This patch adds two new parameters to the preallocate() function so we
will be able to use it not just for preallocating a new image but also
for preallocated image growth.

The offset parameter allows the caller to specify a virtual offset from
which to start preallocating. For newly created images this is always 0,
but for preallocating growth this will be the old image length.

The new_length parameter specifies the supposed new length of the image
(basically the "end offset" for preallocation). During image truncation,
bdrv_getlength() will return the old image length so we cannot rely on
its return value then.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20170613202107.10125-10-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 block/qcow2.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/block/qcow2.c b/block/qcow2.c
index 0f44780..1ee8452 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -2476,17 +2476,24 @@ static int qcow2_set_up_encryption(BlockDriverState *bs, const char *encryptfmt,
 }
 
 
-static int preallocate(BlockDriverState *bs)
+/**
+ * Preallocates metadata structures for data clusters between @offset (in the
+ * guest disk) and @new_length (which is thus generally the new guest disk
+ * size).
+ *
+ * Returns: 0 on success, -errno on failure.
+ */
+static int preallocate(BlockDriverState *bs,
+                       uint64_t offset, uint64_t new_length)
 {
     uint64_t bytes;
-    uint64_t offset;
     uint64_t host_offset = 0;
     unsigned int cur_bytes;
     int ret;
     QCowL2Meta *meta;
 
-    bytes = bdrv_getlength(bs);
-    offset = 0;
+    assert(offset <= new_length);
+    bytes = new_length - offset;
 
     while (bytes) {
         cur_bytes = MIN(bytes, INT_MAX);
@@ -2830,7 +2837,7 @@ static int qcow2_create2(const char *filename, int64_t total_size,
     if (prealloc != PREALLOC_MODE_OFF) {
         BDRVQcow2State *s = blk_bs(blk)->opaque;
         qemu_co_mutex_lock(&s->lock);
-        ret = preallocate(blk_bs(blk));
+        ret = preallocate(blk_bs(blk), 0, total_size);
         qemu_co_mutex_unlock(&s->lock);
         if (ret < 0) {
             error_setg_errno(errp, -ret, "Could not preallocate metadata");
-- 
2.9.4