Re: bisect / history preserving on rename + update

On Tue, 14 Aug 2007, Thomas Gleixner wrote:

That "should just work".

[ However, there does seem to be a bug in the "-B" logic, so it doesn't 
  actually work as well as it should! See below ]

BUT! By default, rename detection isn't on at all, mostly because it results in patches that non-git "patch" cannot apply, but partly also because it can slow certain things down.

So to get nice diffs, use

	git show -B -C

where the magic is:

 - "-B" means "break file associations when a file is *too* dissimilar" 

   Normally, git will assume that if a filename stays around, it's the 
   same file. However, with "-B", it does similarity analysis even for 
   files that are the same, and if they are very different, git will 
   decide that maybe they weren't the same file after all!

 - "-C" is "find code movement and copying".

However, nobody ever actually uses "-B" (it's so rare as to effectively not exist, and it does slow things down a bit), so it seems to have bit-rotted (or maybe it had this bug even originally: as I said, I don't think anybody has ever really _used_ this functionality).

Junio, look at this:

	# create a repo in "testing"
	cd
	mkdir testing
	cd testing/
	git init

	# copy a file from the git repo
	cp ~/git/revision.c .
	git add revision.c
	git commit -a -m "Add file 'A'"

	# move it around, copy another file in its stead
	git mv revision.c old-revision.c
	cp ~/git/Makefile revision.c
	git add revision.c
	git commit -a -m "Move file 'A' to 'B', create new 'A'"
	git show -B -C

and notice how "-B" *did* actually work, and we get a nice:

	diff --git a/revision.c b/old-revision.c
	similarity index 100%
	rename from revision.c
	rename to old-revision.c

but then it breaks: instead of creating the new "revision.c", we get:

	diff --git a/revision.c b/revision.c
	dissimilarity index 98%
	index 038693c..4eb4637 100644
	--- a/revision.c
	+++ b/revision.c
	@@ -1,1572 +1,1117 @@
	-#include "cache.h"
	...

which uses "reivision.c" as the base, even though it was already broken up! I think it *should* have looked like

	diff --git a/old-revision.c b/old-revision.c
	new file mode 100644
	index 0000000..4eb4637
	--- /dev/null
	+++ b/revision.c
	+# The default target of this Makefile is...
	...

so I think there is a bug there where the "-B" thing doesn't really "stick", and some part still uses the old file content even though it was dis-associated with the new content!

Hmm?

			Linus