block: Add bdrv_open_image()

Add a common function for opening images to be used for block drivers
specified through BlockdevRefs in an option QDict. The difference from
bdrv_file_open() is that this function may invoke bdrv_open() instead,
allowing auto-detection of the driver to be used; and second, it
automatically extracts the BlockdevRef from the option QDict.

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

block.c

@@ -1040,6 +1040,79 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict *options, Error **errp)
     return 0;
 }
 
+/*
+ * Opens a disk image whose options are given as BlockdevRef in another block
+ * device's options.
+ *
+ * If force_raw is true, bdrv_file_open() will be used, thereby preventing any
+ * image format auto-detection. If it is false and a filename is given,
+ * bdrv_open() will be used for auto-detection.
+ *
+ * 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.
+ *
+ * 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
+ * itself, all options starting with "${bdref_key}." are considered part of the
+ * BlockdevRef.
+ *
+ * The BlockdevRef will be removed from the options QDict.
+ */
+int bdrv_open_image(BlockDriverState **pbs, const char *filename,
+                    QDict *options, const char *bdref_key, int flags,
+                    bool force_raw, bool allow_none, Error **errp)
+{
+    QDict *image_options;
+    int ret;
+    char *bdref_key_dot;
+    const char *reference;
+
+    bdref_key_dot = g_strdup_printf("%s.", bdref_key);
+    qdict_extract_subqdict(options, &image_options, bdref_key_dot);
+    g_free(bdref_key_dot);
+
+    reference = qdict_get_try_str(options, bdref_key);
+    if (!filename && !reference && !qdict_size(image_options)) {
+        if (allow_none) {
+            ret = 0;
+        } else {
+            error_setg(errp, "A block device must be specified for \"%s\"",
+                       bdref_key);
+            ret = -EINVAL;
+        }
+        goto done;
+    }
+
+    if (filename && !force_raw) {
+        /* If a filename is given and the block driver should be detected
+           automatically (instead of using none), use bdrv_open() in order to do
+           that auto-detection. */
+        BlockDriverState *bs;
+
+        if (reference) {
+            error_setg(errp, "Cannot reference an existing block device while "
+                       "giving a filename");
+            ret = -EINVAL;
+            goto done;
+        }
+
+        bs = bdrv_new("");
+        ret = bdrv_open(bs, filename, image_options, flags, NULL, errp);
+        if (ret < 0) {
+            bdrv_unref(bs);
+        } else {
+            *pbs = bs;
+        }
+    } else {
+        ret = bdrv_file_open(pbs, filename, reference, image_options, flags,
+                             errp);
+    }
+
+done:
+    qdict_del(options, bdref_key);
+    return ret;
+}
+
 /*
  * Opens a disk image (raw, qcow2, vmdk, ...)
  *

include/block/block.h

@@ -186,6 +186,9 @@ int bdrv_parse_discard_flags(const char *mode, int *flags);
 int bdrv_file_open(BlockDriverState **pbs, const char *filename,
                    const char *reference, QDict *options, int flags,
                    Error **errp);
+int bdrv_open_image(BlockDriverState **pbs, const char *filename,
+                    QDict *options, const char *bdref_key, int flags,
+                    bool force_raw, bool allow_none, Error **errp);
 int bdrv_open_backing_file(BlockDriverState *bs, QDict *options, Error **errp);
 int bdrv_open(BlockDriverState *bs, const char *filename, QDict *options,
               int flags, BlockDriver *drv, Error **errp);