pg_upgrade version checking questions

While poking around trying to find an explanation for the pg_upgrade
failure described here:
https://www.postgresql.org/message-id/flat/CACmJi2JUhGo2ZxqDkh-EPHNjEN1ZA1S64uHLJFWHBhUuV4492w%40mail.gmail.com
I noticed a few things that seem a bit fishy about pg_upgrade.
I can't (yet) connect any of these to Tomasz' problem, but:

1. check_bin_dir() does validate_exec() for pg_dumpall and pg_dump,
but not for pg_restore, though pg_upgrade surely calls that too.
For that matter, it's not validating initdb and vacuumdb, though
it's grown dependencies on those as well.  Seems like there's little
point in checking these if we're not going to check all of them.

2. check_cluster_versions() insists that the target version be the
same major version as pg_upgrade itself, but is that really good enough?
As things stand, it looks like pg_upgrade 11.3 would happily use pg_dump
11.1, or vice versa.  With this rule, we cannot safely make any fixes
in minor releases that rely on synchronized changes in the behavior of
pg_upgrade and pg_dump/pg_dumpall/pg_restore.  I've not gone looking
to see if we've already made such changes in the past, but even if we
never have, that's a rather tight-looking straitjacket.  I think we
should insist that the new_cluster.bin_version be an exact match
to pg_upgrade's own PG_VERSION_NUM.

3. Actually, I'm kind of wondering why pg_upgrade has a --new-bindir
option at all, rather than just insisting on finding the new-version
executables in the same directory it is in.  This seems like, at best,
a hangover from before it got into core.  Even if you don't want to
remove the option, we could surely provide a useful default setting
based on find_my_exec.  (I'm amused to notice that pg_upgrade
currently takes the trouble to find out its own path, and then does
precisely nothing with the information.)

Thoughts?

            regards, tom lane