Do not use call_16 by default

The call_16 became obsolete when tarantool-1.6 was gone. There're
no reasons to continue using the old version by default.

Change discovery function requirements in part of single
value support. This is done to be consistent with other Tarantool
connectors.

Closes: #196

Author: nicktorwald

README.md

@@ -237,8 +237,7 @@ tarantool> function get_cluster_nodes() return { 'host1:3301', 'host2:3302', 'ho
 
 You need to pay attention to a function contract we are currently supporting:
 * The client never passes any arguments to a discovery function.
-* A discovery function _should_ return a single result of strings (i.e. single 
-  string `return 'host:3301'` or array of strings `return {'host1:3301', 'host2:3301'}`).
+* A discovery function _must_ return an array of strings (i.e `return {'host1:3301', 'host2:3301'}`).
 * A discovery function _may_ return multi-results but the client takes
   into account only first of them (i.e. `return {'host:3301'}, discovery_delay`, where 
   the second result is unused). Even more, any extra results __are reserved__ by the client

src/main/java/org/tarantool/AbstractTarantoolOps.java

@@ -4,7 +4,7 @@
 public abstract class AbstractTarantoolOps<Space, Tuple, Operation, Result>
     implements TarantoolClientOps<Space, Tuple, Operation, Result> {
 
-    private Code callCode = Code.OLD_CALL;
+    private Code callCode = Code.CALL;
 
     protected abstract Result exec(Code code, Object... args);
 

src/main/java/org/tarantool/TarantoolClientConfig.java

@@ -44,10 +44,12 @@ public class TarantoolClientConfig {
     public long writeTimeoutMillis = 60 * 1000L;
 
     /**
-     * Use old call command https://github.com/tarantool/doc/issues/54,
-     * please ensure that you server supports new call command.
+     * Use new call method instead of obsolete
+     * {@code call_16} which used to work in Tarantool v1.6.
+     *
+     * Since 1.9.3, this flag has become enabled by default
      */
-    public boolean useNewCall = false;
+    public boolean useNewCall = true;
 
     /**
      *  Max time to establish connection to the server

src/main/java/org/tarantool/TarantoolClientImpl.java

@@ -115,11 +115,11 @@ private void initClient(SocketChannelProvider socketProvider, TarantoolClientCon
         this.syncOps = new SyncOps();
         this.composableAsyncOps = new ComposableAsyncOps();
         this.fireAndForgetOps = new FireAndForgetOps();
-        if (config.useNewCall) {
-            setCallCode(Code.CALL);
-            this.syncOps.setCallCode(Code.CALL);
-            this.fireAndForgetOps.setCallCode(Code.CALL);
-            this.composableAsyncOps.setCallCode(Code.CALL);
+        if (!config.useNewCall) {
+            setCallCode(Code.OLD_CALL);
+            this.syncOps.setCallCode(Code.OLD_CALL);
+            this.fireAndForgetOps.setCallCode(Code.OLD_CALL);
+            this.composableAsyncOps.setCallCode(Code.OLD_CALL);
         }
     }
 

src/main/java/org/tarantool/cluster/TarantoolClusterStoredFunctionDiscoverer.java

@@ -55,7 +55,7 @@ private void checkResult(List<?> result) {
         if (result == null || result.isEmpty()) {
             throw new IllegalDiscoveryFunctionResult("Discovery function returned no data");
         }
-        if (!((List<Object>)result.get(0)).stream().allMatch(item -> item instanceof String)) {
+        if (!(result.get(0) instanceof List)) {
             throw new IllegalDiscoveryFunctionResult("The first value must be an array of strings");
         }
     }

src/test/java/org/tarantool/ClientReconnectClusterIT.java

@@ -359,7 +359,7 @@ void testDelayFunctionResultFetch() {
         instances.get(SRV1)
             .executeLua("co = coroutine.create(function() " + functionBody + " end)");
         instances.get(SRV1)
-            .executeLua("function getAddressesFunction() local c, r = coroutine.resume(co); return r end");
+            .executeLua("function getAddressesFunction() local c, r = coroutine.resume(co); return {r} end");
 
         String infoFunctionScript = makeDiscoveryFunction(infoFunctionName, Collections.singletonList(service3Address));
         instances.get(SRV2).executeLua(infoFunctionScript);

src/test/java/org/tarantool/cluster/ClusterServiceStoredFunctionDiscovererIT.java

@@ -153,6 +153,18 @@ public void testWrongTypeResultData() {
         assertThrows(IllegalDiscoveryFunctionResult.class, discoverer::getInstances);
     }
 
+    @Test
+    @DisplayName("fetched with an exception when a single string returned")
+    public void testSingleStringResultData() {
+        String functionCode = makeDiscoveryFunction(ENTRY_FUNCTION_NAME, "'host1:3301'");
+        control.openConsole(INSTANCE_NAME).exec(functionCode);
+
+        TarantoolClusterStoredFunctionDiscoverer discoverer =
+            new TarantoolClusterStoredFunctionDiscoverer(clusterConfig, client);
+
+        assertThrows(IllegalDiscoveryFunctionResult.class, discoverer::getInstances);
+    }
+
     @Test
     @DisplayName("fetched with an exception using no return function")
     public void testFunctionWithNoReturn() {