freebsd stable - Apr 2021 - NFSv4 issues related to delegations and the back channel

Hi,

A problem with the NFSv4.1/4.2 server was identified, where it
erroneously bound the back channel on a new TCP connection
was identified, that could break the Linux client.
Fixing this has cascaded into a fair number of patches to
make the back channel work correctly. Since use of delegations
requires a working back channel, this mostly affects use of
delegations.

I now think I have resolved everything and the patches should
be in "main" and then stable/12 and stable/13 in a few weeks.
They should all be in the next FreeBSD12 and FreeBSD 13 releases.

Until you have a patched NFSv4 client and/or server system, I
am making the following recommendations to avoid trouble:
- Do not use the Linux "nconnects" mount option.
- Do not enable delegations.  (This also applies to a patched
  NFSv4.1/4.2 server if any of your clients are Linux systems
  running a kernel version prior to 5.3.)
  - For the NFSv4 server, leave vfs.nfsd.issue_delegations == 0.
  - For the NFSv4 client, do not run the nfscbd(8) daemon.

Most of the issues are NFSv4.1/4.2 specific, but some affect
NFSv4.0 as well, so the above recommendations apply to all
NFSv4 versions.

Fortunately, delegations seldom improve performance,
so following the above recommendations should not be
a big problem.

rick