[Tarantool-patches] [PATCH rfc 0/1] replication: stop resetting existing connections

[Olga Arkhangelskaia <arkholga@tarantool.org>]

Every time we want to change replication configuration even slightly we
need to reset all the existing connections and bootstrap replica from
every beginning.
Such behavior has some shortcomings:
https://github.com/tarantool/tarantool/issues/4669
https://github.com/tarantool/tarantool/issues/4668

In this rfc I tried to get rid of extra job and create and delete only
those appliers that has to be deleted (no more in cfg file) or to be
created (new one).
I used  straightforward way to check if it is possible and revel all
problem that we will face in case we decide to change schema.
(And may be add smth like box_add_replica_source or box_remove_replica
cllbacks)

Quorum problem:
In case quorum < total and replica did not get it's UUID (in anon list)
replica will stuck in anon list forever. I mean next time we have this
source in cfg it will treated as existing one.
At the moment I clean anon list every time.
Anon problem:
While anon - nonanon transition we need to remove anon replica and
re-connect it. From config point of view replica exists, so i need to
preserve anon state for some time and add such replica to remove array
and to new appliers array. 
Connection to self:
Because we only throw away replicas whose sources are in remove array
replica with connection to self will assert. I just added special check for
self connection while removing replicas from the replicaset.

I use applier->source to distinguish whether applier should be created.

Another way to solve part of problems caused by #4669 #4668 is too check
existing and new configuration for exact match.
    
I need your help to see if I have missed something. And would be happy
to read your thoughts and suggestions. 

Olga Arkhangelskaia (1):
  replication: stop resetting existing connections

 src/box/box.cc                    | 106 +++++++++++++++++++++++++-----
 src/box/box.h                     |   1 +
 src/box/replication.cc            |  56 +++++++++++-----
 src/box/replication.h             |   3 +-
 test/replication/misc.result      |   6 ++
 test/replication/misc.test.lua    |   2 +
 test/replication/quorum.result    |   3 +
 test/replication/quorum.test.lua  |   1 +
 test/replication/replica_self.lua |  11 ++++
 test/replication/self.result      |  68 +++++++++++++++++++
 test/replication/self.test.lua    |  25 +++++++
 11 files changed, 248 insertions(+), 34 deletions(-)
 create mode 100644 test/replication/replica_self.lua
 create mode 100644 test/replication/self.result
 create mode 100644 test/replication/self.test.lua

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2.20.1 (Apple Git-117)