feat(DNS): allow specifying port to override the discovered one (#6764)

It makes the workaround in https://github.com/apache/apisix/issues/6732#issuecomment-1082597439
work.
Signed-off-by: spacewander <spacewanderlzx@gmail.com>

apisix/discovery/dns/init.lua

@@ -37,10 +37,10 @@ function _M.nodes(service_name)
     local nodes = core.table.new(#records, 0)
     for i, r in ipairs(records) do
         if r.address then
-            local node_port = r.port
-            if not node_port or node_port == 0 then
+            local node_port = port
+            if not node_port and r.port ~= 0 then
                 -- if the port is zero, fallback to use the default
-                node_port = port
+                node_port = r.port
             end
 
             nodes[i] = {host = r.address, weight = r.weight or 1, port = node_port}

docs/en/latest/discovery/dns.md

@@ -135,3 +135,6 @@ being selected.
 
 We treat weight 0 record has a weight of 1 so the node "have a very small chance of
 being selected", which is also the common way to treat this type of record.
+
+For SRV record which has port 0, we will fallback to use the upstream protocol's default port.
+You can also specify the port in the "service_name" field directly, like "srv.blah.service:8848".

docs/zh/latest/discovery/dns.md

@@ -128,3 +128,6 @@ Upstream 配置是这样的：
 > 当没有任何候选服务器时，域管理员应使用权重为 0 的，使 RR 更为易读（噪音更少）。当存在权重大于 0 的记录时，权重为 0 的记录被选中的可能性很小。
 
 我们把权重为 0 的记录当作权重为 1，因此节点“被选中的可能性很小”，这也是处理此类记录的常用方法。
+
+对于端口为 0 的 SRV 记录，我们会使用上游协议的默认端口。
+你也可以在 “service_name” 字段中直接指定端口，比如 “srv.blah.service:8848”。

t/discovery/dns/sanity.t

@@ -293,3 +293,19 @@ connect() failed
 qr/proxy request to \S+/
 --- grep_error_log_out
 proxy request to 127.0.0.1:80
+
+
+
+=== TEST 15: SRV (override port)
+--- apisix_yaml
+upstreams:
+    - service_name: "port.srv.test.local:1980"
+      discovery_type: dns
+      type: roundrobin
+      id: 1
+--- grep_error_log eval
+qr/upstream nodes: \{[^}]+\}/
+--- grep_error_log_out eval
+qr/upstream nodes: \{("127.0.0.1:1980":60,"127.0.0.2:1980":20|"127.0.0.2:1980":20,"127.0.0.1:1980":60)\}/
+--- response_body
+hello world