BUG #3326: Invalid lower bound of autovacuum_cost_limit

The following bug has been logged online:

Bug reference: 3326
Logged by: Galy Lee
Email address: lee(dot)galy(at)oss(dot)ntt(dot)co(dot)jp
PostgreSQL version: 8.3
Operating system: Red Hat 4.
Description: Invalid lower bound of autovacuum_cost_limit
Details:

Hello

I found some bugs which relative to the autovacuum_cost_limit GUC
parameter.

* Bug-1: Invalid lower bound of autovacuum_cost_limit

autovacuum_vacuum_cost_limit should be the following value:
autovacuum_vacuum_cost_limit = -1, or [1, 10000]
(0 should be prohibited. )

But 0 can also be accepted for autovacuum_vacuum_cost_limit now.

This causes zero-division error for autovacuum:

ERROR: floating-point exception
DETAIL: An invalid floating-point operation was signaled. This
probably means an out-of-range result or an invalid operation,
such as division by zero.

* Bug-2: 0-cost-limit for autovacuum worker

When autovacuum_max_workers > autovacuum_vacuum_cost_limit, the above
zero-division error also happened.

* Bug-3: no GUC constrain check for pg_autovacuum

The settings in pg_autovacuum are not checked enough now. Invalid value
can be passed to autovacuum, this also causes some columns in
pg_autovacuum has inconsistent upper and lower bound with their original
GUC constrain.

pg_autovacuum colum | definition | GUC constrain
--------------------+------------+--------------------------
vac_base_thresh | integer | [0, INT_MAX]
vac_scale_factor | real | [0.0, 100.0]
anl_base_thresh | integer | [0, INT_MAX]
anl_scale_factor | real | [0.0, 100.0]
vac_cost_delay | integer | [-1, 1000]
vac_cost_limit | integer | [-1, 10000]
freeze_min_age | integer | [0, 1000000000]
freeze_max_age | integer | [100000000, 2000000000]

The above table shows the wrong mapping between pg_autovacuum columns
and GUC constrain.