[Unison-hackers] Proposed mode for unison: "Salvage"

[Ryan Newton]

The biggest complaint I hear from friends and family about unison is
the ease of duplicating files.  This happens most often when running
unison without saved archives (e.g. because things get moved around,
mixed up, moved to new machines, etc).

A typical scenario that is difficult to handle with unison is that you
come across an old copy of folder X that *might* contain something
that you forgot to extract or move into your current, primary copy.
But of course you don't know what's there and checking is manually
hard.  Further, performing a simple unison is the WRONG answer,
because the organization may have changed substantially, making it
very hard to tell if supposedly new files in the old copy are really
new or have just been moved (duplication danger).

In this case it would be very useful to run unison in mode where:

  (1) only copies from old->new are considered.  The goal is not two
identical archives, but to retrieve things from the old copy.
  (2) only files which do not exist ANYWHERE in the new archive are
considered, the new archive is just a flat set of files for the
purpose of this check.

Secondary questions include where to put the new files (presumably the
same path as in the old archive) and what to do with
conflicts/collisions resulting from, for example, modified files
(presumably they're treated in the normal unison way).

The interface could perhaps be a "-salvage X" flag, where X is one of
the roots (just like -force).

Best,
-Ryan

P.S. I actually wrote a separate tool at some point (in ocaml) that
could accomplish the above which I could provide to the curious.  Its
goal was to replace all the files in a folder X, that exist in a
folder Y, with symlinks into folder Y.  One could run this on the old
archive and then use "find" to see all the files that were not turned
to symlinks.  However, I think leveraging unison for this purpose
would be much more desirable.