Re: Git commit generation numbers

On Thu, Jul 14, 2011 at 11:55:39AM -0700, Linus Torvalds wrote:

I'm actually much more nervous about a cache being inconsistent than I
would be about having generation numbers in the tree. The latter we
can (and should - but my patch didn't) add a fsck test for, and then
you would never get into some situation where there's some really
subtle issue with merge base calculation due to a corrupt cache.

Interesting. I'm nervous about that, too, which is why I _favor_ the
cache. Because we calculate the cache ourselves, we know its accurate
according to the parent pointers. If we find a bug, we fix it and bump
the cache version, which forces it to regenerate.

Contrast that with a bogus generation number that makes its way into an
actual commit object. That's there for eternity, just like the commit
timestamp skew we already have. I find it much less likely to happen
than skew in the commit timestamp, if only because generations are a
dirt-simple concept. But it is a case where there is duplicated
information in the actual DAG, and if that information doesn't match up
we are screwed.

-Peff