softmmu: fix device deletion events with -device JSON syntax

The -device JSON syntax impl leaks a reference on the created
DeviceState instance. As a result when you hot-unplug the
device, the device_finalize method won't be called and thus
it will fail to emit the required DEVICE_DELETED event.

A 'json-cli' feature was previously added against the
'device_add' QMP command QAPI schema to indicated to mgmt
apps that -device supported JSON syntax. Given the hotplug
bug that feature flag is not usable for its purpose, so
we add a new 'json-cli-hotplug' feature to indicate the
-device supports JSON without breaking hotplug.

Fixes: 5dacda5167560b3af8eadbce5814f60ba44b467e
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/802
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20220105123847.4047954-2-berrange@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Tested-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

qapi/qdev.json

@@ -44,6 +44,9 @@
 # @json-cli: If present, the "-device" command line option supports JSON
 #            syntax with a structure identical to the arguments of this
 #            command.
+# @json-cli-hotplug: If present, the "-device" command line option supports JSON
+#                    syntax without the reference counting leak that broke
+#                    hot-unplug
 #
 # Notes:
 #
@@ -74,7 +77,7 @@
 { 'command': 'device_add',
   'data': {'driver': 'str', '*bus': 'str', '*id': 'str'},
   'gen': false, # so we can get the additional arguments
-  'features': ['json-cli'] }
+  'features': ['json-cli', 'json-cli-hotplug'] }
 
 ##
 # @device_del:

softmmu/vl.c

@@ -2684,6 +2684,7 @@ static void qemu_create_cli_devices(void)
     qemu_opts_foreach(qemu_find_opts("device"),
                       device_init_func, NULL, &error_fatal);
     QTAILQ_FOREACH(opt, &device_opts, next) {
+        DeviceState *dev;
         loc_push_restore(&opt->loc);
         /*
          * TODO Eventually we should call qmp_device_add() here to make sure it
@@ -2692,7 +2693,8 @@ static void qemu_create_cli_devices(void)
          * from the start, so call qdev_device_add_from_qdict() directly for
          * now.
          */
-        qdev_device_add_from_qdict(opt->opts, true, &error_fatal);
+        dev = qdev_device_add_from_qdict(opt->opts, true, &error_fatal);
+        object_unref(OBJECT(dev));
         loc_pop(&opt->loc);
     }
     rom_reset_order_override();

tests/qtest/device-plug-test.c

@@ -77,6 +77,23 @@ static void test_pci_unplug_request(void)
     qtest_quit(qtest);
 }
 
+static void test_pci_unplug_json_request(void)
+{
+    QTestState *qtest = qtest_initf(
+        "-device '{\"driver\": \"virtio-mouse-pci\", \"id\": \"dev0\"}'");
+
+    /*
+     * Request device removal. As the guest is not running, the request won't
+     * be processed. However during system reset, the removal will be
+     * handled, removing the device.
+     */
+    device_del(qtest, "dev0");
+    system_reset(qtest);
+    wait_device_deleted_event(qtest, "dev0");
+
+    qtest_quit(qtest);
+}
+
 static void test_ccw_unplug(void)
 {
     QTestState *qtest = qtest_initf("-device virtio-balloon-ccw,id=dev0");
@@ -145,6 +162,8 @@ int main(int argc, char **argv)
      */
     qtest_add_func("/device-plug/pci-unplug-request",
                    test_pci_unplug_request);
+    qtest_add_func("/device-plug/pci-unplug-json-request",
+                   test_pci_unplug_json_request);
 
     if (!strcmp(arch, "s390x")) {
         qtest_add_func("/device-plug/ccw-unplug",