Solaris tar issues, or other reason why margay fails 010_pg_basebackup?

Hi,

I noticed that margay (Solaris) has started running more of the tests
lately, but is failing in pg_basebaseup/010_pg_basebackup. It runs
successfully on wrasse (in older branches, Solaris 11.3 is desupported
in 17/master), and also on pollock (illumos, forked from common
ancestor Solaris 10 while it was open source).

Hmm, wrasse is using "/opt/csw/bin/gtar xf ..." and pollock is using
"/usr/gnu/bin/tar xf ...", while margay is using "/usr/bin/tar xf
...". The tar command is indicating success (it's run by
system_or_bail and it's not bailing), but the replica doesn't want to
come up:

pg_ctl: directory
"/home/marcel/build-farm-15/buildroot/HEAD/pgsql.build/src/bin/pg_basebackup/tmp_check/t_010_pg_basebackup_replica_data/pgdata"
is not a database cluster directory"

So one idea would be that our tar format is incompatible with Sun tar
in some way that corrupts the output, or there is some still
difference in the nesting of the directory structure it creates, or
something like that. I wonder if this is already common knowledge in
the repressed memories of this list, but I couldn't find anything
specific. I'd be curious to know why exactly, if so (in terms of
POSIX conformance etc, who is doing something wrong).