Support mem3_shards hot upgrade

[jaydoane]

I tested the upgrade and downgrade paths by first making sure mem3_shards was compiled from tag 3.9.2, then updating the source with this patch, recompiling, then in a remsh

f(_change_code), _change_code = fun(Module, OldVsn) ->
    ok = sys:suspend(Module),
    {module, Module} = l(Module),
    ok = sys:change_code(Module, Module, OldVsn, []),
    ok = sys:resume(Module)
end.

f(_table_exists), _table_exists = fun(Tab) ->
    try ets:tab2list(Tab) of
        Val ->
            true
    catch
        error:badarg ->
            false
    end
end.

f(_state_size), _state_size = fun(Name) ->
    size(sys:get_state(Name)) - 1 % exclude tag
end.

f(_validate), _validate = fun
    (pre) ->
        false = _table_exists(mem3_openers),
        3 == _state_size(mem3_shards),
        ok;
    (post)->
        true = _table_exists(mem3_openers),
        5 == _state_size(mem3_shards),
        ok
end.

ok = _validate(pre).
ok = _change_code(mem3_shards, 1).
ok = _validate(post).
ok = _change_code(mem3_shards, {down, 2}).
ok = _validate(pre).

[rnewson]

I think your emulation must be wrong, or my understanding of release handler is wrong (could genuinely be either).

What should happen, I think, is that, when downgrading, the current version of the module is called with code_change with {down, 2}, this returns a state that the old version (1) can understand.

If it isn't that, I don't see how you're supposed to write downgrade clauses.

[rnewson]

given this matches the arity of the current state record, this clause will be executed on all future hot upgrades, activating the downgrade code and crashing. You'll need to match the OldVsn here as {down, 2} to be sure we only do this on actual downgrade.

[rnewson]

I would, for safety, match OldVsn as 1.

[jaydoane]

@rnewson @robfraz this seems to be passing manual testing now, after I improved my understanding of sys:change_code/4

[rnewson]

this log line was just for you while figuring this out or is it useful?

[jaydoane]

It was helpful for debugging, and since it should only happen once, I figured I might as well leave it in. Do you want me to remove the logging?

[rnewson]

let's keep a code change clause that matches on #st{} so we don't screw ourselves over on the next release.

[jaydoane]

good call -- PR updated with default clause