block: Fix copy-on-read crash with partial final cluster

If the virtual disk size isn't aligned to full clusters,
bdrv_co_do_copy_on_readv() may get pnum == 0 before having the full
cluster completed, which will let it run into an assertion failure:

qemu-io: block/io.c:1203: bdrv_co_do_copy_on_readv: Assertion `skip_bytes < pnum' failed.

Check for EOF, assert that we read at least as much as the read request
originally wanted to have (which is true at EOF because otherwise
bdrv_check_byte_request() would already have returned an error) and
return success early even though we couldn't copy the full cluster.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/io.c

@@ -1200,6 +1200,12 @@ static int coroutine_fn bdrv_co_do_copy_on_readv(BdrvChild *child,
             pnum = MIN(cluster_bytes, max_transfer);
         }
 
+        /* Stop at EOF if the image ends in the middle of the cluster */
+        if (ret == 0 && pnum == 0) {
+            assert(progress >= bytes);
+            break;
+        }
+
         assert(skip_bytes < pnum);
 
         if (ret <= 0) {

tests/qemu-iotests/197

@@ -109,6 +109,15 @@ $QEMU_IO -f qcow2 -c map "$TEST_WRAP"
 _check_test_img
 $QEMU_IMG compare -f $IMGFMT -F qcow2 "$TEST_IMG" "$TEST_WRAP"
 
+echo
+echo '=== Partial final cluster ==='
+echo
+
+_make_test_img 1024
+$QEMU_IO -f $IMGFMT -C -c 'read 0 1024' "$TEST_IMG" | _filter_qemu_io
+$QEMU_IO -f $IMGFMT -c map "$TEST_IMG"
+_check_test_img
+
 # success, all done
 echo '*** done'
 status=0

tests/qemu-iotests/197.out

@@ -23,4 +23,12 @@ can't open device TEST_DIR/t.wrap.qcow2: Can't use copy-on-read on read-only dev
 1023.938 MiB (0x3fff0000) bytes not allocated at offset 3 GiB (0xc0010000)
 No errors were found on the image.
 Images are identical.
+
+=== Partial final cluster ===
+
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1024
+read 1024/1024 bytes at offset 0
+1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+1 KiB (0x400) bytes     allocated at offset 0 bytes (0x0)
+No errors were found on the image.
 *** done