[Bug 235473] gconcat online append

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=235473

            Bug ID: 235473
           Summary: gconcat online append
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: noah.bergbauer at tum.de

Created attachment 201701
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=201701&action=edit
Initial patch draft

It really shouldn't be necessary to rebuild a gconcat volume from scratch just
to add another disk. This is particularly inconvenient because it forces one to
remount/restart any filesystems/geoms stacked on top of gconcat for no apparent
reason.

I attached my first attempt at hacking on this. It works! But since I have
never worked with geoms (or rather any part of the kernel at all) before I am
very unsure about correctness, especially when it comes to locking and race
conditions in general.

Since the existing mutex appears to have a very specific purpose, I created a
new one. One obvious race is keeping the `g_access` levels in sync, so I'm
locking over there as well. I would like to avoid locking a mutex in the
regular IO path (g_concat_start) as this looks like a performance footgun to
me, but given that I have to do array reallocation it seems to me that the only
alternative is to leak memory - perhaps an rwlock would be appropriate to solve
this?

Even after using FreeBSD for several years I never understood the hardcode
option so I didn't implement that either. Incrementing G_CONCAT_VERSION didn't
work for some reason (userland tool wouldn't pick up the new version), did I
miss something? Also I still need to update the manpage.

I mean, the patch is quite literally a copy-pasted mix from gmultipath
(resize/insert capability), gmirror (metadata update) and gconcat itself. It
works but it really needs someone who actually knows how this stuff works to
advise me on how to proceed.

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