fix: last_err can be nil when the reconnection is successful (#8377)

apisix/core/config_etcd.lua

@@ -574,7 +574,7 @@ local function _automatic_fetch(premature, self)
                 if err ~= self.last_err then
                     self.last_err = err
                     self.last_err_time = ngx_time()
-                else
+                elseif self.last_err then
                     if ngx_time() - self.last_err_time >= 30 then
                         self.last_err = nil
                     end
@@ -604,6 +604,12 @@ local function _automatic_fetch(premature, self)
     end
 end
 
+-- for test
+_M.test_automatic_fetch = _automatic_fetch
+function _M.inject_sync_data(f)
+    sync_data = f
+end
+
 
 ---
 -- Create a new connection to communicate with the control plane.

t/core/config_etcd.t

@@ -317,3 +317,33 @@ qr/healthy check use \S+ \w+/
 --- grep_error_log_out eval
 qr/healthy check use round robin
 (healthy check use ngx.shared dict){1,}/
+
+
+
+=== TEST 10: last_err can be nil when the reconnection is successful
+--- config
+    location /t {
+        content_by_lua_block {
+            local config_etcd = require("apisix.core.config_etcd")
+            local count = 0
+            config_etcd.inject_sync_data(function()
+                if count % 2 == 0 then
+                    count = count + 1
+                    return nil, "has no healthy etcd endpoint available"
+                else
+                    return true
+                end
+            end)
+            config_etcd.test_automatic_fetch(false, {
+                running = true,
+                resync_delay = 1,
+            })
+            ngx.say("passed")
+        }
+    }
+--- request
+GET /t
+--- error_log
+reconnected to etcd
+--- response_body
+passed