MXS-1786: Fix hang on COM_STATISTICS

The commands needs to be handled separately from the rest of the result
types.

Added a test case that reproduces the problem and verifies that the change
in code fixes it.

maxscale-system-test/CMakeLists.txt

@@ -637,6 +637,10 @@ add_test_executable(mxs1773_failing_ldli.cpp mxs1773_failing_ldli replication LA
 # https://jira.mariadb.org/browse/MXS-1776
 add_test_executable(mxs1776_ps_exec_hang.cpp mxs1776_ps_exec_hang replication LABELS readwritesplit REPL_BACKEND)
 
+# MXS-1786: Hang with COM_STATISTICS
+# https://jira.mariadb.org/browse/MXS-1786
+add_test_executable(mxs1786_statistics.cpp mxs1786_statistics replication LABELS readwritesplit REPL_BACKEND)
+
 # 'namedserverfilter' test
 add_test_executable(namedserverfilter.cpp namedserverfilter namedserverfilter LABELS namedserverfilter LIGHT REPL_BACKEND)
 

maxscale-system-test/mxs1786_statistics.cpp

@@ -0,0 +1,24 @@
+/**
+ * MXS-1786: Hang with COM_STATISTICS
+ */
+
+#include "testconnections.h"
+
+int main(int argc, char* argv[])
+{
+    TestConnections test(argc, argv);
+
+    test.maxscales->connect();
+
+    for (int i = 0; i < 10; i++)
+    {
+        test.set_timeout(10);
+        mysql_stat(test.maxscales->conn_rwsplit[0]);
+        test.try_query(test.maxscales->conn_rwsplit[0], "SELECT 1");
+    }
+
+    test.maxscales->disconnect();
+
+
+    return test.global_result;
+}

server/modules/routing/readwritesplit/readwritesplit.cc

@@ -534,6 +534,12 @@ bool reply_is_complete(SRWBackend& backend, GWBUF *buffer)
             backend->set_reply_state(REPLY_STATE_DONE);
         }
     }
+    else if (backend->current_command() == MXS_COM_STATISTICS)
+    {
+        // COM_STATISTICS returns a single string and thus requires special handling
+        LOG_RS(backend, REPLY_STATE_DONE);
+        backend->set_reply_state(REPLY_STATE_DONE);
+    }
     else if (backend->get_reply_state() == REPLY_STATE_START &&
         (!mxs_mysql_is_result_set(buffer) || GWBUF_IS_COLLECTED_RESULT(buffer)))
     {