Detecting unchanged files
*************************

Many operations need to know which working files have changed compared
to the basis revision.  (We also sometimes want to know which files
have changed between two revisions, but since we know the text-ids
and hashes that is much easier.)  

The simplest way is to just directly compare the files.  This is
simple and reliable, but has the disadvantage that we need to read in
both files.  For a large tree like the kernel or even samba, this can
use a lot of cache memory and/or be slow.  Some people have machines
that do not have enough memory to hold even one copy of the tree at a
time, and this would use two copies.

So it is nice to know which files have not changed without actually
reading them.  Possibilities:

* Make the working files be hardlinks to the file store.  Easy to see
  if they are still the same file or not by simply stat'ing them.  For
  extra protection, make the stored files readonly.  Has the
  additional advantage of reducing the disk usage of the working copy.

  Disadvantage is that some people have editors that do not handle
  this safely.  In that case the changes will go undetected, and they
  could corrupt history.  Pretty bad.

  We can provide a ``bzr edit`` command that breaks the link and makes
  the working copy writable.

* As above, but link to a temporary pristine directory, not to the
  real store.  They can get a wrong answer, but at least cannot
  corrupt the store.

* Check the mtime and size of the file; compare them to those of the
  previous stored version.  

  The mtime doesn't need to be the time the previous revision was
  committed.

  There is a possibility of a race here where the file is modified but
  does not change size, all in the second after the checkout.  Many
  filesystems don't report sub-second modification times, but Linux
  now allows for it and it may be supported in future.

* Read in all the working files, but first compare them to the
  size and text-hash of the previous file revision; only do the diff
  if they have actually changed.  Means only reading one tree, not
  two, but we still have to scan all the source.

* Copy the file, but use an explicit edit command to remember which
  ones have been changed.  Uneditable files should be readonly to
  prevent inadvertent changes.

The problem with almost all of these models is that it is possible for
people to change a file without tripping them.  The only way to make
this perfectly safe is to actually compare.  So perhaps there should
be a paranoia option.  

It is crucial that no failure can lose history.  Does that mean
hardlinks directly into the file store are just too risky?

It is most important that ``diff``, ``status`` and similar things be
fast, because they are invoked often.  It may be that the ``commit``
command can tolerate being somewhat slower -- but then it would be
confusing if ``commit`` saw something different to what ``diff`` does,
so they should be the same.

For the mooted `edit command`__, we will know whether a file is
checked out as r/o or r/w; if a file is known to be read-only it can
be assumed to be unmodified.

__ optional-edit.html

The ``check`` command can make sure that any files that would be
assumed to be unmodified are actually unmodified.

File times can skew, particularly on network filesystems.  We should
not make any assumptions that mtimes are the same as the system clock
at the time of creation (and that would probably be racy anyhow).

Proposal
--------

::
  
    if file is not editable:
        unmodified

    if not paranoid:
        if same inum and device as basis:
            unmodified
        elif present as read-only:
            unmodified
        elif same mtime and size:
            unmodified

    read working file, calculate sha1
    if same size and sha-1 as previous inventory-entry:
        unmodified

    possibly-modified