router: fix discovery service being nullified too late

If user tries to reconfigure vshard very often, it may happen, that
discovery service didn't manage to notice, that it's cancelled, before
the new one is started. In such case the new fiber may encounter
assertion failure and will be restarted several times.

The approach, which makes the service nullification, when the current
one is the one stored globally was chosen, since with blocking service
cancellation reconfiguration of vshard may take huge amount of time.

Part of #9

NO_DOC=bugfix

Author: Serpentian

test/router-luatest/router_2_2_test.lua

@@ -1140,3 +1140,28 @@ g.test_retryable_transfer_in_progress = function(g)
         _G.bucket_gc_wait()
     end, {bid, g.replica_2_a:replicaset_uuid()})
 end
+
+--
+-- gh-ee-9: service objects are nullified too late.
+--
+g.test_discovery_can_be_restarted_fast = function(g)
+    local new_cfg_template = table.deepcopy(cfg_template)
+    new_cfg_template.discovery_mode = 'once'
+    local new_global_cfg = vtest.config_new(new_cfg_template)
+    vtest.clear_test_cfg_options(new_global_cfg)
+
+    g.router:exec(function(cfg)
+        local internal = ivshard.router.internal
+        internal.errinj.ERRINJ_LONG_DISCOVERY = true
+        ilt.helpers.retrying({timeout = 20}, function()
+            ivshard.router.discovery_wakeup()
+            ilt.assert_equals(internal.errinj.ERRINJ_LONG_DISCOVERY, 'waiting')
+        end)
+        ivshard.router._bucket_reset(1)
+        ivshard.router.cfg(cfg)
+        internal.errinj.ERRINJ_LONG_DISCOVERY = false
+    end, {new_global_cfg})
+
+    t.assert_not(g.router:grep_log('assertion failed'))
+    vtest.router_cfg(g.router, global_cfg)
+end

vshard/router/init.lua

@@ -403,19 +403,20 @@ local function discovery_service_f(router, service)
             end
             while M.errinj.ERRINJ_LONG_DISCOVERY do
                 M.errinj.ERRINJ_LONG_DISCOVERY = 'waiting'
-                lfiber.sleep(0.01)
+                -- Note, that the errinj won't throw FiberIsCancelled error.
+                pcall(lfiber.sleep, 0.01)
             end
         until next(router.replicasets)
     end
 end
 
 local function discovery_f(router)
-    assert(not router.discovery_service)
     local service = lservice_info.new('discovery')
     router.discovery_service = service
     local ok, err = pcall(discovery_service_f, router, service)
-    assert(router.discovery_service == service)
-    router.discovery_service = nil
+    if router.discovery_service == service then
+        router.discovery_service = nil
+    end
     if not ok then
         error(err)
     end