Lost records from the Subscribe() channel when records arrive concurrently

[merlinran]

Reproducible by commit 66af150 . Run following and you'll see some record appears only 2 times, rather than expected 3.

GOTRACEBACK=all  testground run single --wait -p go-threads -t sync-threads -b exec:go -r local:exec -i 2 -tp records=10 -tp verbose=2 | grep "record #" | cut -d ":" -f 4- | sort | uniq -c | sort -r

It doesn't seem to relate to #525 .

[merlinran]

Note that the logic has changed a bit from the original implementation to surface the issue.