On Wed, Dec 18, 2013 at 11:27:10AM -0500, Eric S. Raymond wrote:
> For use in reposurgeon I have defined a generic cross-VCS reference to
> commit I call an "action stamp"; it consists of an RFC3339 date followed by
> a committer email address. Here's an example:
>
> 2013-02-06T09:35:[email protected]
>
> In any VCS with changesets (git, Subversion, bzr, Mercurial) this
> almost always suffices to uniquely identify a commit. The "almost" is
> because in these systems it is possible for a user to do multiple commits
> in the same second.
FWIW, this has quite a few collisions in git.git:
$ git log --format='%ct %ce' | sort | uniq -c | sort -rn | head
22 1172221032 [email protected]
22 1172221031 [email protected]
22 1172221029 [email protected]
21 1190197351 [email protected]
21 1172221030 [email protected]
20 1190197350 [email protected]
17 1172221033 [email protected]
15 1263457676 [email protected]
15 1193717011 [email protected]
14 1367447590 [email protected]
In git, it may happen quite a bit during "git am" or "git rebase", in
which a large number of commits are replayed in a tight loop. You can
use the author timestamp instead, but it also collides (try "%at %ae" in
the above command instead).
> And now you know why I wish git had subsecond timestamp resolution! If it
> did, uniqueness of these in a git stream could be guaranteed.
It's still not guaranteed. Even with sufficient resolution that no two
operations could possibly complete in the same time unit, clocks do not
always march forward. They get reset, they may skew from machine to
machine, the same operation may happen on different machines, etc. The
probability of such collisions is significantly reduced, though, if only
because the extra precision adds an essentially random factor.
But in some cases you might even see the same commit "replayed" on top
of different parts of the graph, or affecting different paths (e.g., by
filter-branch). I.e., no matter what your precision, multiple hacked-up
views of the changeset will still always have that same timestamp.
-Peff
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