[perl #128937] Supply.throttle should return a Supply of pairs

[via RT]

# New Ticket Created by  刘刊 
# Please include the string:  [perl #128937]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=128937 >


Supply.throttle returns a Supply of promises, which in practice may be 
inadequate. It should return a Supply of pairs, keys being the elements, value 
being the corresponding Promises. Consider the following senario,

my @a = 1 .. 10;
my $c = -> {
    if $i == 10.rand { die 'boom' }
    if $i == 10.rand { while 1 {} }
    $_;
};

my $s = Supply.from-list( |@a ).throttle: 5, -> $i { &$c( $i ) };

as is, we have no way of knowing which element's corresponding promise failed, 
not having implemented the logic inside $c.

To circumvent this limitation, one could perceivably do the following, which is 
combersome.

my $s2 = Supply.from-list( |@a ).throttle: 5, -> $i { $i => start { &$c( $i ) } 
};

Furthermore, throttle should provide a timeout logic. Without it, to implement 
something equivalent, which is just convoluted.

my $s3 = Supply.from-list( |@a ).throttle: 5, -> $i {
    my $p = start { &$c( $i ) };
    await $timeout
    ?? Promise.anyof: $p, Promise.in: $timeout
    !! Promise.allof: $p;
    return $i => $p;
}

The code is also not stable when the resulting supply is tapped. when the thrott
le limit exceeds a cerntain threshold, it just hangs.


- kan
________________________________
From: 刘刊
Sent: Friday, August 12, 2016 3:35 PM
To: rakudo...@perl.org
Subject: [perl #128906]


[15:01] <kanl> Supply . throttle returns a supply of promises. How does each 
promise correspond to the original items? e.g. my $s = Supply.from-list( |@a 
).throttle: 3, -> $v { do-something-with( $a ) }; $s.tap: -> $v { 
how-to-figure-out-relation-here() }
[15:04] <kanl> this matters because when running a number of things in 
parallel, one supposedly would need to know whose promise got kept and whose 
got broken, not just which.
[15:15] <kanl> i don't suppose it's safe to stick things into a hash inside 
throttle's pointy block, not knowing it's act() or tap(), Someone care to 
share, please?
[15:17] == sssd [~ac1f0106@38.72.195.196] has quit [Quit: 
http://www.kiwiirc.com/ - A hand-crafted IRC client]
[15:20] <kanl> m: my $s = Supply.from-list(^6).throttle: 3, { sleep 3; die if 
$_ == 1 }; $s.act: -> $v { $v.status.say }; $s.wait;
[15:20] <+camelia> rakudo-moar c587b9: OUTPUT«Kept␤Broken␤Kept␤Kept␤Kept␤Memory 
allocation failed; could not allocate 4 bytes␤»
[15:21] <kanl> m: my $s = Supply.from-list(^6).throttle: 3, { sleep 3; die if 
$_ == 1 }; $s.act: -> $v { $v.status.say }; $s.wait;
[15:21] <+camelia> rakudo-moar c587b9: OUTPUT«Broken␤Kept␤Kept␤Kept␤Kept␤Memory 
allocation failed; could not allocate 32 bytes␤Memory allocation failed; could 
not allocate 4 bytes␤»
[15:21] <kanl> m: my $s = Supply.from-list(^6).throttle: 3, { sleep 3; die if 
$_ == 1 }; $s.act: -> $v { $v.status.say }; $s.wait;
[15:21] <+camelia> rakudo-moar c587b9: OUTPUT«Kept␤Broken␤Kept␤Kept␤Memory 
allocation failed; could not allocate 4 bytes␤»
[15:22] == TheLemonMan [~root@unaffiliated/thelemonman] has joined #perl6
[15:22] <kanl> The same code produces weirder results on my machine.
[15:24] == labster [~Adium@miraheze/Labster] has joined #perl6
[15:25] <gfldex> m: my $s = Supply.from-list(^6).throttle: 3, { sleep 3; 
Failure.new("booboo") if $_ == 1 }; $s.act: { .status.say }; $s.wait;
[15:25] <+camelia> rakudo-moar c587b9: 
OUTPUT«Kept␤Kept␤Kept␤Kept␤Kept␤Kept␤Kept␤Kept␤»
[15:25] == camelia [~p6e...@ns1.niner.name]
[15:25] ==  realname : combined, experimental evalbot
[15:25] ==  channels : +#perl6
[15:25] ==  server   : leguin.freenode.net [Umeå, SE, EU]
[15:25] ==  account  : camelia
[15:25] == End of WHOIS
[15:25] <gfldex> m: my $s = Supply.from-list(^6).throttle: 3, { sleep 3; 
Failure.new("booboo") if $_ == 1 }; $s.act: { .status.say }; $s.wait;
[15:25] <+camelia> rakudo-moar c587b9: 
OUTPUT«Kept␤Kept␤Kept␤Kept␤Kept␤Kept␤Memory allocation failed; could not 
allocate 32 bytes␤»
[15:26] <gfldex> nice new bug for jnthn :)
________________________________
[15:29] == darutoko [~Vandal@176.209.234.142] has joined #perl6
[15:31] == wbill [~wb...@96-38-107-68.dhcp.jcsn.tn.charter.com] has joined 
#perl6
[15:31] <kanl> even if said bug is fixed, i still have no way of knowing the 
broken promise belongs to 1.
[15:32] <kanl> well, i do, because i put it there. but if i didn't, then how?

- kan