[PATCH] btrfs-progs: do not use async discard for misc/004

[Qu Wenruo <wqu@suse.com>]

[BUG]
There is a long long existing failure in my local VM that with any newer
kernel (6.x) the test case misc/004 would fail with ENOSPC during
balance:

    [TEST]   misc-tests.sh
    [TEST/misc]   004-shrink-fs
 failed: /home/adam/btrfs-progs/btrfs balance start -mconvert=single -sconvert=single -f /home/adam/btrfs-progs/tests/mnt
 test failed for case 004-shrink-fs
 make: *** [Makefile:547: test-misc] Error 1

[CAUSE]
With more testing, it turns out that just before the balance, the
filesystem still have several empty data block groups.

The reason is the new default discard=async behavior, as it also changes
the empty block groups to be async, this leave the empty block groups
there, resulting no extra space for the convert balance.

[FIX]
I do not understand why for loopback block devices we also enable
discard, but at least disable discard for the test case so that we can
ensure the empty block groups get cleaned up properly.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 tests/misc-tests/004-shrink-fs/test.sh | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/tests/misc-tests/004-shrink-fs/test.sh b/tests/misc-tests/004-shrink-fs/test.sh
index c7473649020e..4fb42a024b81 100755
--- a/tests/misc-tests/004-shrink-fs/test.sh
+++ b/tests/misc-tests/004-shrink-fs/test.sh
@@ -32,7 +32,9 @@ shrink_test()
 
 run_check truncate -s 20G "$IMAGE"
 run_check "$TOP/mkfs.btrfs" -f "$IMAGE"
-run_check $SUDO_HELPER mount "$IMAGE" "$TEST_MNT"
+# Disable the new default async discard, which makes empty block group cleanup
+# async.
+run_check $SUDO_HELPER mount -o nodiscard "$IMAGE" "$TEST_MNT"
 run_check $SUDO_HELPER chmod a+rw "$TEST_MNT"
 
 # Create 7 data block groups, each with a size of 1Gb.
-- 
2.45.2