block: Introduce bdrv_open_child()

It is the same as bdrv_open_image(), except that it doesn't only return
success or failure, but the newly created BdrvChild object for the new
child node.

As the BdrvChild object already contains a BlockDriverState pointer (and
this is supposed to become the only pointer so that bdrv_append() and
friends can just change a single pointer in BdrvChild), the pbs
parameter is removed for bdrv_open_child().

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>

block.c

@@ -1102,7 +1102,7 @@ static int bdrv_fill_options(QDict **options, const char **pfilename,
     return 0;
 }
 
-static void bdrv_attach_child(BlockDriverState *parent_bs,
+static BdrvChild *bdrv_attach_child(BlockDriverState *parent_bs,
                                     BlockDriverState *child_bs,
                                     const BdrvChildRole *child_role)
 {
@@ -1113,6 +1113,8 @@ static void bdrv_attach_child(BlockDriverState *parent_bs,
     };
 
     QLIST_INSERT_HEAD(&parent_bs->children, child, next);
+
+    return child;
 }
 
 void bdrv_set_backing_hd(BlockDriverState *bs, BlockDriverState *backing_hd)
@@ -1229,7 +1231,7 @@ free_exit:
  * device's options.
  *
  * If allow_none is true, no image will be opened if filename is false and no
- * BlockdevRef is given. *pbs will remain unchanged and 0 will be returned.
+ * BlockdevRef is given. NULL will be returned, but errp remains unset.
  *
  * bdrev_key specifies the key for the image's BlockdevRef in the options QDict.
  * That QDict has to be flattened; therefore, if the BlockdevRef is a QDict
@@ -1237,21 +1239,20 @@ free_exit:
  * BlockdevRef.
  *
  * The BlockdevRef will be removed from the options QDict.
- *
- * To conform with the behavior of bdrv_open(), *pbs has to be NULL.
  */
-int bdrv_open_image(BlockDriverState **pbs, const char *filename,
+BdrvChild *bdrv_open_child(const char *filename,
                            QDict *options, const char *bdref_key,
-                    BlockDriverState* parent, const BdrvChildRole *child_role,
+                           BlockDriverState* parent,
+                           const BdrvChildRole *child_role,
                            bool allow_none, Error **errp)
 {
+    BdrvChild *c = NULL;
+    BlockDriverState *bs;
     QDict *image_options;
     int ret;
     char *bdref_key_dot;
     const char *reference;
 
-    assert(pbs);
-    assert(*pbs == NULL);
     assert(child_role != NULL);
 
     bdref_key_dot = g_strdup_printf("%s.", bdref_key);
@@ -1260,28 +1261,60 @@ int bdrv_open_image(BlockDriverState **pbs, const char *filename,
 
     reference = qdict_get_try_str(options, bdref_key);
     if (!filename && !reference && !qdict_size(image_options)) {
-        if (allow_none) {
+        if (!allow_none) {
-            ret = 0;
-        } else {
             error_setg(errp, "A block device must be specified for \"%s\"",
                        bdref_key);
-            ret = -EINVAL;
         }
         QDECREF(image_options);
         goto done;
     }
 
-    ret = bdrv_open_inherit(pbs, filename, reference, image_options, 0,
+    bs = NULL;
+    ret = bdrv_open_inherit(&bs, filename, reference, image_options, 0,
                             parent, child_role, NULL, errp);
     if (ret < 0) {
         goto done;
     }
 
-    bdrv_attach_child(parent, *pbs, child_role);
+    c = bdrv_attach_child(parent, bs, child_role);
 
 done:
     qdict_del(options, bdref_key);
-    return ret;
+    return c;
+}
+
+/*
+ * This is a version of bdrv_open_child() that returns 0/-EINVAL instead of
+ * a BdrvChild object.
+ *
+ * If allow_none is true, no image will be opened if filename is false and no
+ * BlockdevRef is given. *pbs will remain unchanged and 0 will be returned.
+ *
+ * To conform with the behavior of bdrv_open(), *pbs has to be NULL.
+ */
+int bdrv_open_image(BlockDriverState **pbs, const char *filename,
+                    QDict *options, const char *bdref_key,
+                    BlockDriverState* parent, const BdrvChildRole *child_role,
+                    bool allow_none, Error **errp)
+{
+    Error *local_err = NULL;
+    BdrvChild *c;
+
+    assert(pbs);
+    assert(*pbs == NULL);
+
+    c = bdrv_open_child(filename, options, bdref_key, parent, child_role,
+                        allow_none, &local_err);
+    if (local_err) {
+        error_propagate(errp, local_err);
+        return -EINVAL;
+    }
+
+    if (c != NULL) {
+        *pbs = c->bs;
+    }
+
+    return 0;
 }
 
 int bdrv_append_temp_snapshot(BlockDriverState *bs, int flags, Error **errp)

include/block/block.h

@@ -12,6 +12,7 @@
 /* block.c */
 typedef struct BlockDriver BlockDriver;
 typedef struct BlockJob BlockJob;
+typedef struct BdrvChild BdrvChild;
 typedef struct BdrvChildRole BdrvChildRole;
 
 typedef struct BlockDriverInfo {
@@ -208,6 +209,11 @@ int bdrv_open_image(BlockDriverState **pbs, const char *filename,
                     QDict *options, const char *bdref_key,
                     BlockDriverState* parent, const BdrvChildRole *child_role,
                     bool allow_none, Error **errp);
+BdrvChild *bdrv_open_child(const char *filename,
+                           QDict *options, const char *bdref_key,
+                           BlockDriverState* parent,
+                           const BdrvChildRole *child_role,
+                           bool allow_none, Error **errp);
 void bdrv_set_backing_hd(BlockDriverState *bs, BlockDriverState *backing_hd);
 int bdrv_open_backing_file(BlockDriverState *bs, QDict *options, Error **errp);
 int bdrv_append_temp_snapshot(BlockDriverState *bs, int flags, Error **errp);

include/block/block_int.h

@@ -335,11 +335,11 @@ struct BdrvChildRole {
 extern const BdrvChildRole child_file;
 extern const BdrvChildRole child_format;
 
-typedef struct BdrvChild {
+struct BdrvChild {
     BlockDriverState *bs;
     const BdrvChildRole *role;
     QLIST_ENTRY(BdrvChild) next;
-} BdrvChild;
+};
 
 /*
  * Note: the function bdrv_append() copies and swaps contents of