If pututxline() is repeated after it fails with EINTR or EAGAIN, the utmp file may end up containing some stale entries, as shown below.

There are several issues with pututxline() that have been fixed upstream, and we start with the following commit, which has several dependencies.

commit 61d3db428176d9d0822e4e680305fe34285edff2
Author: Florian Weimer <fweimer>
Date:   Wed Aug 28 11:59:45 2019 +0200

    login: pututxline could fail to overwrite existing entries [BZ #24902]
    
    The internal_getut_r function updates the file_offset variable and
    therefore must always update last_entry as well.
    
    Previously, if pututxline could not upgrade the read lock to a
    write lock, internal_getut_r would update file_offset only,
    without updating last_entry, and a subsequent call would not
    overwrite the existing utmpx entry at file_offset, instead
    creating a new entry.  This has been observed to cause unbounded
    file growth in high-load situations.
    
    This commit removes the buffer argument to internal_getut_r and
    updates the last_entry variable directly, along with file_offset.
    
    Initially reported and fixed by Ondřej Lysoněk.
    
    Reviewed-by: Gabriel F. T. Gomes <gabrielftg.com>

We also want:

commit 1a7fe2ebe52b3c8bf465d1756e69452d05c1c103
Author: Florian Weimer <fweimer>
Date:   Mon Aug 5 15:54:10 2019 +0200

    login: Remove utmp backend jump tables [BZ #23518]
    
    There is just one file-based implementation, so this dispatch
    mechanism is unnecessary.  Instead of the vtable pointer
    __libc_utmp_jump_table, use a non-negative file_fd as the indicator
    that the backend is initialized.

commit a33b817f13170b5c24263b92e7e09880fe797d7e
Author: Florian Weimer <fweimer>
Date:   Tue Aug 13 12:09:32 2019 +0200

    login: Assume that _HAVE_UT_* constants are true
    
    Make the GNU version of bits/utmp.h the generic version because
    all remaining ports use it (with a sysdeps override for
    Linux s390/s390x).

commit 5a3afa9738f3dbbaf8c0a35665318c1af782111b
Author: Florian Weimer <fweimer>
Date:   Tue Aug 13 15:53:19 2019 +0200

    login: Replace macro-based control flow with function calls in utmp

commit 341da5b4b6253de9a7581a066f33f89cacb44dec
Author: Florian Weimer <fweimer>
Date:   Thu Aug 15 10:30:23 2019 +0200

    login: Fix updwtmp, updwtmx unlocking
    
    Commit 5a3afa9738f3dbbaf8c0a35665318c1af782111b (login: Replace
    macro-based control flow with function calls in utmp) introduced
    a regression because after it, __libc_updwtmp attempts to unlock
    the wrong file descriptor.

commit 628598be7e1bfaa04f34df71ef6678f2c5103dfd
Author: Florian Weimer <fweimer>
Date:   Thu Aug 15 16:09:05 2019 +0200

    login: Disarm timer after utmp lock acquisition [BZ #24879]
    
    If the file processing takes a long time for some reason, SIGALRM can
    arrive while the file is still being processed.  At that point, file
    access will fail with EINTR.  Disarming the timer after lock
    acquisition avoids that.  (If there was a previous alarm, it is the
    responsibility of the caller to deal with the EINTR error.)

commit 0d5b2917530ccaf8ad312dfbb7bce69d569c23ad
Author: Florian Weimer <fweimer>
Date:   Thu Aug 15 16:09:20 2019 +0200

    login: Use struct flock64 in utmp [BZ #24880]
    
    Commit 06ab719d30b01da401150068054d3b8ea93dd12f ("Fix Linux fcntl OFD
    locks for non-LFS architectures (BZ#20251)") introduced the use of
    fcntl64 into the utmp implementation.  However, the lock file
    structure was not updated to struct flock64 at that point.

commit c2adefbafcdd2519ff43eca6891c77cd7b29ab62
Author: Florian Weimer <fweimer>
Date:   Thu Aug 15 16:09:43 2019 +0200

    login: Add nonstring attributes to struct utmp, struct utmpx [BZ #24899]
    
    Commit 7532837d7b03b3ca5b9a63d77a5bd81dd23f3d9c ("The
    -Wstringop-truncation option new in GCC 8 detects common misuses")
    added __attribute_nonstring__ to bits/utmp.h, but it did not update
    the parallel bits/utmpx.h header.  In struct utmp, the nonstring
    attribute for ut_id was missing.