Formatting variables and then passing them into the logger functions leads
to performance issues. i.e.
```erlang
logger:debug("RECV ~s", [emqx_packet:format(Packet)])
```
Above message will only be printed when the current log level set to
`debug`, but the function emqx_packet:format/1 will always be evaluated no
matter what the current log level is.
OTP 21 provides a special meta-data named `report_cb`, which can be used
for lazy evaluation. The fun is only evaluated if the primary/handler log level
check passes, and is therefore recommended if it is expensive to generate
the message.
Prior to this change, emqx_client:start_link does 2 works in one call:
- init an erlang process for emqx_client
- send MQTT CONNECT to remote broker
But this solution have some drawbacks:
- the return value of `start_link` compiles the return values of the 2
works: {ok, Pid, MqttResult}. It is inconsistent with the return value
of `gen_statem:start_link`, may causes confusions.
- the return mode of the 2 works are different:
`start_link` should always return {ok, Pid} or {error, Reason}, but
connecting to mqtt may throw out exceptions as it handles the
socket. But the caller couldn't have thought of the exception, he would
pattern match on the result of `emqx_client:start_link`, but it crashed!
- If the init work succeed but the connection failed, the caller couldn't
get a Pid from the return value, but indeed it was created inside the
emqx_client. This hides the fact that the Pid was created, and when the
Pid dies, the caller would receive an message from a Pid it doesn' know about.
This change divived these 2 work into 2 APIs:
- `start_link/1` is to build and verify the options, and returns {ok,Pid}
(on success) or {error, Reason} (on failure).
- `connect/1` is to send MQTT CONNECT, and returns {ok, MQTTResult::properties()} or
{error, MQTTReason}. MQTT reason codes will contains in the `MQTTReason`.
Prior to this change, there are ok, {binary, _Data}. {datagram, _Peer,
_Dara} case clauses, and the {binary, _Data} and {datagram, _Peer,
_Data} are unnecessary cases
This change delete these two cases and add ok in the end of funtion in
send_fun of emqx_ws_connection.
Fixed bugs:
- Priority queue lack of a `len + 1` logic in `in/2`
Changed behaviors:
- Topics not found in priority table (from config) will be treated with default priority,
instead of hasing topic name to a priority number.
- Default priority is now configurable (it was always lower than all configured priorities)
- The dropped message due to reaching `max_len` is now returned from `in/2`,
so the queue owner (`in/2` caller) can perform autopsy on it
Prior to this change, you can just use CN or EN field from the client
certificate as username.
This change add a new option to allow user to use Certificate directly as
username.
terry-xiaoyu