[PATCH v2] fstests: btrfs: add a regression test for incorrect inode incompressible flag

[Qu Wenruo <wqu@suse.com>]

[BUG]
Since kernel commit 59615e2c1f63 ("btrfs: reject single block sized
compression early"), a single block write at file offset 0, which can
not be inlined due the inode size, will mark the inode incompressible.

[REGRESSION TEST]
The new regression test will do:

- Create and mount the fs with compression,max_inline=2k

- Do the following operations:
  * Truncate the inode to 2 * blocksize
    This will rule out any future inlined writes.

  * Buffered write [0, 2K)
    Which will not be inlined.

  * Sync
    For affected kernels, this will set the inode with NOCOMPRESS
    and reject all future compression on that inode.

  * Buffered write [1M, 2M)
    For affected kernels, the range will not be compressed due
    to the NOCOMPRESS inode flag.

- Unmount the fs

- Make sure that:
  * The inode has no NOCOMPRESS flag
  * File extent at file offset 1M is being compressed

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
---
Changelog:
v2:
- Remove an unnessary sentence
  Which is confusing because I missed the "and" to connect the two
  sentences, and it's not needed after the first paragraph anyway.

- Use full "btrfs inspect-internal" group name instead

- Add missing punctures
---
 tests/btrfs/343     | 47 +++++++++++++++++++++++++++++++++++++++++++++
 tests/btrfs/343.out |  2 ++
 2 files changed, 49 insertions(+)
 create mode 100755 tests/btrfs/343
 create mode 100644 tests/btrfs/343.out

diff --git a/tests/btrfs/343 b/tests/btrfs/343
new file mode 100755
index 00000000..78079eff
--- /dev/null
+++ b/tests/btrfs/343
@@ -0,0 +1,47 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (c) 2026 SUSE S.A.  All Rights Reserved.
+#
+# FS QA Test 343
+#
+# A regression test to make sure a single-block write at file offset 0 won't
+# incorrectly mark the inode incompressible.
+#
+. ./common/preamble
+_begin_fstest auto quick compress
+
+_require_scratch
+
+_fixed_by_kernel_commit xxxxxxxxxxxx \
+	"btrfs: fix the inline compressed extent check in inode_need_compress()"
+
+_scratch_mkfs >>$seqres.full 2>&1
+_scratch_mount "-o compress,max_inline=2048"
+
+blocksize=$(_get_block_size $SCRATCH_MNT)
+
+# Create a sparse file which is 2 * blocksize, then try a small write at
+# file offset 0 which should not be inlined.
+# Sync so that [0, 2K) range is written, then write a larger range which
+# should be able to be compressed.
+$XFS_IO_PROG -f -c "truncate $((2 * $blocksize))" -c "pwrite 0 2k" -c sync \
+		-c "pwrite 1m 1m" $SCRATCH_MNT/foobar >> $seqres.full
+ino=$(stat -c "%i" $SCRATCH_MNT/foobar)
+_scratch_unmount
+
+# Dump the fstree into seqres.full for debug
+$BTRFS_UTIL_PROG inspect dump-tree -t 5 $SCRATCH_DEV >> $seqres.full
+
+# Check the NOCOMPRESS flag of the inode.
+$BTRFS_UTIL_PROG inspect dump-tree -t 5 $SCRATCH_DEV |\
+grep -A 4 -e "item .* key ($ino INODE_ITEM 0)" | grep -q NOCOMPRESS
+[ $? -eq 0 ] && echo "inode $ino has NOCOMPRESS flag"
+
+# Check the file extent at fileoffset 1m
+$BTRFS_UTIL_PROG inspect dump-tree -t 5 $SCRATCH_DEV |\
+grep -A 4 -e "item .* key ($ino EXTENT_DATA 1048576)" | grep -q "compression 0"
+[ $? -eq 0 ] && echo "inode $ino file offset 1M is not compressed"
+
+echo "Silence is golden"
+# success, all done
+_exit 0
diff --git a/tests/btrfs/343.out b/tests/btrfs/343.out
new file mode 100644
index 00000000..2eb30e4f
--- /dev/null
+++ b/tests/btrfs/343.out
@@ -0,0 +1,2 @@
+QA output created by 343
+Silence is golden
-- 
2.51.2