softmmu/qdev-monitor: add error handling in qdev_set_id

qdev_set_id() is mostly used when the user adds a device (using
-device cli option or device_add qmp command). This commit adds
an error parameter to handle the case where the given id is
already taken.

Also document the function and add a return value in order to
be able to capture success/failure: the function now returns the
id in case of success, or NULL in case of failure.

The commit modifies the 2 calling places (qdev-monitor and
xen-legacy-backend) to add the error object parameter.

Note that the id is, right now, guaranteed to be unique because
all ids came from the "device" QemuOptsList where the id is used
as key. This addition is a preparation for a future commit which
will relax the uniqueness.

Signed-off-by: Damien Hedde <damien.hedde@greensocs.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20211008133442.141332-10-kwolf@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

hw/xen/xen-legacy-backend.c

@@ -276,7 +276,8 @@ static struct XenLegacyDevice *xen_be_get_xendev(const char *type, int dom,
     xendev = g_malloc0(ops->size);
     object_initialize(&xendev->qdev, ops->size, TYPE_XENBACKEND);
     OBJECT(xendev)->free = g_free;
-    qdev_set_id(DEVICE(xendev), g_strdup_printf("xen-%s-%d", type, dev));
+    qdev_set_id(DEVICE(xendev), g_strdup_printf("xen-%s-%d", type, dev),
+                &error_fatal);
     qdev_realize(DEVICE(xendev), xen_sysbus, &error_fatal);
     object_unref(OBJECT(xendev));
 

include/monitor/qdev.h

@@ -9,6 +9,29 @@ void qmp_device_add(QDict *qdict, QObject **ret_data, Error **errp);
 
 int qdev_device_help(QemuOpts *opts);
 DeviceState *qdev_device_add(QemuOpts *opts, Error **errp);
-void qdev_set_id(DeviceState *dev, char *id);
+
+/**
+ * qdev_set_id: parent the device and set its id if provided.
+ * @dev: device to handle
+ * @id: id to be given to the device, or NULL.
+ *
+ * Returns: the id of the device in case of success; otherwise NULL.
+ *
+ * @dev must be unrealized, unparented and must not have an id.
+ *
+ * If @id is non-NULL, this function tries to setup @dev qom path as
+ * "/peripheral/id". If @id is already taken, it fails. If it succeeds,
+ * the id field of @dev is set to @id (@dev now owns the given @id
+ * parameter).
+ *
+ * If @id is NULL, this function generates a unique name and setups @dev
+ * qom path as "/peripheral-anon/name". This name is not set as the id
+ * of @dev.
+ *
+ * Upon success, it returns the id/name (generated or provided). The
+ * returned string is owned by the corresponding child property and must
+ * not be freed by the caller.
+ */
+const char *qdev_set_id(DeviceState *dev, char *id, Error **errp);
 
 #endif

softmmu/qdev-monitor.c

@@ -593,22 +593,35 @@ static BusState *qbus_find(const char *path, Error **errp)
 }
 
 /* Takes ownership of @id, will be freed when deleting the device */
-void qdev_set_id(DeviceState *dev, char *id)
+const char *qdev_set_id(DeviceState *dev, char *id, Error **errp)
 {
-    if (id) {
-        dev->id = id;
-    }
+    ObjectProperty *prop;
 
-    if (dev->id) {
-        object_property_add_child(qdev_get_peripheral(), dev->id,
-                                  OBJECT(dev));
+    assert(!dev->id && !dev->realized);
+
+    /*
+     * object_property_[try_]add_child() below will assert the device
+     * has no parent
+     */
+    if (id) {
+        prop = object_property_try_add_child(qdev_get_peripheral(), id,
+                                             OBJECT(dev), NULL);
+        if (prop) {
+            dev->id = id;
+        } else {
+            g_free(id);
+            error_setg(errp, "Duplicate device ID '%s'", id);
+            return NULL;
+        }
     } else {
         static int anon_count;
         gchar *name = g_strdup_printf("device[%d]", anon_count++);
-        object_property_add_child(qdev_get_peripheral_anon(), name,
-                                  OBJECT(dev));
+        prop = object_property_add_child(qdev_get_peripheral_anon(), name,
+                                         OBJECT(dev));
         g_free(name);
     }
+
+    return prop->name;
 }
 
 DeviceState *qdev_device_add(QemuOpts *opts, Error **errp)
@@ -691,7 +704,13 @@ DeviceState *qdev_device_add(QemuOpts *opts, Error **errp)
         }
     }
 
-    qdev_set_id(dev, g_strdup(qemu_opts_id(opts)));
+    /*
+     * set dev's parent and register its id.
+     * If it fails it means the id is already taken.
+     */
+    if (!qdev_set_id(dev, g_strdup(qemu_opts_id(opts)), errp)) {
+        goto err_del_dev;
+    }
 
     /* set properties */
     if (qemu_opt_foreach(opts, set_property, dev, errp)) {