Synchronous requests are not allowed in net.box triggers

[TarantoolBot]

Related dev. issue: tarantool/tarantool#5358
Product: Tarantool
Since: 2.4.2
Audience/target: administrators, developers
Root document: https://www.tarantool.io/en/doc/latest/reference/reference_lua/net_box/#triggers
SME: @ locker

Details

An attempt to issue a synchronous request (e.g. call) from
net.box triggers on_connect and on_schema_reload now raises
an error: "Synchronous requests are not allowed in net.box trigger"
(Before tarantool/tarantool#5358 was
fixed, it silently hung.)

Invoking an asynchronous request (see is_async option) is allowed,
but the request will not be processed until the trigger returns and
an attempt to wait for the request completion with future:pairs()
or future:wait_result() will raise the same error.
Requested by @locker in tarantool/tarantool@0d944f9.

Definition of done

• net.box triggers section explains that synchronous requests are not allowed
• net.box triggers section explains how asynchronous requests are processed