btrfs-progs: tests: add test case for ext4 unwritten extents

The test case verifies behavior of ext4 unwritten extents:

- Create a unwritten (preallocated) extent on ext4

- Fill the on-disk extent with random garbage
  This is to make sure if btrfs tries to read the on-disk data, it would
  definitely get some garbage.
  As I found sometimes mkfs.ext4 can fill the unused bg with zeros.

- Fill the preallocated file range with some data
  This is to make sure btrfs-convert can handle mixed written and
  unwritten ranges.

- Save the checksum of the file.

- Convert the fs

- Verify the checksum
  For older btrfs-convert, there would be only one regular file extent,
  and reading the file would read out some garbage and cause checksum to
  mismatch.

  For the fixed btrfs-convert, we punch holes for unwritten extents,
  thus only the written part would be read out and match the checksum.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

tests/convert-tests/025-ext4-uninit-written/test.sh

@@ -0,0 +1,53 @@
+#!/bin/bash
+# Make sure btrfs is handling the ext4 uninit (preallocated) extent correctly
+
+source "$TEST_TOP/common" || exit
+source "$TEST_TOP/common.convert" || exit
+
+setup_root_helper
+prepare_test_dev 1G
+
+check_global_prereq mkfs.ext4
+check_global_prereq fallocate
+check_global_prereq filefrag
+check_global_prereq awk
+check_global_prereq md5sum
+check_prereq btrfs-convert
+check_prereq btrfs
+
+convert_test_prep_fs ext4 mke2fs -t ext4 -b 4096
+
+# Create a preallocated extent first.
+run_check $SUDO_HELPER fallocate -l 32K "$TEST_MNT/file"
+sync
+# Get the real on-disk location and write some data into it.
+physical=$(run_check_stdout $SUDO_HELPER filefrag -v "$TEST_MNT/file" | grep unwritten | awk '{print $4}' | grep -o "[[:digit:]]*")
+
+if [ -z "$physical" ]; then
+	_fail "unable to get the physical address of the file"
+fi
+
+# Now fill the underlying range with non-zeros.
+# For properly converted fs, we should not read the contents anyway
+run_check $SUDO_HELPER dd if=/dev/urandom of=$TEST_DEV bs=4096 seek="$physical" conv=notrunc count=8
+
+# Write some thing into the file range.
+run_check $SUDO_HELPER dd if=/dev/zero of="$TEST_MNT/file" bs=4096 count=1 conv=notrunc
+run_check $SUDO_HELPER dd if=/dev/zero of="$TEST_MNT/file" bs=4096 count=1 seek=3 conv=notrunc
+sync
+md5_before=$(md5sum "$TEST_MNT/file" | cut -f1 -d' ')
+_log "md5sum before convert: $md5_before"
+run_check_umount_test_dev
+
+# Btrfs-convert should handle the unwritten part correctly, either punching a hole
+# or a proper preallocated extent, so that we won't read the on-disk data.
+convert_test_do_convert
+
+run_check_mount_test_dev
+md5_after=$(md5sum "$TEST_MNT/file" | cut -f1 -d' ')
+_log "md5sum after convert: $md5_after"
+run_check_umount_test_dev
+
+if [ "$md5_before" != "$md5_after" ]; then
+	_fail "contents mismatch"
+fi