PERFORCE change 176831 for review

[Tim Kientzle]

>>>> But I really do believe that single-pass direct
>>>> install is feasible and is eventually where we want
>>>> to be.  The key insight for me was when Florent recently
>>>> pointed out that you could just read the +CONTENTS,
>>>> then do a verify pass, then extract everything.
>>>> (Any conflict during the extraction pass
>>>> would be a fatal error:  delete everything
>>>> extracted so far and scream loudly.)

>>> Agreed on the final point. I'm kind of interested though about the
>>> first point -- the verify pass -- are you verifying that the contents
>>> are sane on the disk or in the tarball? If so, why not just verify the
>>> contents after the fact...

That is:
   * Read and parse +CONTENTS
   * Check the package database to verify that
     dependent packages exist and that no conflicting
     packages exist.  If there's a problem, ask the
     user what to do.  (This gets complicated.)
   * Check the disk to verify that none of the files
     listed in +CONTENTS already exist on disk.  If so,
     there's a conflict and we must ask the user
     what to do (this gets complicated).
   * Extract all of the files, failing if there is a
     file in the tarball that wasn't listed in +CONTENTS
     or if there is a file in the tarball that already
     exists on disk (which must have been created in the
     intervening window).