Re: Symlink indirection

[Andrew Walrond <andrew@walrond.org>]

junkio@cox.net wrote:
> "AW" == Andrew Walrond <andrew@walrond.org> gives an example of
> a/{x,y,z}, b/{y,z}, c/z mounted on d/. in that order, later
> mounts covering the earlier ones.
> 
> AW> echo "d/w" > d/w would create a new file in directory a.
> 
> Personally I'd rather expect this to happen in c/.  Imagine a/
> being on read-only medium like CD-ROM containing bunch of source
> files, b/ to hold patched source, and c/ to hold binaries
> resulting from compilation.  That is,
> 
>     rm -fr a b c d
>     mkdir a b c d
>     mount /cdrom a
>     mount --bind a d
>     mount --bind --overlay b d
>     (cd b && bzip2 -d <../patch-2.9.91.bz2 | patch -p1)
>     mount --bind --overlay c d
>     (cd c && make mrproper && cat ../.config >.config &&
>     make oldconfig && make dep && make bzImage)

A nice example.

Lets ditch --overlay and replace it with:

--transparent
	just like --overlay
--read-transparent
	a bit like a pane of glass. You can see stuff behind it (read) but 
throw a tomato at it and it sticks to the glass (write)

Then, simplifying your example a bit, we can mount the source cd then 
mount a directory --read-transparent over the top to hold both our 
patched source and compiled binaries.

Or if you want to keep patched-source and binaries seperate,

mount /cdrom stuff
mount --bind --read-transparent patched-src stuff

cd stuff
patch the src

mount --bind --read-transparent binaries stuff

compile your code

> 
> Back to your example; what do you wish to happen when we do
> this?
> 
>     $ mv d/z d/zz && test -f d/z && cat d/z
> 
> Here we rename d/z (which is really c/z) to zz.  Does this
> reveal z that used to be hidden by that, namely b/z, and "cat
> d/z" now shows "b/z"?

Yes - exactly

> 
> Or just like the case of creating a new file, does the union
> "remember" the fact that the directory "d" should not contain
> "z" anymore, and "test -f d/z" fails?
> 

No. Thats not necessary.