Re: "git-send-pack"

["H. Peter Anvin" <hpa@zytor.com>]

Linus Torvalds wrote:
> 
> On Fri, 1 Jul 2005, H. Peter Anvin wrote:
> 
>>Any reason not to simply append objects to an existing packfile?
> 
> 
> What happens when somebody screws up in the middle?
> 
> The one thing I care about more than anything else is consistency. We are 
> careful about writing objects in the right order, and we can re-create the 
> state from the originator etc. But if we start appending stuff and 
> something goes wrong in the middle, I'm just not going to touch it. A 
> "truncate and hope for the best" algorithm? 
> 
> Besides, the result is not a valid git archive any more. 
> 

It's a log.  It's a standard technique to append entries to a log.  The 
requirements for this to always be consistent is that a) it's possible 
to know when the entry/entries at the end are inconsistent and b) it's 
always possible to roll back the log to a consistent state.

This is normally done with commit records (write data - fdatasync - 
write commit record - fdatasync), but in the case of git, the commit 
record isn't required because each git record is self-validating.  This 
is an incredibly powerful property.

If the log is written in topological sort order, then even a truncated 
log file is a valid (subset) git object store.

	-hpa