On Wed, Jan 15, 2014 at 11:37:08AM +0100, David Kastrup wrote: > The question is what guarantees I have with regard to the commit date of > a commit in relation to that of its parent commits: > > a) none > b) commitdate(child) >= commitdate(parent) > c) commitdate(child) > commitdate(parent)
a) none > Obviously, I can rely on c) being true "almost always": Actually, b) is quite often the case in automated processes (e.g., "git am" or "git rebase"). The author dates are different, but the committer dates may be in the same second. And of course a) is the result of clock skew and software bugs. > it's definitely > good for a heuristic used for improving performance (meaning as an > ordering criterion for a commit priority queue). The problem is how > much I should cater for graceful behavior for the cases where it's not. Yes, this is exactly how git uses it. We generally walk breadth-first through the graph, relying on commit times for performance but not correctness. There are some parts of the code that will behave badly with clock skew. For example, "--since" will stop traversing when we hit a certain point. It requires a fixed number of "too old" commits before quitting, though, in an attempt to bypass small runs of skewed clocks. The "git describe --contains" algorithm will also produce wrong results in the face of skew. I believe it uses a "slop" of 24 hours in its skew. The current "tag --contains" algorithm is currently correct in the face of skew, but it can go much faster if you accept that skew will cause it to go wrong. I suspect there are other algorithms that could be sped up, too, if we had trustworthy generation numbers (I implemented and timed the "--contains" algorithm, but haven't done so for other algorithms). I've played with calculating and caching the generation numbers at repack time, but there aren't any patches currently under consideration. > Does git do any actual checks before pushing? No. One problem with checking the commit relationships is that it may not be the new commit which is broken (by skewing backwards), but rather the commit it builds on (which has skewed forwards). If you are building on such a fast-forward commit, it is often to late to fix that commit. So you have to fast-forward yourself, propagating the bogus value. If the receiving machine checked the incoming commits against its own internal clock, that could work. You would still want to check the commit relationships to catch new commits that erroneously claim to be from the past, though (that is, before their parents). -Peff -- To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
