rm_offset_get_from_fd merges logically separate extents

[cebtenzzre]

Description

When extents are physically adjacent, rmlint assumes they can safely be merged, even if they are not logically adjacent.

Steps to reproduce

Build rmlint from the develop branch with _RM_OFFSET_DEBUG=1 and fixes for #527, #528, and #529 applied.
Then run these commands:

$ dd if=/dev/urandom of=foo_src bs=200K oflag=sync count=1
$ xfs_io -f -c 'copy_range -l 100K foo_src' -c 'fsync' foo
$ xfs_io -f -c 'copy_range -l 100K -s 100K -d 200K foo_src' -c 'fsync' foo
$ cp --reflink foo bar
$ filefrag -vb1 foo bar
Filesystem type is: 9123683e
File size of foo is 307200 (307200 blocks of 1 bytes)
 ext:     logical_offset:        physical_offset: length:   expected: flags:
   0:        0..  102399: 734768922624..734769025023: 102400:             shared
   1:   204800..  307199: 734769025024..734769127423: 102400:             last,shared,eof
foo: 1 extent found
File size of bar is 307200 (307200 blocks of 1 bytes)
 ext:     logical_offset:        physical_offset: length:   expected: flags:
   0:        0..  102399: 734768922624..734769025023: 102400:             shared
   1:   204800..  307199: 734769025024..734769127423: 102400:             last,shared,eof
bar: 1 extent found
$ rmlint --is-reflink -vv foo bar
DEBUG: Testing if foo is clone of bar
DEBUG: Checking link type for foo vs bar
DEBUG: rm_offset_get_fiemap: fd=3, n_extents=1, file_offset=0
DEBUG: rm_offset_get_fiemap: fd=3, n_extents=1, file_offset=102400
DEBUG: rm_offset_get_fiemap: fd=4, n_extents=1, file_offset=0
DEBUG: rm_offset_get_fiemap: fd=4, n_extents=1, file_offset=102400
DEBUG: Offsets match at fd1=3, fd2=4, logical=0, physical=734768922624
DEBUG: Files are clones (share same data)
Link type for 'foo' and 'bar', result:
Reflink

Actual result

rmlint only checks one pair of physical offsets, which is probably harmless to --is-reflink but indicates that rm_offset_get_fiemap is merging the representation of these two extents, which is confusing and incorrect (and can cause --is-reflink false positives).

Expected result

$ rmlint --is-reflink -vv foo bar
DEBUG: Testing if foo is clone of bar
DEBUG: Checking link type for foo vs bar
DEBUG: rm_offset_get_fiemap: fd=3, n_extents=1, file_offset=0
DEBUG: rm_offset_get_fiemap: fd=3, n_extents=1, file_offset=102400
DEBUG: rm_offset_get_fiemap: fd=4, n_extents=1, file_offset=0
DEBUG: rm_offset_get_fiemap: fd=4, n_extents=1, file_offset=102400
DEBUG: Offsets match at fd1=3, fd2=4, logical=0, physical=734768922624
DEBUG: rm_offset_get_fiemap: fd=3, n_extents=1, file_offset=102400
DEBUG: rm_offset_get_fiemap: fd=4, n_extents=1, file_offset=102400
DEBUG: Offsets match at fd1=3, fd2=4, logical=102400, physical=734769025024
DEBUG: Files are clones (share same data)
Link type for 'foo' and 'bar', result:
Reflink

Note that rmlint reports two matching offsets, just like how filefrag reports two extents.

Versions

rmlint version 2.10.1 built from develop commit bdb591f, with _RM_OFFSET_DEBUG=1 and fixes for #527, #528, and #529 applied.