Re: [Zero-install-devel] Zero Install 0.1.17

[Joachim K. <joa...@i1...>]

Joachim Kupke:

> Besides, there are some issues with arguably higher priority:
[...]

Oh, and not to forget: The issue of accessing outdated sites, that is
the scenario where:

 * Alice accesses /uri/0install/site.dom on Oct 28 and finds
   /uri/0install/site.dom/sitelib/Linux-ix86/1.2.3{,-or-later}
 * Bob accesses /uri/0install/someapp.com on Nov 28 and wants to execute
   the brand-new /uri/0install/someapp.com/app/Linux-ix86/latest/AppRun
   which is linked against
   /uri/0install/site.dom/sitelib/Linux-ix86/1.2.4-or-later/blahblah and
   therefore fails to run unless
    - either Bob uses a different machine (which is unlikely to be his
      favored choice)
    - or Bob issues "0refresh" manually, even though he may never have
      heard about the command line or zero install in particular
    - or /lib/ld-linux* does so, even though it is definitely not its
      job and we should probably not have the dynamic loader do zero
      install specific things
    - OR: 0refresh is executed on a regular basis, say once in 24 hours,
      as to give "absolutely aggressive caching" up.

The last option sounds like the most sensible one. It should be made
clear that this means that if a site chooses to withdraw a certain file
from its zero install repository, applications may cease working, and a
user is more at the mercy of some packager, application author, or the
like. I would therefore suggest to have 0build issue intimidating
warnings to a packager if they dare removing files. Things can still
break, of course, by replacing files (or symlinks). But in order to
circumvent this, you would probably have to use (and to enforce usage
of) a versioned file system, like in:

  /uri/0install/<site name>/<mtime>/<arbitrary hierarchy>

where <mtime> would be (say, the decimal representation of) the time at
which 0build was called, and where /uri/0install/<site name>/current
would be a symlink always pointing to the most current version of the
zero install tree. Regular files that remain unchanged between two runs
of 0build can be dealt with by setting symlinks appropriately, that is:
if you already have

  /uri/0install/site.dom/1066479254/somefile

and "now" is 1067346879 (seconds after Jan 1, 1970 UTC...), and somefile
has not changed, then you would have

  /uri/0install/site.dom/1067346879/somefile

point to ../../1066479254/somefile. (Another run of 0build will create
another link to the same file, not to the link, in order not to inflict
too many levels of indirection.) The upshot of this is that a client can
decide never to remove old versions of files (unless it is known that
they are definitely no longer used) in order to facilitate keeping an
application runnable with a library disappearing.

I am not sure whether we would want all that fuss. Probably not.

Joachim

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