[9fans] union directories exploration

[Bakul Shah <bakul@bitblocks.com>]

I think I understand plan9 union directories but find them
limiting for some uses.  Consider

term% mkdir -p a1/b/c a1/b/d a2/b/c a2/b/e
term% touch a1/b1 a2/b2 a1/b/c1 a2/b/c2
term% du -a a1
0	a1/b1
0	a1/b/c
0	a1/b/d
0	a1/b/c1
0	a1/b
0	a1
term% du -a a2
0	a2/b/e
0	a2/b/c2
0	a2/b/c
0	a2/b
0	a2/b2
0	a2
term% bind -a a1 a2
term% du -a a2
0	a2/b/e
0	a2/b/c2
0	a2/b/c
0	a2/b2
0	a2/b1
0	a2/b
0	a2
[Note: redundant entries are deleted]

Notice that after the binding, b/d and b/c1 don't show up in
a2.  This happens because b exists in both directories and
once a2/b is traversed, further search is limited to the
original a2/ tree. Even if you ls a2/b/c1, it will fail.

The behavior I want (making all the file in a1 and a2 visible
after binding, with preference to the "before" directory) can
be achieved by binding *every* in a1 directories that has a
correponding dir in a2, starting with the leaf directories and
moving up.

term% unmount a2
term% bind -a a1/b/c a2/b/c
term% bind -a a1/b a2/b
term% bind -a a1 a2
term% du -a a2
0	a2/b/e
0	a2/b/c
0	a2/b/d
0	a2/b2
0	a2/b1
0	a2/b/c2
0	a2/b/c1
0	a2/b
0	a2
[Note: redundant entries are deleted]

What I want to do idea is to overlay one tree on another.
This idea can be used to overlay a r/w tree on a readonly
source tree and do a build. Or even do a concurrent build for
different architectures.  But there are hundreds of
directories in /sys so this can get quite unwieldy and a
walk(9) can become more of a shuffle or search.  May be this
should be called a "labor" union!

Has anyone tried this and has a script for this? Is there a
better way to handle this (overlay one partial tree on another
to provie a different 'view')? Then there are issues with what
happens when directories are added or deleted. One almost
wants a `Watch' command that watches such changes and does the
right thing.

Since binds are only allowed on directories, deleting a file
foo in the "before" directory can make any file foo in the
"after" directory. I guess this is why *BSD unionfs has a
`whiteout' file type [this is analogous to a `blackhole' route
in a network forwarding table or a `blacklisted' email address
in a spam filter. How funny]

[Thinking about this makes me appreciate the complexity of
*BSD unionfs and why it has never been fully shaken out.  May
be worth prototyping on plan9]