block: New bdrv_nb_sectors()

A call to retrieve the image size converts between bytes and sectors
several times:

* BlockDriver method bdrv_getlength() returns bytes.

* refresh_total_sectors() converts to sectors, rounding up, and stores
  in total_sectors.

* bdrv_getlength() converts total_sectors back to bytes (now rounded
  up to a multiple of the sector size).

* Callers wanting sectors rather bytes convert it right back.
  Example: bdrv_get_geometry().

bdrv_nb_sectors() provides a way to omit the last two conversions.
It's exactly bdrv_getlength() with the conversion to bytes omitted.
It's functionally like bdrv_get_geometry() without its odd error
handling.

Reimplement bdrv_getlength() and bdrv_get_geometry() on top of
bdrv_nb_sectors().

The next patches will convert some users of bdrv_getlength() to
bdrv_nb_sectors().
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NBenoit Canet <benoit@irqsave.net>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

block.c

@@ -701,6 +701,7 @@ static int find_image_format(BlockDriverState *bs, const char *filename,
 
 /**
  * Set the current 'total_sectors' value
+ * Return 0 on success, -errno on error.
  */
 static int refresh_total_sectors(BlockDriverState *bs, int64_t hint)
 {
@@ -3536,11 +3537,12 @@ int64_t bdrv_get_allocated_file_size(BlockDriverState *bs)
 }
 
 /**
- * Length of a file in bytes. Return < 0 if error or unknown.
+ * Return number of sectors on success, -errno on error.
  */
-int64_t bdrv_getlength(BlockDriverState *bs)
+int64_t bdrv_nb_sectors(BlockDriverState *bs)
 {
     BlockDriver *drv = bs->drv;
+
     if (!drv)
         return -ENOMEDIUM;
 
@@ -3550,19 +3552,26 @@ int64_t bdrv_getlength(BlockDriverState *bs)
             return ret;
         }
     }
-    return bs->total_sectors * BDRV_SECTOR_SIZE;
+    return bs->total_sectors;
+}
+
+/**
+ * Return length in bytes on success, -errno on error.
+ * The length is always a multiple of BDRV_SECTOR_SIZE.
+ */
+int64_t bdrv_getlength(BlockDriverState *bs)
+{
+    int64_t ret = bdrv_nb_sectors(bs);
+
+    return ret < 0 ? ret : ret * BDRV_SECTOR_SIZE;
 }
 
 /* return 0 as number of sectors if no device present or error */
 void bdrv_get_geometry(BlockDriverState *bs, uint64_t *nb_sectors_ptr)
 {
-    int64_t length;
-    length = bdrv_getlength(bs);
-    if (length < 0)
-        length = 0;
-    else
-        length = length >> BDRV_SECTOR_BITS;
-    *nb_sectors_ptr = length;
+    int64_t nb_sectors = bdrv_nb_sectors(bs);
+
+    *nb_sectors_ptr = nb_sectors < 0 ? 0 : nb_sectors;
 }
 
 void bdrv_set_on_error(BlockDriverState *bs, BlockdevOnError on_read_error,

include/block/block.h

@@ -275,6 +275,7 @@ BlockDriverState *bdrv_find_backing_image(BlockDriverState *bs,
     const char *backing_file);
 int bdrv_get_backing_file_depth(BlockDriverState *bs);
 int bdrv_truncate(BlockDriverState *bs, int64_t offset);
+int64_t bdrv_nb_sectors(BlockDriverState *bs);
 int64_t bdrv_getlength(BlockDriverState *bs);
 int64_t bdrv_get_allocated_file_size(BlockDriverState *bs);
 void bdrv_get_geometry(BlockDriverState *bs, uint64_t *nb_sectors_ptr);