Re: [ANNOUNCE] chronoversion: chronological archiving script with temporary commits

From: Shawn O. Pearce <spearce@spearce.org>
Date: Mar 13, 2007, 15:45 UTC
Message-ID: <20070313154511.GA18890@spearce.org>
In-Reply-To: <e1dab3980703130526t4b573f18h793a065d54c9369@mail.gmail.com>

David Tweed <david.tweed@gmail.com> wrote:

Show 14 quoted lines

> A question for those who understand things: I stash the last written
> _tree_'s hash in a tag and then when a new "commit's" directory tree
> is written starts look to see if it's the same SHA value. If it is I
> know I can avoid the commit. At the moment I'm using
> 
>    if os.path.exists(lastTreeFile) and
> tree==open(lastTreeFile,"r").read()[:40]:
> 
> to be safe just in case a user, eg, goes mad and manually deletes that
> record. Clearly this is going to hit trouble if git ever decides to
> put this tag into a packed refs file.
> Is there any neat way of using builtin stuff like git-rev-parse to ask
> if a ref has a given SHA1 value and return an easily parsed yes/no
> answer?

The common idiom if you want to compare trees to see if you need to make a commit is:

	oldc=`git rev-parse $tagname^{commit}`
	oldt=`git rev-parse $oldc^{tree}`
	newt`git write-tree`
	if [ X$oldt = X$newt ]; then
		: nothing to save
	else
		newc=`git commit-tree $newt -p $oldc`
		git update-ref $tagname $newc $oldc
	fi

Yes, a little ugly. But its safe; if another program were to alter the value of $tagname (e.g. "git branch -f", "git tag -f") while your script is running the update-ref in the else will fail, letting you know that $tagname changed in the process.

-- 
Shawn.