=encoding utf8

=head1 TITLE

Synopsis 17: Concurrency [DRAFT]

=head1 AUTHOR

 Elizabeth Mattijsen <liz@dijkmat.nl>
 Audrey Tang <autrijus@autrijus.org>

=head1 VERSION

 Maintainer: Elizabeth Mattijsen <liz@dijkmat.nl>
 Date: 13 Jun 2005
 Last Modified: 13 Nov 2005
 Number: 0
 Version: 1

=head1 SKETCH

This is a rough sketch of how concurrency works in Perl 6.

(actually these are just random notes, put here under the release-early
release-often principle, slowly being integrated in a more textual format.
Patches welcome!)

=head1 OVERVIEW

Concurrency can take many forms in Perl 6.  With varying degrees of
explicitness and control capabilities.  This document attempts to describe
what these capabilities are and in which form they can be accessed in Perl 6.

=head2 Processes, threads, fibers?

Concurrency comes in many shapes and forms.  Most Perl users are used to
the concept of a "process" or a "thread" (usually depending on the OS they
work on).  Some systems even are familiar with very lightweight threads
called "fibers".

When discussing issues about concurrency with different people, it soon
becomes apparent that everybody has his own set of "understandings" about
what each word means, which doesn't make it any easier to describe
Perl 6 concurrency.

It seemed the most natural to use the word "thread" to describe a process
which has its own context, but also shares context with 0 or more
concurrently running processes.  Depending on your OS, or even specific
version of your OS, this could still be a single "process" from the OS's
point of view.  Or it could contain an OS process for each thread.  Or
any mixture of these two implementations.

In this document we try to be agnostic about this: all we know in Perl 6
are "threads", which have their own context and share context with other
concurrently running "threads".  Whether they be process, threads or fibres
at the OS level should not matter at the Perl 6 level.

And for sake of consistency, an unthreaded "normal" program is considered
to be also running in a single thread.

=head2 Variables

In the past, there have been two models for concurrent processes in Perl.
In general, these are referred to as "5.005 threads" (C<perldoc perlothrtut>)
and "ithreads" (C<perldoc perlthrtut>).

The main difference between these two models from a programmer's point of
view, is that variables in "5.005 threads" are shared by default.  Whereas
in the "ithreads" model, only variables that have been indicated to be
"shared", are actually shared between threads.  All other variable values
are actually copies of the variable's value in the "parent" thread.

With regards to variables, the concurrency model of Perl 6 is closer to the
"5.005 threads" model than it is to the "ithreads" model.  In fact, all
variables "visible" to a particular scope in Perl 6 will be accessible and
modifiable from all of the concurrent processes that start from that scope.
In that sense, one could consider the "ithreads" model as a historical
diversion: the Perl 6 concurrency picks up where the "5.005 threads" path
left off.

(EM: maybe point out that the "ithreads" behaviour can be simulated with
some kind of copy-on-write magic to be automagically added to all variable
access inside a thread, except for those with an explicit "is shared"
attribute?)

=head2 No user accessible locks

Differently from any current concurrent process implementation in Perl, there
are no user accessible locks.  Instead, the concept of Software
Transactionable Memory is used.  This is in concept similar to the use of

 BEGIN TRANSACTION
 ... do your uninterruptible actions
 COMMIT

in the database world.  More interestingly, this also includes the concept of
rollback:

 BEGIN TRANSACTION
 ... do your stuff, but impossible to complete: ROLLBACK

This causes the state of the process to be reverted to the state at the moment
the BEGIN TRANSACTION was executed.

Perl 6 supports this concept through Code blocks which are marked "is atomic".
These sections are guaranteed to either be completed totally (when the Code
block is exited), or have their state reverted to the state at the start of
the Code block (with the L<retry> statement).

(EM: maybe point out if / how old style locks can be "simulated", for those
needing a migration path?)

=head2 Atomic Code blocks

    my ($x, $y);
    sub c is atomic {
        $x -= 3;
        $y += 3;
        if $x < 10 { retry }
    };

    $e = &c.retry_with( &d ); # 
    $e();

    if $i { is atomic; ...  } else { ...; }

A Code block can be marked as "is atomic".  This means that code executed
inside that scope is guaranteed not to be interrupted in any way.

The start of a block marked "is atomic" also becomes a "checkpoint" to
which execution can return (in exactly the same state) if a problem occurs
(a.k.a. a L<retry> is done) inside the scope of the Code block.

=head3 retry

The C<retry> function basically restores the state of the thread at the
last checkpoint and will wait there until an external event allows it to
potentially run that atomic section of code again without having to retry
again.

If there are no external events possible that could restart execution, an
exception will be raised.

The last checkpoint is either the last atomic / non-atomic boundary, or
the most immediate caller constructed with C<retry_with>.

=head3 retry_with

The C<retry_with> method on an atomic Code object causes a checkpoint to
be made for C<retry>, creating an alternate execution path to be followed
when a C<retry> is done.

=head3 limitations

Because Perl 6 must be able to revert its state to the state it had at the
checkpoint, it is not allowed to perform any non-revertable actions.  These
would include reading / writing from file handles that do not support
C<seek> (such as sockets).  Attempting to do so will cause a fatal error
to occur.

If you're not interested in revertability, but are interested in
uninteruptability, you could use the "is critical" trait.

=head2 Critical Code blocks

 sub tricky is critical {
     # code accessing external info, not to be interrupted
 }

 if ($update) {
     is critical;
     # code accessing external info, not to be interrupted
 }

A Code block marked "is critical" can not be interrupted in any way.  But
since it is able to access non-revertible data structures (such as
non-seekable file handles), it cannot do a C<retry> as it would be impossible
to restore the state to the beginning of the Code block.

=head2 Mixing Atomic and Critical

Both "atomic" as well as "critical" propagate down the call chain.  This means
that any subroutine that in itself is not "atomic" or "critical" becomes
uninterruptible if called inside a code block that is marked as "atomic" or
"critical".

Atomic Code blocks called inside the call chain of a "critical" code block
do not pose a problem, as they are more restrictive.

Any code that attempts to perform any non-revertible action (e.g. reading
from a socket) will cause a fatal error when called inside the call chain
of an Atomic Code block.

=head2 Co-Routines

The execution of co-routine (or "coro" for short) could be considered as a
short "side-step" from the normal path of execution, much like the normal
calling of a subroutine.  

The main difference with a normal subroutine, is that the co-routine supports
a special type of return, called "yield".

(EM: not sure whether the "threads->yield" causes so much mental interference
that we should use something else for "yield" in the coro context.  And
whether we should have a seperate "coro" keyword at all: after all, the "yield"
could be in a normal subroutine called from a coro, so it's not like the
compiler would be allowed to flag "yield" in a sub as an error).


#######################################################################
Below here still the more or less unorganized stuff

CORE::GLOBAL::exit; # kills all the threads

# We intententionally do not list cross-machine parallelism Conc:: classes here.
# Consult your local 6PAN mirror with a time machine.
use Conc::Processes; # fork() or createProcess based implementation
use Conc::Threads;   # maybe it just exports &async to override the default one, yay
use Conc::Multiplex; # this is default

my $thr = async {
    ...do something...
    END { }
};

Conc::Thread.this
Conc::Proc.this

Conc object # name is still up for grabs!
- numify to TIDs (as in pugs)
- stringify to something sensible (eg. "<Conc:tid=5>");
- enumerable with Conc.list
- Conc.yield (if this is to live but deprecated, maybe call it sleep(0)?)
- sleep() always respects other threads, thank you very much
- standard methods:
    - .join    # wait for invocant to finish (always item cxt)
    - .die     # throw exception in the invocant thread
    - .alarm   # set up alarms
    - .alarms  # query existing alarms
    - .suspend # pause a thread; fail if already paused
    - .resume  # revive a thread; fail if already running
    - .detach  # survives parent thread demise (promoted to process)
               # process-local changes no longer affects parent
               # tentatively, the control methods still applies to it
               # including wait (which will always return undef)
               # also needs to discard any atomicity context
- attributes:
    - .started  # time
    - .finished # time
    - .waiting  # suspened (not diff from block on wakeup signal)
                # waiting on a handle, a condition, a lock, et cetera
                # otherwise returns false for running threads
                # if it's finished then it's undef(?)
    - .current_continuation
                # the CC currently running in that thread

- "is throttled" trait

    method throttled::trait_auxillary:<is> ($limit=1, :$key=gensym()) {
        # "is throttled" limits max connection to this Code object
        # the throttling is shared among closures with the same key
        # the limit may differ on closures with the same key.
        # if the counter with the "key" equals or exceeds a closure's limit,
        # the closure can't be entered until it's released
        # (this can be trivially implmented using atomic+retry)
    }

    class Foo {
        method a is throttled(:limit(3) :key<blah>) { ... }
        method b is throttled(:limit(2) :key<blah>) { ... }
    }
    my Foo $f .= new;
    async { $f.a }
    async { $f.b }

- Thread::Status
- IO objects and containers gets concurrency love!
    - $obj.wake_on_readable
    - $obj.wake_on_writable
    - $obj.wake_on_either_readable_or_writable_or_passed_time(3); # fixme fixme
    - $obj.wake_on:{.readable} # busy wait, probably

    my @a is Array::Chan = 1..Inf;
    async { @a.push(1) };
    async { @a.blocking_shift({ ... }) };
    async { @a.unshift({ ... }) };

Communication abstractions
- shared, transactional variables by default

# program will wait for _all_ threads
# unjoined threads will be joined at the beginning of the END block batch
# of the parent thread that spawned them

### INTERFACE BARRIER ###
module Blah;
{

    is atomic;   # retry/orelse/whatever other rollback stuff
                 # limitation: no external IO (without lethal warnings anyway)
                 # can't do anything irreversible

    is critical; # free to do anything irreversible
                 # means "don't interrupt me"
                 # in system with critical section, no interrupts from
                 # other threads will happen during execution
                 # you can't suspend me

    my $boo is export;
    $boo = 1;

    # We decree that this part forms the static interface
    # it's run once during initial compilation under the
    # Separate Compilation doctrine and the syms sealed off
    # to form part fo bytecode syms headers
    %CALLER::<&blah> = { 1 }; # work - adds to export set
    die "Eureka!" if %CALLER::<$sym>; # never dies

    # BEGIN { $boo = time };

    sub IMPORT {
        # VERY DYNAMIC!

        our $i = time;
        %CALLER::<&blah> = { 1 }; # work - adds to export set
        die "Eureka!" if %CALLER::<$sym>; # probes interactively
    }
}
### INTERFACE BARRIER ###

my $sym;
threads.new({
    use Blah;
    BEGIN { require(Blah).import }

    my $boo; BEGIN { eval slurp<Blah.pm>; $boo := $Blah::boo };

    ...
});

=head2 Signals

Asynchronous exceptions are just like user-initiated exceptions with C<die>,
so you can also catch it with regular C<CATCH> blocks as specified in S04.

To declare your main program catches INT signals, put a CATCH block anywhere
in the toplevel to handle exceptions like this:

 CATCH {
     when Error::Signal::INT { ... }
 }

=head2 Alarm

An alarm is just a pre-arranged exception to be delivered to your program.

By the time alarm has arrived, the current block may have already finished
executing, so you would need to set up CATCH blocks in places where an alarm
can rise to handle it properly.

You can request an alarm using the number of seconds, or with a target date.
It returns a proxy alarm object that you can do interesting things with.

    multi Alarm *alarm (Num $seconds = $CALLER::_, &do = {die Sig::ALARM}, :$repeat = 1)
    multi Alarm *alarm (Date $date, &do = {die Sig::ALARM}, :$repeat = 1)

Perl 6's C<alarm> has three additional features over traditional alarms:

=head3 Multiple and Lexical Alarms

One can set up multiple alarms using repeated alarm calls:

    {
        my $a1 = alarm(2);
        my $a2 = alarm(2);
        sleep 10;
        CATCH {
            is critical; # if you don't want $a2 to be raised inside this
            when Sig::ALARM { ... } 
        }
    }

To stop an alarm, call C<$alarm.stop>.  The C<alarms> method for Conc objects
(including process and threads) returns a list of alarms currently scheduled
for that concurrent context.

When an alarm object is garbage collected, the alarm is stopped automatically.
Under void context, the implicit alarm object can only be stopped by querying
C<.alarms> on the current process.

We are not sure what C<alarm(0)> would mean.  Probably a deprecation warning?

=head3 Repeated Alarms

If you request a repeated alarm using the C<repeated> named argument, it will
attempt to fire off the alarm that many times.  However, the alarm will be
supressed when inside a C<CATCH> block that's already handling the exception
raised by I<same> alarm.

To repeat 0 times is to not fire off any alarms at all.  To repeat +Inf times
is to repeat over and over again.

=head3 Callbacks in Alarms

You can arrange a callback (like JavaScript's setTimeOut) in C<alarm>, which
will then be invoked with the then-current code as caller.

If you set up such a callback to another Conc object, what happens is just like
when you called C<.die> on behalf of that object -- namely, the callback
closure, along with anything it referenced, is shared to the target Conc
context.

Unlike in Perl 5's ithreads where you cannot share anything after the fact,
this allows passing shared objects in an C<ad-hoc> fashion across concurrent
parts of the program.  Under the default (multiplexing) concurrency model, this
is basically a no-op.

=head2 Continuations

=head3 Coroutines

## braindump of coro meeting by Liz and Autri, more to follow

- Coros are _like_ processes

coro dbl { yield $_ * 2; yield $_; return };
my @x = 1..10;
my %y = map &dbl, @x;
# 2 => 2, 6 => 4, 10 => 6, ...

coro perm (@x) {
    @x.splice(rand(@x),1).yield while @x;
}

my &p1 := &perm.start(1..10);
my &p2 := &perm.start(1..20);

p1(); p1();
p2(); p2();

coro foo { yield 42 };

(1..10).pick;

coro foo ($x) {
    yield $x;
    yield $x+2;
    cleanup();
    while (2) {
        while (1) {
            &?SUB.kill; # seppuku
        }
    }
} # implicit falloff return + return() means startover without yielding
  # return() means yielding and restart + no implicit falloff (I LIKE THIS)

&foo.finished; # true on return() and false on midway yield()

foo(4); # and that's all she wrote

coro foo ($x) {
    yield $x;
    # this point with $x bound to 10
    yield $x+1;
    return 5;
    ... # this is never reached, I think we all agree
}

# If you don't want your variables to get rebound, use "is copy":
coro foo ($x is copy) {...}
# which is sugar for
coro foo ($x) {
  {
    my $x := $OUTER::x;
    ...;
    # Further calls of &foo rebound $OUTER::x, not $x.
  }
}

sub foo {
    return undef if rand;
    ...
}

use overload {
    '&{}' => sub { ... }
}

class Coro is Conc::Multiplex does Code {
    method postcircumfix:<( )> {
        # start the thread, block stuff (we are in the caller's context)
    }
}

class Hash is extended {
    method postcircumfix:<( )> (&self: *@_) {
        &self = ./start(@_);
    }
    method start {
        # remember self
        # upon return() or normal falloff, restore self
    }
}

%ENV(123);

&foo_continued := &foo.start(10);
&foo.start(20);

foo(10);    # returns 10

foo();      # be "insufficient param" error or just return 11?
foo(20);    # returns 21

# continuation coros
multi foo () { ...no rebinding... }
multi foo ($x) { ...rebinding... }

&foo.kill;


my $first_ret = zoro( type => <even> );
&zoro.variant(:type<even>).kill;
&zoro.variant(type => 'even').kill;

zoro( type => <odd> );

zoro( even => 1 );
zoro( odd => 1 );

multi coro zoro ($type where 'even') {}
multi coro zoro ($type where 'odd') {}

multi coro zoro ($even is named) {}
multi coro zoro ($odd is named) {}


# iblech's thoughts:
# Coroutine parameters should never be rebound. Instead, yield(...)s return
# value is an Arglist object containing the new arguments:
coro bar ($a, $b) {
    ...;
    my $new_set_of_args = yield(...);
    my $sum_of_old_a_and_new_a = $a + $new_set_of_args<$a>;
    ...;
}
bar(42, 23);  # $a is 42, $b is 23
bar(17, 19);  # $a still 42, $b still 19,
              # $new_set_of_args is \(a => 17, b => 19)

=head2 Junctive Autothreading and Hyper Operations

Live in userland for the time being.

=head2 Interprocess Communication

=head2 I/O Considerations

=head3 File Descriptors

=head3 Sockets

=cut
