fix(gproc_pool): fix `rand:uniform/1` range (#193)
Since
[`rand:uniform/1`](https://www.erlang.org/doc/man/rand.html#uniform-1)'s
return value is a number in a closed interval (`0 =< N =< Sz`), we
shouldn't call it with the pool size + 1, otherwise we will get a
non-uniform distribution of results where one of the workers receive
substantially more work than all others.
To verify this, I've started a pool with 8 workers using the `random`
load-balancing algorithm and measured the frequencies of each PID in
`gproc_pool`.
```erlang
PoolName = my_pool.
PoolSize = 8.
ok = gproc_pool:new(PoolName, random, [{size, PoolSize}]).
lists:foreach(fun(I) ->
gproc_pool:add_worker(PoolName, {PoolName, I}, I),
spawn(fun() ->
true = gproc_pool:connect_worker(PoolName, {PoolName, I}),
receive
stop -> ok
end
end),
ok
end, lists:seq(1, PoolSize)).
MeasureFreqs = fun(Pool, NumPoints) ->
Pids = lists:map(fun(_) -> gproc_pool:pick_worker(Pool) end, lists:seq(1, NumPoints)),
lists:foldl(
fun(Pid, Acc) ->
maps:update_with(Pid, fun(N) -> N + 1 end, 1, Acc)
end,
#{},
Pids
)
end.
MeasureFreqs(PoolName, 100_000).
```
Results prior to the fix (note how `<0.172.0>` has almost double the
frequency of other PIDs):
```erlang
#{<0.172.0> => 22222,<0.173.0> => 11138,<0.174.0> => 11000,
<0.175.0> => 11046,<0.176.0> => 11141,<0.177.0> => 11165,
<0.178.0> => 11166,<0.179.0> => 11122}
```
After the fix:
```erlang
#{<0.172.0> => 12460,<0.173.0> => 12367,<0.174.0> => 12541,
<0.175.0> => 12494,<0.176.0> => 12649,<0.177.0> => 12399,
<0.178.0> => 12439,<0.179.0> => 12651}
```
Thales Macedo Garitezi