git svn: reset invalidates the memoized mergeinfo caches

git svn: reset invalidates the memoized mergeinfo caches

Since v1.7.0-rc2~11 (git-svn: persistent memoization, 2010-01-30),
git-svn has maintained some private per-repository caches in
.git/svn/.caches to avoid refetching and recalculating some
mergeinfo-related information with every 'git svn fetch'.

This memoization can cause problems, e.g consider the following case:

SVN repo:

  ... - a - b - c - m  <- trunk
          \        /
            d  -  e    <- branch1

The Git import of the above repo is at commit 'a' and doesn't know about
the branch1. In case of an 'git svn rebase', only the trunk of the
SVN repo is imported. During the creation of the git commit 'm', git svn
uses the svn:mergeinfo property and tries to find the corresponding git
commit 'e' to create 'm' with 'c' and 'e' as parents. But git svn rebase
only imports the current branch so commit 'e' is not imported.
Therefore git svn fails to create commit 'm' as a merge commit, because one
of its parents is not known to git. The imported history looks like this:

  ... - a - b - c - m  <- trunk

A later 'git svn fetch' to import all branches can't rewrite the commit 'm'
to add 'e' as a parent and to make it a real git merge commit, because it
was already imported.

That's why the imported history misses the merge and looks like this:

  ... - a - b - c - m  <- trunk
          \
            d  -  e    <- branch1

Right now the only known workaround for importing 'm' as a merge is to
force reimporting 'm' again from SVN, e.g. via

  $ git svn reset --revision $(git find-rev $c)
  $ git svn fetch

Sadly, this is where the behavior has regressed: git svn reset doesn't
invalidate the old mergeinfo cache, which is no longer valid for the
reimport, which leads to 'm' beeing imprted with only 'c' as parent.

As solution to this problem, this commit invalidates the mergeinfo cache
to force correct recalculation of the parents.

During development of this patch, several ways for invalidating the cache
where considered. One of them is to use Memoize::flush_cache, which will
call the CLEAR method on the underlying Memoize persistency implementation.
Sadly, neither Memoize::Storable nor the newer Memoize::YAML module
introduced in 68f532f4ba888 could optionally be used implement the
CLEAR method, so this is not an option.

Reseting the internal hash used to store the memoized values has the same
problem, because it calls the non-existing CLEAR method of the
underlying persistency layer, too.

Considering this and taking into account the different implementations
of the memoization modules, where Memoize::Storable is not in our control,
implementing the missing CLEAR method is not an option, at least not if
Memoize::Storable is still used.

Therefore the easiest solution to clear the cache is to delete the files
on disk in 'git svn reset'. Normally, deleting the files behind the back
of the memoization module would be problematic, because the in-memory
representation would still exist and contain wrong data. Fortunately, the
memoization is active in memory only for a small portion of the code.
Invalidating the cache by deleting the files on disk if it isn't active
should be safe.

Signed-off-by: Peter Baumann <waste.manager@gmx.de>
Signed-off-by: Steven Walter <stevenrwalter@gmail.com>
Signed-off-by: Eric Wong <normalperson@yhbt.net>