fsx: test copy_file_range() using non-zero length copy

The copy_file_range() test detection code performs a zero-length
copy to determine whether to perform such calls during the test run.
While this detects the common case of syscall availability,
copy_file_range() has a somewhat variable implementation on the
kernel side that can depend on certain per-filesystem features, etc.
In some implementations, a zero length copy can shortcut and return
success before ever invoking per-filesystem functionality and thus
not thoroughly testing the copy mechanism on the current system.
This can cause the test detection code to pass only to run into an
immediate failure on the first copy_file_range() call during the

Tweak test_copy_range() to perform a small single byte copy to avoid
this problem. Also fix a typo bug in the errno check of the clone
range detection logic.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>
1 file changed