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[PATCH 0/1] difftool: --no-index is incompatible with --dir-diff

While debugging why a straight-forward backport of the js/difftool-no-index 
branch onto Git for Windows' master does not work as expected (spoiler: it's
due to git diff --no-index now implying --ext-diff, a change that is not yet
in Git for Windows' master), I ran into quite a few segmentation faults
while trying to make --no-index work nicely with --dir-diff.

And this was a much bigger project than I thought it would be: the worktree 
is NULL and causes a segmentation fault, make_transient_cache_entry() 
returns NULL instead of a cache entry, while looking up replacement objects
there is a crash because git_replace_ref_base is NULL, during 
checkout_entry()'s life time, the prepare_alt_odb() function tries to
dereference the_repository.objects->odb (which is NULL, of course). I'll
stop here with all the segmentation faults.

Even worse, when trying to compare ../1 to ../2 (as we do in the regression
test for difftool --no-index outside any repository), we run afoul of 
verify_path() complaining that this is not a valid path for an index entry.

After figuring all of that out, I took a step back and guess what: it turns
out that it does not even make sense to combine those two options. The 
--dir-diff option exists to enable diff'ing subdirectories of a worktree
while pretending that untracked and ignored files in them do not even exist.
And without a worktree, there simply are no untracked or ignored files, so 
--dir-diff is not necessary.

Still, we should address those segmentation faults. And I think we address
them best by explicitly telling the user that they cannot use those two
options together.

Johannes Schindelin (1):
  difftool --no-index: error out on --dir-diff (and don't crash)

 builtin/difftool.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

base-commit: 20de316e33446f37200e51aa333ba7d824dfd478
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-186%2Fdscho%2Fdifftool-no-index-extra-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-186/dscho/difftool-no-index-extra-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/186