block: Produce zeros when protocols reading beyond end of file

While Asias is debugging an issue creating qcow2 images on top of
non-file protocols.  It boils down to this example using NBD:

$ qemu-io -c 'open -g nbd+unix:///?socket=/tmp/nbd.sock' -c 'read -v 0 512'

Notice the open -g option to set bs->growable.  This means you can
read/write beyond end of file.  Reading beyond end of file is supposed
to produce zeroes.

We rely on this behavior in qcow2_create2() during qcow2 image
creation.  We create a new file and then write the qcow2 header
structure using bdrv_pwrite().  Since QCowHeader is not a multiple of
sector size, block.c first uses bdrv_read() on the empty file to fetch
the first sector (should be all zeroes).

Here is the output from the qemu-io NBD example above:

$ qemu-io -c 'open -g nbd+unix:///?socket=/tmp/nbd.sock' -c 'read -v 0 512'
00000000:  ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab  ................
00000010:  ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab  ................
00000020:  ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab  ................
...

We are not zeroing the buffer!  As a result qcow2 image creation on top
of protocols is not guaranteed to work even when file creation is
supported by the protocol.

[Adapted this patch to use bs->zero_beyond_eof.
-- Stefan]

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Asias He <asias@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block.c

@@ -2571,7 +2571,35 @@ static int coroutine_fn bdrv_co_do_readv(BlockDriverState *bs,
         }
     }
 
-    ret = drv->bdrv_co_readv(bs, sector_num, nb_sectors, qiov);
+    if (!(bs->zero_beyond_eof && bs->growable)) {
+        ret = drv->bdrv_co_readv(bs, sector_num, nb_sectors, qiov);
+    } else {
+        /* Read zeros after EOF of growable BDSes */
+        int64_t len, total_sectors, max_nb_sectors;
+
+        len = bdrv_getlength(bs);
+        if (len < 0) {
+            ret = len;
+            goto out;
+        }
+
+        total_sectors = len >> BDRV_SECTOR_BITS;
+        max_nb_sectors = MAX(0, total_sectors - sector_num);
+        if (max_nb_sectors > 0) {
+            ret = drv->bdrv_co_readv(bs, sector_num,
+                                     MIN(nb_sectors, max_nb_sectors), qiov);
+        } else {
+            ret = 0;
+        }
+
+        /* Reading beyond end of file is supposed to produce zeroes */
+        if (ret == 0 && total_sectors < sector_num + nb_sectors) {
+            uint64_t offset = MAX(0, total_sectors - sector_num);
+            uint64_t bytes = (sector_num + nb_sectors - offset) *
+                              BDRV_SECTOR_SIZE;
+            qemu_iovec_memset(qiov, offset * BDRV_SECTOR_SIZE, 0, bytes);
+        }
+    }
 
 out:
     tracked_request_end(&req);