[Bug 255061] [PATCH] Proof of concept nested mounts for automounter -hosts map

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=255061

            Bug ID: 255061
           Summary: [PATCH] Proof of concept nested mounts for automounter
                    -hosts map
           Product: Base System
           Version: 12.2-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: kreuter at progn.net

Created attachment 224112
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=224112&action=edit
Extend automountd, autounmountd, and autofs to understand nested automounted
mount points.

(This is perhaps a duplicate of bug #195564, but I don't know whether the
project prefers reopening things vs. new reports.)

I'm interested in enhancing the automounter to be able to handle
nested/hierarchical keys in the -hosts map (mostly as a complement to zfs,
which makes it convenient to organize data sets in nested hierarchies), so I'm
attaching a working proof-of-concept implementation of the capability.

This picture shows the net result on a test machine that exports some zfs
shares rooted at /t to itself:

# mount | grep /net
map -hosts on /net (autofs)
localhost:/t/a on /net/localhost/t/a (nfs, nosuid, automounted)
map -hosts on /net/localhost/t/a/b (autofs, automounted)
localhost:/t/a/b on /net/localhost/t/a/b (nfs, nosuid, automounted)
map -hosts on /net/localhost/t/a/b/c (autofs, automounted)

That is, I've added the idea of "automounted autofs" mount points. Because
they're automounted, they are created dynamically by automountd and destroyed
by autounmountd. Because they're autofs mounts, they eventually cause something
else to get mounted.

The attachment is a tar file, here's a description of its contents:

report.txt -- this PR writeup.

notes.txt -- detailed description of the approach, limitations, future work

automountd.diff -- add the notion of automounted autofs mounts

autounmountd.diff -- teach autounmountd about unmount order dependencies.

autofs.diff -- have an automounted autofs pass getattr calls through.

These diffs apply against the releng-12.2 branch (it's what I was running when
I started), but should be pretty cleanly applicable to CURRENT or HEAD, since
not much has changed in the automounter.

Note that the diffs are named "logically"; in fact the first 2 diffs touch
multiple files in usr.sbin/autofs, and and must be applied in the order I've
mentioned them.

This set of changes is lightly tested, but does what I want it to so far under
repeated, but manual, testing. That said, the approach I've taken isn't the
only conceivable way to do things; it was just the shortest path I could think
of from what was there to the behavior I wanted to see.

Anyhow, there's work to do before this might be merge-worthy (e.g., I haven't
gotten to "automount -u" yet.) If there's an interest in incorporating this
approach, I'd be very happy to iterate on this effort, ideally with some
guidance/help from those who know this stuff.

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