Add test coverage for source_selection_rate and tidy up how we reflect
being throttled due to source_selection_rate; in the first cut
we made it look like the ready queue was full, but now we're a
bit more deliberate:
* If any of the candidate queues were full, we report that the
ready queue was full so that the message can be retried again
"soon".
* If we hit a source_selection_rate throttle, we use the throttle
delay to adjust the min delay period and if we run out of candidate
sources, we'll have the same sort of return value as we would
for a suspension with a definite duration; the message will
log that transient failure in the same way with the same sort
of retry schedule. I've revised the wording in that case to be
"no sources for SITE pool=`POOL` are eligible for selection at this time"
(the important part being `at this time`) to suggest at the
transient nature of the unavailability.
* If we exhaust all sources for other reasons, we'll treat this
as the NoSources case. In practice, this should mean that the
source has zero weight. We'll now report "no sources available for SITE
pool=`POOL`" in this case instead of the more precise but wordy
"no non-zero-weighted sources available for SITE" that we used
prior to this commit.
Wez Furlong
0e339a2b9e