block: Add blk_new_open()

blk_new_with_bs() creates a BlockBackend with an empty BlockDriverState
attached to it. Empty BDSs are not nice, therefore add an alternative
function which combines blk_new_with_bs() with bdrv_open().

Note: In contrast to bdrv_open() which takes a BlockDriver parameter,
blk_new_open() does not take such a parameter. This is because
bdrv_open() opens a BlockDriverState, therefore it is natural to be able
to set the BlockDriver for that BDS. The fact that bdrv_open() can open
more than a single BDS is merely some form of a byproduct.

blk_new_open() on the other hand is intended to be used to create a
whole tree of BlockDriverStates. Therefore, setting a single BlockDriver
does not make much sense. Instead, the drivers to be used for each of
the nodes must be configured through the "options" QDict; including the
driver of the root BDS.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-id: 1423162705-32065-3-git-send-email-mreitz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block/block-backend.c

@@ -91,6 +91,40 @@ BlockBackend *blk_new_with_bs(const char *name, Error **errp)
     return blk;
 }
 
+/*
+ * Calls blk_new_with_bs() and then calls bdrv_open() on the BlockDriverState.
+ *
+ * Just as with bdrv_open(), after having called this function the reference to
+ * @options belongs to the block layer (even on failure).
+ *
+ * TODO: Remove @filename and @flags; it should be possible to specify a whole
+ * BDS tree just by specifying the @options QDict (or @reference,
+ * alternatively). At the time of adding this function, this is not possible,
+ * though, so callers of this function have to be able to specify @filename and
+ * @flags.
+ */
+BlockBackend *blk_new_open(const char *name, const char *filename,
+                           const char *reference, QDict *options, int flags,
+                           Error **errp)
+{
+    BlockBackend *blk;
+    int ret;
+
+    blk = blk_new_with_bs(name, errp);
+    if (!blk) {
+        QDECREF(options);
+        return NULL;
+    }
+
+    ret = bdrv_open(&blk->bs, filename, reference, options, flags, NULL, errp);
+    if (ret < 0) {
+        blk_unref(blk);
+        return NULL;
+    }
+
+    return blk;
+}
+
 static void blk_delete(BlockBackend *blk)
 {
     assert(!blk->refcnt);

include/sysemu/block-backend.h

@@ -62,6 +62,9 @@ typedef struct BlockDevOps {
 
 BlockBackend *blk_new(const char *name, Error **errp);
 BlockBackend *blk_new_with_bs(const char *name, Error **errp);
+BlockBackend *blk_new_open(const char *name, const char *filename,
+                           const char *reference, QDict *options, int flags,
+                           Error **errp);
 void blk_ref(BlockBackend *blk);
 void blk_unref(BlockBackend *blk);
 const char *blk_name(BlockBackend *blk);