[07/15] block: add bdrv_aio_copy_backing()

diff --git a/block.c b/block.c
index 3d074af..8225758 100644
--- a/block.c
+++ b/block.c
@@ -2240,6 +2240,43 @@  BlockDriverAIOCB *bdrv_aio_writev(BlockDriverState *bs, int64_t sector_num,
     return ret;
 }
 
+/**
+ * Attempt to copy unallocated sectors from backing file.
+ *
+ * @sector_num - the first sector to start from
+ * @cb - completion callback
+ * @opaque - data to pass completion callback
+ *
+ * Returns NULL if the image format not support the operation, the image is
+ * read-only, or no image is open.
+ *
+ * The intention of this function is for a user to execute it once with a
+ * sector_num of 0 and then upon receiving a completion callback, to remember
+ * the number of sectors copied, and then to call this function again with
+ * an offset adjusted by the number of sectors previously copied.
+ *
+ * This allows a user to progressive stream in an image at a pace that makes
+ * sense.  In general, this function tries to do the smallest amount of I/O
+ * possible to do some useful work.
+ *
+ * This function only really makes sense in combination with a block format
+ * that supports copy on read and has it enabled.  If copy on read is not
+ * enabled, a block format driver may return NULL.
+ *
+ * If an I/O error occurs the completion callback is invoked with -errno in the
+ * nb_sectors argument.
+ */
+BlockDriverAIOCB *bdrv_aio_copy_backing(BlockDriverState *bs,
+                                        int64_t sector_num,
+                                        BlockDriverCopyBackingCB *cb,
+                                        void *opaque)
+{
+    if (!bs->drv || bs->read_only || !bs->drv->bdrv_aio_copy_backing) {
+        return NULL;
+    }
+
+    return bs->drv->bdrv_aio_copy_backing(bs, sector_num, cb, opaque);
+}
 
 typedef struct MultiwriteCB {
     int error;
diff --git a/block.h b/block.h
index f6ffa93..aee11e6 100644
--- a/block.h
+++ b/block.h
@@ -113,6 +113,7 @@  typedef struct BlockDriverAIOCB BlockDriverAIOCB;
 typedef void BlockDriverCompletionFunc(void *opaque, int ret);
 typedef void BlockDriverDirtyHandler(BlockDriverState *bs, int64_t sector,
                                      int sector_num);
+typedef void BlockDriverCopyBackingCB(void *opaque, int nb_sectors);
 BlockDriverAIOCB *bdrv_aio_readv(BlockDriverState *bs, int64_t sector_num,
                                  QEMUIOVector *iov, int nb_sectors,
                                  BlockDriverCompletionFunc *cb, void *opaque);
@@ -121,6 +122,10 @@  BlockDriverAIOCB *bdrv_aio_writev(BlockDriverState *bs, int64_t sector_num,
                                   BlockDriverCompletionFunc *cb, void *opaque);
 BlockDriverAIOCB *bdrv_aio_flush(BlockDriverState *bs,
                                  BlockDriverCompletionFunc *cb, void *opaque);
+BlockDriverAIOCB *bdrv_aio_copy_backing(BlockDriverState *bs,
+                                        int64_t sector_num,
+                                        BlockDriverCopyBackingCB *cb,
+                                        void *opaque);
 void bdrv_aio_cancel(BlockDriverAIOCB *acb);
 
 typedef struct BlockRequest {
diff --git a/block_int.h b/block_int.h
index 69dd5a7..8b10083 100644
--- a/block_int.h
+++ b/block_int.h
@@ -74,6 +74,8 @@  struct BlockDriver {
         BlockDriverCompletionFunc *cb, void *opaque);
     BlockDriverAIOCB *(*bdrv_aio_flush)(BlockDriverState *bs,
         BlockDriverCompletionFunc *cb, void *opaque);
+    BlockDriverAIOCB *(*bdrv_aio_copy_backing)(BlockDriverState *bs,
+        int64_t sector_num, BlockDriverCopyBackingCB *cb, void *opaque);
     int (*bdrv_discard)(BlockDriverState *bs, int64_t sector_num,
                         int nb_sectors);