fix(DNS): prefer SRV in service discovery (#4992)

Fix #4986

Signed-off-by: spacewander <spacewanderlzx@gmail.com>

apisix/discovery/dns.lua

@@ -78,7 +78,7 @@ function _M.init_worker()
         hosts = {},
         resolvConf = {},
         nameservers = servers,
-        order = {"last", "A", "AAAA", "SRV", "CNAME"},
+        order = {"last", "SRV", "A", "AAAA", "CNAME"},
     }
 
     local client, err = core.dns_client.new(opts)

t/coredns/db.test.local

@@ -45,3 +45,7 @@ split-weight.srv   86400 IN    SRV 10      0     1980 C
 
 priority.srv   86400 IN    SRV 10       60     1979 A
 priority.srv   86400 IN    SRV 20       60     1980 B
+
+; a domain has both SRV & A records
+srv-a   86400 IN    SRV 10       60     1980 A
+srv-a         IN    A   127.0.0.1

t/discovery/dns/mix.t

@@ -110,6 +110,9 @@ upstreams:
                     end
                 end
 
+                -- It is expected to have 5 DNS queries
+                -- the first turn: one for global resolver & two for discovery (SRV, then A)
+                -- the second turn: each one for both global resolver & discovery
                 if i < 2 then
                     ngx.sleep(1.1)
                 end
@@ -125,3 +128,4 @@ connect to 127.0.0.1:1053
 connect to 127.0.0.1:1053
 connect to 127.0.0.1:1053
 connect to 127.0.0.1:1053
+connect to 127.0.0.1:1053

t/discovery/dns/sanity.t

@@ -284,3 +284,17 @@ qr/proxy request to \S+/
 --- grep_error_log_out
 proxy request to 127.0.0.1:1979
 proxy request to 127.0.0.2:1980
+
+
+
+=== TEST 14: prefer SRV than A
+--- apisix_yaml
+upstreams:
+    - service_name: "srv-a.test.local"
+      discovery_type: dns
+      type: roundrobin
+      id: 1
+--- error_log
+proxy request to 127.0.0.1:1980
+--- response_body
+hello world