The `mode` option for `call()` and `filter()`

[TarantoolBot]

Related dev. issue(s): tarantool/tarantool#9930
Parent doc. issue(s): #4129

Product: Tarantool
Since: 3.1
Root document: not created yet (#4129)
SME: @ ImeevMA

Details

The new mode option is now supported by the call() and filter()
functions from the experimental.connpool module. This option allows to
filter candidates based on their read-only status.

The filter() function supports three values of the mode option:

1. nil means that the read_only status of the instance is not
   checked;
2. ro means that only instances with read_only == true are
   considered;
3. rw means that only instances with read_only == false are
   considered.

The call() function supports five values of the mode option:

1. nil means that the read_only status of the instance is not
   checked when instance is selected to execute call();
2. ro means that only instances with read_only == true are
   considered when instance is selected to execute call();
3. rw means that only instances with read_only == false are
   considered when instance is selected to execute call().
4. prefer_ro means that call() will only be executed on instances
   with read_only == false if it is not possible to execute it on
   instances with read_only == true;
5. prefer_rw means that call() will only be executed on instances
   with read_only == true if it is not possible to execute it on
   instances with read_only == false.

Note that if this option is not nil, a connection will be attempted to
each instance in the config if a connection does not exist. This means
that any of these functions can potentially block for a maximum of
<number of instances> * 10 seconds.
Requested by @ ImeevMA in tarantool/tarantool@60fdffb.