tests: qmp-test: verify command batching

OOB introduced DROP event for flow control.  This should not affect old
QMP clients.  Add a command batching check to make sure of it.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180309090006.10018-23-peterx@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>

tests/qmp-test.c

@@ -82,6 +82,7 @@ static void test_qmp_protocol(void)
     QTestState *qts;
     const QListEntry *entry;
     QString *qstr;
+    int i;
 
     qts = qtest_init_without_qmp_handshake(common_args);
 
@@ -139,6 +140,27 @@ static void test_qmp_protocol(void)
     g_assert_cmpint(qdict_get_int(resp, "id"), ==, 2);
     QDECREF(resp);
 
+    /*
+     * Test command batching.  In current test OOB is not enabled, we
+     * should be able to run as many commands in batch as we like.
+     * Using 16 (>8, which is OOB queue length) to make sure OOB won't
+     * break existing clients.  Note: this test does not control the
+     * scheduling of QEMU's QMP command processing threads so it may
+     * not really trigger batching inside QEMU.  This is just a
+     * best-effort test.
+     */
+    for (i = 0; i < 16; i++) {
+        qtest_async_qmp(qts, "{ 'execute': 'query-version' }");
+    }
+    /* Verify the replies to make sure no command is dropped. */
+    for (i = 0; i < 16; i++) {
+        resp = qtest_qmp_receive(qts);
+        /* It should never be dropped.  Each of them should be a reply. */
+        g_assert(qdict_haskey(resp, "return"));
+        g_assert(!qdict_haskey(resp, "event"));
+        QDECREF(resp);
+    }
+
     qtest_quit(qts);
 }