block: Add bdrv_has_zero_init_truncate()

No .bdrv_has_zero_init() implementation returns 1 if growing the file
would add non-zero areas (at least with PREALLOC_MODE_OFF), so using it
in lieu of this new function was always safe.

But on the other hand, it is possible that growing an image that is not
zero-initialized would still add a zero-initialized area, like when
using nonpreallocating truncation on a preallocated image.  For callers
that care only about truncation, not about creation with potential
preallocation, this new function is useful.

Alternatively, we could have added a PreallocMode parameter to
bdrv_has_zero_init().  But the only user would have been qemu-img
convert, which does not have a plain PreallocMode value right now -- it
would have to parse the creation option to obtain it.  Therefore, the
simpler solution is to let bdrv_has_zero_init() inquire the
preallocation status and add the new bdrv_has_zero_init_truncate() that
presupposes PREALLOC_MODE_OFF.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190724171239.8764-4-mreitz@redhat.com
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

block.c

@@ -5078,6 +5078,27 @@ int bdrv_has_zero_init(BlockDriverState *bs)
     return 0;
 }
 
+int bdrv_has_zero_init_truncate(BlockDriverState *bs)
+{
+    if (!bs->drv) {
+        return 0;
+    }
+
+    if (bs->backing) {
+        /* Depends on the backing image length, but better safe than sorry */
+        return 0;
+    }
+    if (bs->drv->bdrv_has_zero_init_truncate) {
+        return bs->drv->bdrv_has_zero_init_truncate(bs);
+    }
+    if (bs->file && bs->drv->is_filter) {
+        return bdrv_has_zero_init_truncate(bs->file->bs);
+    }
+
+    /* safe default */
+    return 0;
+}
+
 bool bdrv_unallocated_blocks_are_zero(BlockDriverState *bs)
 {
     BlockDriverInfo bdi;

include/block/block.h

@@ -443,6 +443,7 @@ int bdrv_pdiscard(BdrvChild *child, int64_t offset, int64_t bytes);
 int bdrv_co_pdiscard(BdrvChild *child, int64_t offset, int64_t bytes);
 int bdrv_has_zero_init_1(BlockDriverState *bs);
 int bdrv_has_zero_init(BlockDriverState *bs);
+int bdrv_has_zero_init_truncate(BlockDriverState *bs);
 bool bdrv_unallocated_blocks_are_zero(BlockDriverState *bs);
 bool bdrv_can_write_zeroes_with_unmap(BlockDriverState *bs);
 int bdrv_block_status(BlockDriverState *bs, int64_t offset,

include/block/block_int.h

@@ -419,9 +419,16 @@ struct BlockDriver {
     /*
      * Returns 1 if newly created images are guaranteed to contain only
      * zeros, 0 otherwise.
+     * Must return 0 if .bdrv_has_zero_init_truncate() returns 0.
      */
     int (*bdrv_has_zero_init)(BlockDriverState *bs);
 
+    /*
+     * Returns 1 if new areas added by growing the image with
+     * PREALLOC_MODE_OFF contain only zeros, 0 otherwise.
+     */
+    int (*bdrv_has_zero_init_truncate)(BlockDriverState *bs);
+
     /* Remove fd handlers, timers, and other event loop callbacks so the event
      * loop is no longer in use.  Called with no in-flight requests and in
      * depth-first traversal order with parents before child nodes.