interact with them
through their C<STDIN>, C<STDOUT> and C<STDERR> handles.

=head1 METHODS

C<System::Command> supports the following methods:

=head2 new( @cmd )

Runs an external command using the list in C<@cmd>.

If C<@cmd> contains a hash reference, it is taken as an I<option> hash.

The recognized keys are:

=over 4

=item C<cwd>

The I<current working directory> in which the command will be run.

=item C<env>

A hashref containing key / values to add to the command environment.

If a value is C<undef>, the variable corresponding to the key will
be I<removed> from the environment.

=item C<input>

A string that is send to the command's standard input, which is then closed.

Using the empty string as C<input> will close the command's standard input
without writing to it.

Using C<undef> as C<input> will not do anything. This behaviour provides
a way to modify previous options populated by some other part of the program.

On some systems, some commands may close standard input on startup,
which will cause a SIGPIPE when trying to write to it. This will raise
an exception.

=back

If several option hashes are passed to C<new()>, they will be merged
together with individual values being overridden by those (with the same
key) from hashes that appear later in the list.

The C<System::Command> object returned by C<new()> has a number of
attributes defined (see below).


=head2 close()

Close all pipes to the child process, collects exit status, etc.
and defines a number of attributes (see below).

=head2 is_terminated()

Returns a true value if the underlying process was terminated.

If the process was indeed terminated, collects exit status, etc.
and defines the same attributes as C<close()>, but does B<not> close
all pipes to the child process,


=head2 spawn( @cmd )

This shortcut method calls C<new()> (and so accepts options in the same
manner) and directly returns the C<pid>, C<stdin>, C<stdout> and C<stderr>
attributes, in that order.


=head2 Accessors

The attributes of a C<System::Command> object are also accessible
through a number of accessors.

The object returned by C<new()> will have the following attributes defined:

=over 4

=item cmdline()

Return the command-line actually executed, as a list of strings.

=item options()

The merged list of options used to run the command.

=item pid()

The PID of the underlying command.

=item stdin()

A filehandle opened in write mode to the child process' standard input.

=item stdout()

A filehandle opened in read mode to the child process' standard output.

=item stderr()

A filehandle opened in read mode to the child process' standard error output.

=back

Regarding the handles to the child process, note that in the following code:

    my $fh = System::Command->new( @cmd )->stdout;

C<$fh> is opened and points to the output handle of the child process,
while the anonymous C<System::Command> object has been destroyed. Once
C<$fh> is destroyed, the subprocess will be reaped, thus avoiding zombies.


After the call to C<close()> or after C<is_terminated()> returns true,
the following attributes will be defined:

=over 4

=item exit()

The exit status of the underlying command.

=item core()

A boolean value indicating if the command dumped core.

=item signal()

The signal, if any, that killed the command.

=back

=head1 CAVEAT EMPTOR

Note that C<System::Command> uses C<waitpid()> to catch the status
information of the child processes it starts. This means that if your
code (or any module you C<use>) does something like the following:

    local $SIG{CHLD} = 'IGNORE';    # reap child processes

C<System::Command> will not be able to capture the C<exit>, C<core>
and C<signal> attributes. It will instead set all of them to the
impossible value C<-1>, and display the warning
C<Child process already reaped, check for a SIGCHLD handler>.

To silence this warning (and accept the impossible status information),
load C<System::Command> with:

    use System::Command -quiet;

It is also possible to more finely control the warning by setting
the C<$System::Command::QUIET> variable (the warning is not emitted
if the variable is set to a true value).

If the subprocess started by C<System::Command> has a short life
expectancy, and no other child process is expected to die during that
time, you could even disable the handler locally (use at your own risks):

    {
        local $SIG{CHLD};
        my $cmd = System::Command->new(@cmd);
        ...
    }

=head1 AUTHOR

Philippe Bruhat (BooK), C<< <book at cpan.org> >>

=head1 ACKNOWLEDGEMENTS

Thanks to Alexis Sukrieh who, when he saw the description of
C<Git::Repository::Command> during my talk at OSDC.fr 2010, asked
why it was not an independent module. This module was started by
taking out of C<Git::Repository::Command> 1.08 the parts that
weren't related to Git.


=head1 BUGS

Please report any bugs or feature requests to C<bug-system-command at rt.cpan.org>, or through
the web interface at L<http://rt.cpan.org/NoAuth/ReportBug.html?Queue=System-Command>.  I will be notified, and then you'll
automatically be notified of progress on your bug as I make changes.

=head1 SUPPORT

You can find documentation for this module with the perldoc command.

    perldoc System::Command


You can also look for information at:

=over 4

=item * RT: CPAN's request tracker

L<http://rt.cpan.org/NoAuth/Bugs.html?Dist=System-Command>

=item * AnnoCPAN: Annotated CPAN documentation

L<http://annocpan.org/dist/System-Command>

=item * CPAN Ratings

L<http://cpanratings.perl.org/d/System-Command>

=item * Search CPAN

L<http://search.cpan.org/dist/System-Command/>

=back


=head1 COPYRIGHT

Copyright 2010-2011 Philippe Bruhat (BooK).

=head1 LICENSE

This program is free software; you can redistribute it and/or modify it
under the terms of either: the GNU General Public License as published
by the Free Software Foundation; or the Artistic License.

See http://dev.perl.org/licenses/ for more information.


=cut

