Re: Careful object writing..

From: Linus Torvalds <torvalds@osdl.org>
Date: May 3, 2005, 19:47 UTC
Message-ID: <Pine.LNX.4.58.0505031242330.26698@ppc970.osdl.org>
In-Reply-To: <20050503192753.GA6435@taniwha.stupidest.org>

On Tue, 3 May 2005, Chris Wedgwood wrote:

> 
> how is this better than a single rename?  i take it there is something
> fundamental from clue.101 i slept though here?

A rename will overwrite any old object, which means that you cannot do any collision checks. In contrast, a "link()" will return EEXIST if somebody else raced with you and created a new object, and you can do collision checks instead of overwriting another persons object.

> also, if you are *really* paranoid you want to fsync *before* you do
> the link/unklink or rename --- which is what MTAs do[1]

Me, I refuse to slow down my habits for old filesystems. You can either fsck, or use a logging filesystem.

I don't see anybody not using logging filesystems these days, so..

> also, shouldn't HEAD (and similar)[2] be updated with a temporary and
> a rename too?

Maybe. Much less important, though.

Show 5 quoted lines

> > NOTE NOTE NOTE! I have _not_ updated all the helper stuff that also
> > write objects.
> 
> i thought this was all common code?  if it's not maybe now is the time
> to change that?

It is all common code, except:
 - things like "fetch from another host" will use rsync/wget/xxx to 
   actually get the files. To those programs, we're not talking about git 
   objects, we're just talking "regular files"
 - rpull.c has a special different routine to write its objects. I don't 
   use it, so..

Anyway, it should be reasonably easily fixable.

		Linus