fix(balancer) avoid needless rebuild of balancer when adding target

kong/runloop/balancer.lua

@@ -416,8 +416,9 @@ local function check_target_history(upstream, balancer)
   -- compare balancer history with db-loaded history
   local last_equal_index = 0  -- last index where history is the same
   for i, entry in ipairs(old_history) do
-    if entry.order ~= (new_history[i] or EMPTY_T).order then
-      last_equal_index = i - 1
+    if new_history[i] and entry.order == new_history[i].order then
+      last_equal_index = i
+    else
       break
     end
   end

spec/01-unit/011-balancer_spec.lua

@@ -8,6 +8,7 @@ describe("Balancer", function()
   local uuid = require("kong.tools.utils").uuid
   local upstream_hc
   local upstream_ph
+  local upstream_ote
 
   teardown(function()
     ngx.log:revert()
@@ -78,9 +79,11 @@ describe("Balancer", function()
       [6] = { id = "f", name = "upstream_f", slots = 10, healthchecks = hc_defaults },
       [7] = { id = "hc", name = "upstream_hc", slots = 10, healthchecks = passive_hc },
       [8] = { id = "ph", name = "upstream_ph", slots = 10, healthchecks = passive_hc },
+      [9] = { id = "ote", name = "upstream_ote", slots = 10, healthchecks = hc_defaults },
     }
     upstream_hc = UPSTREAMS_FIXTURES[7]
     upstream_ph = UPSTREAMS_FIXTURES[8]
+    upstream_ote = UPSTREAMS_FIXTURES[9]
 
     TARGETS_FIXTURES = {
       -- 1st upstream; a
@@ -187,6 +190,14 @@ describe("Balancer", function()
         target = "127.0.0.1:2222",
         weight = 10,
       },
+      -- upstream_ote
+      {
+        id = "ote1",
+        created_at = "001",
+        upstream_id = "ote",
+        target = "localhost:1111",
+        weight = 10,
+      },
     }
 
     local function find_all_in_fixture_fn(fixture)
@@ -374,6 +385,31 @@ describe("Balancer", function()
     end)
   end)
 
+  describe("on_target_event()", function()
+    setup(function()
+      balancer._load_targets_into_memory("ote")
+    end)
+
+    it("adding a target does not recreate a balancer", function()
+      local b1 = balancer._create_balancer(upstream_ote)
+      assert.same(1, #(balancer._get_target_history(b1)))
+
+      table.insert(TARGETS_FIXTURES, {
+        id = "ote2",
+        created_at = "002",
+        upstream_id = "ote",
+        target = "localhost:1112",
+        weight = 10,
+      })
+      balancer.on_target_event("create", { upstream_id = "ote" })
+
+      local b2 = balancer._create_balancer(upstream_ote)
+      assert.same(2, #(balancer._get_target_history(b2)))
+
+      assert(b1 == b2)
+    end)
+  end)
+
   describe("post_health()", function()
     local hc, my_balancer