block: Use JSON null instead of "" to disable backing file

BlockdevRef is an alternate of BlockdevOptions (inline definition) and
str (reference to an existing block device by name).  BlockdevRef
value "" is special: "no block device should be referenced."  It's
actually interpreted that way in just one place: optional member
@backing of COW formats.  Semantics:

* Present means "use this block device" as backing storage

* Absent means "default to the one stored in the image"

* Except "" means "don't use backing storage at all"

The first two are perfectly normal: when the parameter is absent, it
defaults to an implied value, but the value's meaning is the same.

The third one overloads the parameter with a second meaning.  The
overloading is *implicit*, i.e. it's not visible in the types.  Works
here, because "" is not a value block device ID.

Pressing argument values the schema accepts, but are semantically
invalid, into service to mean "do something else entirely" is not
general, as suitable invalid values need not exist.  I also find it
ugly.

To clean this up, we could add a separate flag argument to suppress
@backing, or add a distinct value to @backing.  This commit implements
the latter: add JSON null to the values of @backing, deprecate "".

Because we're so close to the 2.10 freeze, implement it in the
stupidest way possible: have qmp_blockdev_add() rewrite null to ""
before anything else can see the null.  Works, because BlockdevRef
occurs only within arguments of blockdev-add.  The proper way to do it
would be rewriting "" to null, preferably in a cleaner way, but that
requires fixing up code to work with null.  Add a TODO comment for
that.

Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Daniel P. Berrange <[email protected]>
Acked-by: Kevin Wolf <[email protected]>
Reviewed-by: Eric Blake <[email protected]>

diff --git a/blockdev.c b/blockdev.c
index 6469f161df0f367be419ebd9e717be8c157db235..02cd69bc21ad9859c6bfe72d14ee5a3c8fe1a19d 100644 (file)
--- a/blockdev.c
+++ b/blockdev.c
@@ -3887,6 +3887,7 @@ void qmp_blockdev_add(BlockdevOptions *options, Error **errp)
     QObject *obj;
     Visitor *v = qobject_output_visitor_new(&obj);
     QDict *qdict;
+    const QDictEntry *ent;
     Error *local_err = NULL;
 
     visit_type_BlockdevOptions(v, NULL, &options, &local_err);
@@ -3900,6 +3901,19 @@ void qmp_blockdev_add(BlockdevOptions *options, Error **errp)
 
     qdict_flatten(qdict);
 
+    /*
+     * Rewrite "backing": null to "backing": ""
+     * TODO Rewrite "" to null instead, and perhaps not even here
+     */
+    for (ent = qdict_first(qdict); ent; ent = qdict_next(qdict, ent)) {
+        char *dot = strrchr(ent->key, '.');
+
+        if (!strcmp(dot ? dot + 1 : ent->key, "backing")
+            && qobject_type(ent->value) == QTYPE_QNULL) {
+            qdict_put(qdict, ent->key, qstring_new());
+        }
+    }
+
     if (!qdict_get_try_str(qdict, "node-name")) {
         error_setg(errp, "'node-name' must be specified for the root node");
         goto fail;

diff --git a/qapi/block-core.json b/qapi/block-core.json
index ff8e2ba0cb963a04a08a06f74ef77a8aa723af0e..6866ae8a38e9b252601c0b191d8a1c67eec9224a 100644 (file)
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -2273,15 +2273,14 @@
 # besides their data source and an optional backing file.
 #
 # @backing:     reference to or definition of the backing file block
-#               device (if missing, taken from the image file content). It is
-#               allowed to pass an empty string here in order to disable the
-#               default backing file.
+#               device, null disables the backing file entirely.
+#               Defaults to the backing file stored the image file.
 #
 # Since: 2.9
 ##
 { 'struct': 'BlockdevOptionsGenericCOWFormat',
   'base': 'BlockdevOptionsGenericFormat',
-  'data': { '*backing': 'BlockdevRef' } }
+  'data': { '*backing': 'BlockdevRefOrNull' } }
 
 ##
 # @Qcow2OverlapCheckMode:
@@ -3120,9 +3119,7 @@
 # Reference to a block device.
 #
 # @definition:      defines a new block device inline
-# @reference:       references the ID of an existing block device. An
-#                   empty string means that no block device should be
-#                   referenced.
+# @reference:       references the ID of an existing block device
 #
 # Since: 2.9
 ##
@@ -3130,6 +3127,24 @@
   'data': { 'definition': 'BlockdevOptions',
             'reference': 'str' } }
 
+##
+# @BlockdevRefOrNull:
+#
+# Reference to a block device.
+#
+# @definition:      defines a new block device inline
+# @reference:       references the ID of an existing block device.
+#                   An empty string means that no block device should
+#                   be referenced.  Deprecated; use null instead.
+# @null:            No block device should be referenced (since 2.10)
+#
+# Since: 2.9
+##
+{ 'alternate': 'BlockdevRefOrNull',
+  'data': { 'definition': 'BlockdevOptions',
+            'reference': 'str',
+            'null': 'null' } }
+
 ##
 # @blockdev-add:
 #

diff --git a/tests/qemu-iotests/085 b/tests/qemu-iotests/085
index 71efe50d34984be069b6ce1c6405d0032daed6ca..5c7668cf9bacbe5c0e76f2250dd0cece3782bc06 100755 (executable)
--- a/tests/qemu-iotests/085
+++ b/tests/qemu-iotests/085
@@ -106,7 +106,7 @@ function add_snapshot_image()
     snapshot_file="${TEST_DIR}/${1}-${snapshot_virt0}"
     _make_test_img -u -b "${base_image}" "$size"
     mv "${TEST_IMG}" "${snapshot_file}"
-    do_blockdev_add "$1" "'backing': '', " "${snapshot_file}"
+    do_blockdev_add "$1" "'backing': null, " "${snapshot_file}"
 }
 
 # ${1}: unique identifier for the snapshot filename

diff --git a/tests/qemu-iotests/139 b/tests/qemu-iotests/139
index 9ff51d964767a8ba6df99289efe8c49fb9c0105e..50cf40fbd53654b3bccabccd375dc80472073422 100644 (file)
--- a/tests/qemu-iotests/139
+++ b/tests/qemu-iotests/139
@@ -69,7 +69,7 @@ class TestBlockdevDel(iotests.QMPTestCase):
                          '-b', base_img, new_img, '1M')
         opts = {'driver': iotests.imgfmt,
                 'node-name': node,
-                'backing': '',
+                'backing': None,
                 'file': {'driver': 'file',
                          'filename': new_img}}
         result = self.vm.qmp('blockdev-add', conv_keys = False, **opts)