Re: my git problem

From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Apr 27, 2008, 19:15 UTC
Message-ID: <alpine.LFD.1.10.0804271204510.3119@woody.linux-foundation.org>
In-Reply-To: <20080427112938.4dbe8e2a.akpm@linux-foundation.org>

On Sun, 27 Apr 2008, Andrew Morton wrote:

Show 8 quoted lines

> 
> Now, I want to generate a plain patch against mainline which will add the
> patches which are in git-ia64 and which aren't in mainline.  ie: when that
> patch is applied to mainline, we get git-ia64.  Sounds simple.
> 
> A naive
> 
> 	git-diff origin git-ia64

Don't do that.

That will diff between the two branches, and if they both contain stuff (which they obviously do), you'll get all the things that are in origin (but not git-ia64) as a reversed diff.

What you _want_ is the diff from the last common point, aka the "merge base".

With git, you could do that as

	merge_base=$(git merge-base origin git-ia64)
	git diff $merge_base git-ia64

but there is a convenient shorthand for that, which is to use "a...b" (three dots!), so

	git diff -p --stat origin...git-ia64

should generally get you what you want.

I say *generally*, because there might be multiple merge-bases if there are crossing merges between the two branches and there is no well-defined single common point. But that criss-cross case almost never happens for the kernel, because I've been pretty good at trying to teach maintainers to not generate that kind of complex history (it doesn't just obfuscate the above kind of situation, it also makes gitk output harder-to-read than it otherwise would be).

That said, your script (that does a merge) should have been able to get the diff too, and in fact handle even the criss-cross case. It's written a bit strangely (like having that really old-fashioned way of using git merge, passing in HEAD explicitly etc).

			Linus