Re: [Tarantool-patches] [PATCH 1/1] netbox: don't fire on_connect() at schema update

[Alexander Turenko <alexander.turenko@tarantool.org>]

The patch LGTM in the sense that it should work as expected as far as I
see.

However I would discuss points I described below: I think we can write
the patch in a bit more clean way. Vlad, please, let me know what do you
think about this.

WBR, Alexander Turenko.

> diff --git a/src/box/lua/net_box.lua b/src/box/lua/net_box.lua
> index 31a8c16b7..696b30fd9 100644
> --- a/src/box/lua/net_box.lua
> +++ b/src/box/lua/net_box.lua
> @@ -927,14 +927,17 @@ local function new_sm(host, port, opts, connection, greeting)
>      local function callback(what, ...)
>          if what == 'state_changed' then
>              local state, errno, err = ...
> -            if (remote.state == 'active' or remote.state == 'fetch_schema') and
> -               (state == 'error' or state == 'closed' or
> -                state == 'error_reconnect') then
> -               remote._on_disconnect:run(remote)
> -            end
> -            if remote.state ~= 'error' and remote.state ~= 'error_reconnect' and
> -               state == 'active' then
> -                remote._on_connect:run(remote)
> +            local was_connected = remote._is_connected
> +            if state == 'active' then
> +                if not was_connected then
> +                    remote._is_connected = true
> +                    remote._on_connect:run(remote)
> +                end

We splitted states into 'connected', 'neural' and 'disconnected' (a kind
of tags or properties). We fire the trigger when a connection step into
one of 'connected' states from one of 'disconnected' ones ('neural' are
not counted). This looks okay.

> +            elseif errno ~= nil then
> +                if was_connected then
> +                    remote._is_connected = false
> +                    remote._on_disconnect:run(remote)
> +                end

Here we use `errno ~= nil` condition to determine whether a state is
'disconnected' one. The condition is true for 'error', 'error_reconnect'
and 'closed' states. This way should give a correct behaviour.

When I saw the patch my question was whether a connection step into
'fetch_schema' state with `errno ~= nil`. It was not obvious for me what
list of states are always set with some 'errno' value (however it is
easy to deduce from set_state() calls). That is the first point.

The second is that I cannot prove (at least after brief look into the
code) that 'errno' is newer `nil` / `box.NULL` for 'disconnected'
states, because that are places where 'errno' is passed through a
function.

I think we should at least give a comment that by using `errno ~= nil`
we lean on assumption that we always step into 'error',
'error_reconnect' and 'closed' states with non-null 'errno' and that
there is no other states that set 'errno'; but better don't assume this.

Let's consider unix errno: it should not be used as a primary source of
information **whether** an error occurs. You always check a return code
and only if it says that an error occurs we can consider 'errno' as a
source of information **which kind** of error occurs.

That is why I generally against using of errno / diagnostic area as
sources of information whether an error occurs: in context of Unix APIs
this would be an improper usage.

I would mark states as 'connected' and 'disconnected' explicitly:

 | -- XXX: Give a comment why, say, 'fetch_schema' is not here.
 | local function is_state_connected(state)
 |     return state == 'active'
 | end
 |
 | local disconnected_states = {
 |     initial = true,
 |     error = true,
 |     error_reconnect = true,
 |     closed = true,
 | }
 |
 | local function is_state_disconnected(state)
 |     return disconnected_states[state]
 | end

And use this markers instead of `state == 'active'` and `errno ~= nil`
conditions.

What do you think about this way?

>              end
>              remote.state, remote.error = state, err
>              if state == 'error_reconnect' then
> @@ -989,6 +992,7 @@ local function new_sm(host, port, opts, connection, greeting)
>      remote._on_schema_reload = trigger.new("on_schema_reload")
>      remote._on_disconnect = trigger.new("on_disconnect")
>      remote._on_connect = trigger.new("on_connect")
> +    remote._is_connected = false
>      remote._transport = create_transport(host, port, user, password, callback,
>                                           connection, greeting)
>      remote._transport.start()