inconsistent results when broadcasting with empty segments on multilocale

[stress-tess]

A user reported this and I was able to reproduce. Below is what I'm seeing with gasnet and 3 locales

>>> segments = ak.array([3,3,5,6,6,7,7])
>>> values = ak.arange(7)
>>> size = 10
>>> ak.broadcast(segments, values, size)
array([0 0 0 1 1 2 3 4 4 4])

>>> ak.broadcast(segments, values, size)
array([0 0 0 1 1 2 3 4 4 4])

>>> ak.broadcast(segments, values, size)
array([0 0 0 0 0 1 2 3 3 3])

[stress-tess]

This unexpected result is due to a race condition in this forall loop:

arkouda/src/Broadcast.chpl

Lines 151 to 153 in 9ab6f17

	forall (s, d) in zip(segs, diffs) with (var agg = newDstAggregator(t)) {
	agg.copy(expandedVals[s], d);
	}

if we print out the values for segs and diffs we get:

segs: 3 3 5 6 6 7 7
diffs: 0 1 1 1 1 1 1

we see that the seg 3 has two associated diffs, so expandedVals[3] will vary depending on the order that these two segments write

[stress-tess]

I believe the correct result for this case should be array([0 0 0 1 1 2 4 6 6 6])

My reasoning is:

• we don't specify what goes in (0,3) so it remains uninitialized (i.e. 0)
• 0 goes in segment (3,3) which is empty
• 1 goes in segment (3,5)
• 2 goes in segment (5,6)
• 3 goes in segment (6,6) which is empty
• 4 goes in segment (6,7)
• 5 goes in segment (7,7) which is empty
• 6 goes in segment (7,10)