Initial stab at serializing stdout/stderr on P::A

I'm still not sure whether I covered the case that data may come in *after*
the pipe has been closed.  Probably not.  But at least this is an initial
stab at it.  FWIW, the associated test file fails about 1 in 5 times still
with segfaults and all sorts of other strange messages.  So there's still
work to be done!

src/core/Proc/Async.pm

@@ -105,21 +105,52 @@ my class Proc::Async {
 
         my $promise = Promise.new;
         my $lock = Lock.new;
-        my int $last_seq = -1;
         my int $moreing;
+        my int $next_seq;
+        my @buffer; # should be Mu, as data can be Mu
 
         nqp::bindkey(callbacks,
             std ~ ( type ?? '_chars' !! '_bytes' ),
             -> Mu \seq, Mu \data, Mu \err {
+
+                # oh noes!
                 if err {
                     $promise.keep( (supply,err) );
                 }
+
+                # we're done
                 elsif seq < 0 {
                     $promise.keep( supply );
                 }
+
+                # got new data to process
                 else {
-#say "{std}: seq = {seq} with {data}   in {$*THREAD}" if std eq 'stdout';
-                    supply.more(data);
+                    # cannot simply return out of here, so we need a flag
+                    my int $in_charge;
+
+                    $lock.protect( {
+#say "seq = {seq} with {data}   in {$*THREAD}" if std eq 'stdout';
+                        @buffer[ seq - $next_seq ] := data;
+                        $in_charge = $moreing = 1 unless $moreing;
+                    } );
+
+                    if $in_charge {
+                        my int $done;
+                        while @buffer.exists_pos($done) {
+#say "moreing { $next_seq + $done }: {@buffer[$done]}" if std eq 'stdout';
+                            supply.more( @buffer[$done] );
+                            $done = $done + 1;
+                        }
+
+                        $lock.protect( {
+                            if $done {
+#say "discarding from $next_seq for $done" if std eq 'stdout';
+                                @buffer.splice(0,$done);
+                                $next_seq = $next_seq + $done;
+                            }
+                            $moreing = 0;
+                        } );
+                    }
                 }
             }
         );