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Updating more than one copy of Cygwin on the same machine: cautionary note

I hesitate to offer this posting as an instruction or even as a piece of
advice, and maybe somebody high in the Cygwin echelons will be able to
give it a quality stamp; alternatively  having written it down I can see
that it might quite likely be obvious to everybody but me. But having
been caught out a few times before I recognised what was happening, here

If like me you ever have more than one installation of Cygwin hanging
off the same machine (say on drives g: and h: where either or both or
neither might be portable; or in different locations g/ and h/ on the
same drive) then care needs to be taken when updating them.

I have discovered through bitter experience that if you update g/ in the
usual way by making it the Root Install Directory in setup, and then
later seek to update h/ by making it the Root Install Directory in setup
_without_first_unmounting_ g/, then immediately (and I don't really know
why) the file h/etc/setup/installed.db is overwritten by the existing
g/etc/setup/installed.db. As a consequence, your attempt to update h/
will fail with a "Nothing needed" message. You need to unmount g/ before
updating h/ successfully.

Moral: use setup only to update the currently mounted installation or to
mount a new or existing installation from scratch, never to swap from
one current installation to another.

(If you do it not once but twice or more too often, then you can all too
easily have Cygwin installations all with local files
/etc/setup/installed.db reporting them to be up-to-date, when I fact
none of them is.)

If this is obvious (and having written it down it seems to me something
every child would know) I am sorry for wasting your time. I checked it
just now by inventing a location h/ where using setup I attempted a new
Cygwin build without first unmounting my current installation. It turned
out that h/ (containing precisely nothing) was reported as fully up to
date, and needing nothing.


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