From @skids

16​:18 skids I think the Supply work may have made $fh.watch.Promise not fire anymore.
16​:20 watch.tap still works.
16​:21 jnthn The semantics of Supply.Promise changed
16​:21 You'll need $fh.watch.head(1).Promise to get the original ones
16​:21 skids Ah that would explain it. jnthn++
16​:22 Hrm that doesn't seem to do it though.
  2 more elements. Show/hide.
16​:34 jnthn Hm, head doesn't seem to be filing the "done"

These work​:

$ perl6 -e 'my $f = Supplier.new; my $s = $f.head(1).Supply; my $t = $s.tap(-> $ { 42.say }, done => {43.say} ); start { for 0..3 { $f.emit(42) }; $f.done(); }; await $s.Promise; 44.say; '
42
43
44

$ perl6 -e 'my $f = Supplier.new; my $s = $f.head(1).Supply; my $t = $s.tap(-> $ { 42.say }, done => {43.say} ); start { for 0..3 { $f.emit(42) }; }; await $s.Promise; 44.say; '
42
43
44

This neither sends a done nor fulfills a .Promise (input echoed into file from another terminal)​:

$ perl6 -e 'my $fh = open("/tmp/foo.txt", :create); my $s = $fh.watch.head(1); my $t = $s.tap(-> $ { 42.say }, done => {43.say} ); await $s.Promise; 44.say'
42
^C

...and when the .head is off a Supply instead of a Supplier, a done from the original Supply propagates but the Promise never unhooks​:

$ perl6 -e 'my $f = Supplier.new; my $s = $f.Supply.head(1); my $t = $s.tap(-> $ { 42.say }, done => {43.say} ); start { for 0..3 { $f.emit(42) }; $f.done(); }; await $s.Promise; 44.say; '
42
43
^C

$ perl6 -e 'my $f = Supplier.new; my $s = $f.Supply.head(1); my $t = $s.tap(-> $ { 42.say }, done => {43.say} ); start { for 0..3 { $f.emit(42) }; }; await $s.Promise; 44.say; '
42
^C

From @jnthn

On Sat Dec 05 11​:13​:54 2015, bri@​abrij.org wrote​:

16​:18 skids I think the Supply work may have made
$fh.watch.Promise not fire anymore.
16​:20 watch.tap still works.
16​:21 jnthn The semantics of Supply.Promise changed
16​:21 You'll need $fh.watch.head(1).Promise to get the
original ones
16​:21 skids Ah that would explain it. jnthn++
16​:22 Hrm that doesn't seem to do it though.
2 more elements. Show/hide.
16​:34 jnthn Hm, head doesn't seem to be filing the "done"

These work​:

$ perl6 -e 'my $f = Supplier.new; my $s = $f.head(1).Supply; my $t =
$s.tap(-> $ { 42.say }, done => {43.say} ); start { for 0..3 {
$f.emit(42) }; $f.done(); }; await $s.Promise; 44.say; '
42
43
44

$ perl6 -e 'my $f = Supplier.new; my $s = $f.head(1).Supply; my $t =
$s.tap(-> $ { 42.say }, done => {43.say} ); start { for 0..3 {
$f.emit(42) }; }; await $s.Promise; 44.say; '
42
43
44

This neither sends a done nor fulfills a .Promise (input echoed into
file from another terminal)​:

$ perl6 -e 'my $fh = open("/tmp/foo.txt", :create); my $s =
$fh.watch.head(1); my $t = $s.tap(-> $ { 42.say }, done => {43.say} );
await $s.Promise; 44.say'
42
^C

This works reliably now, and so could do with a test. Probably thanks to one of the many Supply fixes over the last months.

...and when the .head is off a Supply instead of a Supplier, a done
from the original Supply propagates but the Promise never unhooks​:

$ perl6 -e 'my $f = Supplier.new; my $s = $f.Supply.head(1); my $t =
$s.tap(-> $ { 42.say }, done => {43.say} ); start { for 0..3 {
$f.emit(42) }; $f.done(); }; await $s.Promise; 44.say; '
42
43
^C

$ perl6 -e 'my $f = Supplier.new; my $s = $f.Supply.head(1); my $t =
$s.tap(-> $ { 42.say }, done => {43.say} ); start { for 0..3 {
$f.emit(42) }; }; await $s.Promise; 44.say; '
42
^C

This code is vulnerable to a race condition, thus it sometimes works, sometimes not. If you add a `sleep 1` before the `await` then it reliably hangs. The reason is that a Supplier is a live supply; if you miss the events (emit/done) then they're gone for good. (Note that `.Promise` taps the supply separately from the `.tap` that is already taking place.) For this to reliably work, the `Promise` should be obtained prior to setting off the `start` block. This works reliably, every time​:

my $f = Supplier.new;
my $s = $f.Supply.head(1);
my $t = $s.tap(-> $ { 42.say }, done => {43.say} );
my $p = $s.Promise;
start {
  for 0..3 {
  $f.emit(42)
  };
  $f.done();
}
await $p;
44.say;

Tagging testneeded for the part that deserves a test.

/jnthn

The RT System itself - Status changed from 'new' to 'open'

From @moritz

Tests now in https://rt-archive.perl.org/perl6//Public/Bug/Display.html?id=126824

@moritz - Status changed from 'open' to 'resolved'