Re: Following history of a copied file from another indirect branch

From: Brian Gernhardt <brian@gernhardtsoftware.com>
Date: Oct 21, 2010, 19:39 UTC
Message-ID: <9089D1F5-A19B-4030-A6ED-463F250E450B@gernhardtsoftware.com>
In-Reply-To: <4CC08AA5.8070502@workspacewhiz.com>

On Oct 21, 2010, at 2:47 PM, Joshua Jensen wrote:

> It has become a necessity to copy a file from one long-lived branch to another.  It is not possible to merge the branches at this time.
> 
> I would like to have 'git gui blame' follow the copy back through its original history, but I don't believe Git has metadata for storing this.  Something along the lines of a 'followparent' in the commit object, for instance, would allow the revision walking code to wander the history down an alternate line.

Git stores no per-file metadata.  The closest we come is .gitattributes and .gitignore.

> By comparison, integrates work at a file level in Perforce.  That means I can integrate a file from one branch to another, and parentage is stored such that I can follow the file back through its history.
> 
> Are there any facilities to do this now?

Git simply does not have the idea of the history of a file.  Nothing in git will help merge "just a file" from one branch to another.  Either we have merged the two commits or not.

HOWEVER...

You can use git-filter-branch to create a new branch that contains only that single file and only the commits that affected it.  Something like the following (untested):

	# Merging "file" from branch "src" to branch "dest"
	git checkout -b temp src
	git filter-branch --prune-empty --index-filter="git read-tree --empty; git add file"
	# Since you describe the branch as "long-running", I'd suspect you'll have to wait a while here.
	git checkout dest
	git merge temp
	git branch -d temp
	git branch -D refs/original/temp

This will go faster if you have a ramdisk/tmpfs to perform the filtering in.  (git-filter-branch is very I/O intensive.)  Something like the following in place of the `git filter-branch` invocation above:

	mkdir /tmp/filter-branch # Assuming /tmp is tmpfs or similar
	git filter-branch -d /tmp/filter-branch --prune-empty --index-filter="git read-tree --empty; git add file"
	rm -rf /tmp/filter-branch

You could use --msg-filter to add the SHA-1 of the original commits to the "file history" branch.  Something like --msg-filter='cat;echo;echo From: $GIT_COMMIT'

I would recommend using cherry-pick to pull any further changes to the file across branches (be careful of commits that touch more than that file!).  I think git-filter-branch could be used to keep the one file branch up to date, but that is likely more effort than it's worth.  I would specifically advise against merging the single file branch into both "src" and "dest", as I think any later merge of the two would find these commits as a merge-base.

~~ Brian Gernhardt