BUG/MINOR: sink: fix a race condition between the writer and the reader

This is the same issue as just fixed in b8e0fb97f ("BUG/MINOR: ring/cli:
fix a race condition between the writer and the reader") but this time
for sinks. They're also sucking the ring and present the same race at
high write loads.

This must be backported to 2.2 as well. See comments in the aforementioned
commit for backport hints if needed.
1 file changed