test: add a test for join with self delete

Deletion of the own entry from _cluster space is allowed during
the join stage, because the remote master could have already had
the joining instance UUID in _cluster space but then deleted it.

Then for the joining instance it looks like deletion of self from
_cluster. But that is fine - in the end of join the master will
register the replica again.

The case is handled, but not covered with a test. The patch adds
one.

NO_DOC=test
NO_CHANGELOG=test

test/replication-luatest/join_test.lua

+local t = require('luatest')
+local server = require('luatest.server')
+local uuid = require('uuid')
+
+local g = t.group('join')
+
+g.before_all(function(lg)
+    lg.master = server:new({
+        alias = 'master',
+        box_cfg = {
+            replication_timeout = 0.1,
+        },
+    })
+    lg.master:start()
+end)
+
+g.after_all(function(lg)
+    lg.master:drop()
+end)
+
+--
+-- New replica (or one being re-bootstrapped) can legally see deletion of self
+-- from _cluster space during join. It is allowed because the master could
+-- delete the replica from _cluster before the replica tried to rejoin or join
+-- first time via a previously existing UUID.
+--
+g.test_fetch_self_delete_during_final_join = function(lg)
+    t.tarantool.skip_if_not_debug()
+    lg.master:exec(function()
+        local s = box.schema.create_space('test')
+        s:create_index('pk')
+        s:replace{1}
+        box.error.injection.set('ERRINJ_RELAY_SEND_DELAY', true)
+    end)
+    local replica_uuid = uuid.str()
+    local box_cfg = table.deepcopy(lg.master.box_cfg)
+    box_cfg.replication = {lg.master.net_box_uri}
+    box_cfg.instance_uuid = replica_uuid
+    local replica = server:new({
+        alias = 'replica',
+        box_cfg = box_cfg,
+    })
+    replica:start({wait_until_ready = false})
+    local msg = ('joining replica %s'):format(replica_uuid):gsub('%-', '%%-')
+    t.helpers.retrying({}, function()
+        t.assert(lg.master:grep_log(msg))
+    end)
+    lg.master:exec(function(replica_uuid)
+        local _cluster = box.space._cluster
+        _cluster:replace{2, replica_uuid}
+        _cluster:delete{2}
+        _cluster:replace{2, replica_uuid}
+        _cluster:delete{2}
+        box.error.injection.set('ERRINJ_RELAY_SEND_DELAY', false)
+    end, {replica_uuid})
+    replica:wait_until_ready()
+    local replica_id = replica:exec(function()
+        t.assert_equals(box.space.test:get{1}, {1})
+        return box.info.id
+    end)
+    replica:drop()
+    lg.master:exec(function(replica_id)
+        box.space.test:drop()
+        box.space._cluster:delete{replica_id}
+    end, {replica_id})
+end