[PATCH] iotests/221: Discard image before qemu-img map

[Max Reitz <mreitz@redhat.com>]

See the new comment for why this should be done.

I do not have a reproducer on master, but when using FUSE block exports,
this test breaks depending on the underlying filesystem (for me, it
works on tmpfs, but fails on xfs, because the block allocated by
file-posix has 16 kB there instead of 4 kB).

Suggested-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 tests/qemu-iotests/221     |  7 +++++++
 tests/qemu-iotests/221.out | 14 ++++++--------
 2 files changed, 13 insertions(+), 8 deletions(-)

diff --git a/tests/qemu-iotests/221 b/tests/qemu-iotests/221
index 0e9096fec7..73c45a4ca6 100755
--- a/tests/qemu-iotests/221
+++ b/tests/qemu-iotests/221
@@ -46,6 +46,13 @@ echo "=== Check mapping of unaligned raw image ==="
 echo
 
 _make_test_img 65537 # qemu-img create rounds size up
+
+# file-posix allocates the first block of any images when it is created;
+# the size of this block depends on the host page size and the file
+# system block size, none of which are constant.  Discard the whole
+# image so we will not see this allocation in qemu-img map's output.
+$QEMU_IO -c 'discard 0 65537' "$TEST_IMG" | _filter_qemu_io
+
 $QEMU_IMG map --output=json "$TEST_IMG" | _filter_qemu_img_map
 
 truncate --size=65537 "$TEST_IMG" # so we resize it and check again
diff --git a/tests/qemu-iotests/221.out b/tests/qemu-iotests/221.out
index dca024a0c3..93846c7dab 100644
--- a/tests/qemu-iotests/221.out
+++ b/tests/qemu-iotests/221.out
@@ -3,18 +3,16 @@ QA output created by 221
 === Check mapping of unaligned raw image ===
 
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=65537
-[{ "start": 0, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 4096, "length": 61952, "depth": 0, "zero": true, "data": false, "offset": OFFSET}]
-[{ "start": 0, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 4096, "length": 61952, "depth": 0, "zero": true, "data": false, "offset": OFFSET}]
+discard 65537/65537 bytes at offset 0
+64.001 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+[{ "start": 0, "length": 66048, "depth": 0, "zero": true, "data": false, "offset": OFFSET}]
+[{ "start": 0, "length": 66048, "depth": 0, "zero": true, "data": false, "offset": OFFSET}]
 wrote 1/1 bytes at offset 65536
 1 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-[{ "start": 0, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 4096, "length": 61440, "depth": 0, "zero": true, "data": false, "offset": OFFSET},
+[{ "start": 0, "length": 65536, "depth": 0, "zero": true, "data": false, "offset": OFFSET},
 { "start": 65536, "length": 1, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
 { "start": 65537, "length": 511, "depth": 0, "zero": true, "data": false, "offset": OFFSET}]
-[{ "start": 0, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 4096, "length": 61440, "depth": 0, "zero": true, "data": false, "offset": OFFSET},
+[{ "start": 0, "length": 65536, "depth": 0, "zero": true, "data": false, "offset": OFFSET},
 { "start": 65536, "length": 1, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
 { "start": 65537, "length": 511, "depth": 0, "zero": true, "data": false, "offset": OFFSET}]
 *** done
-- 
2.28.0