Re: confused about preserved permissions

From: Shawn O. Pearce <spearce@spearce.org>
Date: Aug 21, 2007, 01:35 UTC
Message-ID: <20070821013541.GC27913@spearce.org>
In-Reply-To: <20070820205042.GB10173@glandium.org>

Mike Hommey <mh@glandium.org> wrote:

> > >> > sprintf "%06o %s\0%s", $mode, $file, pack("H[40]", $sha1)
> 
> The question here was why the permissions are encoded with "%06o" while
> the hash is packed. Anyways, it's just a boring detail.

Because text format is simple and pretty much everyone understands it, especially when you are talking about UNIX mode/permission bits in octal, the name is "text", and then oh, wait, those 40 bytes of hex is a lot of data - we'll just make that 20 bytes of binary instead. :-)

OK, I don't know if that's really true. Probably only Linus can speak to why the tree format is what it is. Its just how I have rationalized it myself.

Pack v4 changes the storage of the mode to be in binary, at least within the packfile, but it does still have to convert it back to the "%06o" format when computing the SHA-1 of a tree object. Believe it or not, pack v4 is being worked on again... so maybe in a future version of Git the tree modes will take up a tiny bit less space.

I haven't specifically looked at this, but I would also suspect that within any given tree's zlib dictionary the mode string portion is given a reasonably short bit sequence. So its doubtful that the mode takes up 6 bytes when compressed by zlib, its probably more likely around 1 byte. Pack v4's big win here isn't due to the mode, its due to the filenames being compressed better across trees.

-- 
Shawn.