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Re: resolving ambiguity in action stamps
From: |
Eric S. Raymond |
Subject: |
Re: resolving ambiguity in action stamps |
Date: |
Sun, 14 Sep 2014 10:21:17 -0400 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
Stephen J. Turnbull <address@hidden>:
> Eric S. Raymond writes:
>
> > One is a genuinely funny gotcha. You can't get to git hashes
> > without going through something semantically like my version stamps
> > on the way!
>
> I disagree.
Well, maybe you have some case, but haring off into bug bisection
techniques doesn't make it.
It is still a technical fact that no git translation containing SHA1s
can be built without passing through a VCS-independent representation
of commit refs on the way. This is because the SHA1-containing repo
would have to be built one commit at a time in order for the correct target
hashes to *exist* to be patched into whatever refers to them.
That means the rewrite of the old commit refs has would need at some
point to pass through a mapping of old refs to something that identifies
a commit but cannot be a git hash *because the hash doesn't exist yet!*
That's the gotcha.
This is not limited to git: it would a general problem with hg or any
other target VCS in which parent hashes are mixed into a commit's hash.
I have a design sketch in my head for a tool (a filter on fast-import
streams) to work around this problem, but it can't eliminate the need
for a VCS-independent commit-ref format, only hide the fact that one
was used during the move.
> > The second problem is that it's not future-proof. Someday we might
> > have to change VCSes again; git is the *fifth*, after RCS CVS Arch
> > bzr. It would be unwise to assume that nobody will ever have a
> > better idea.
>
> I don't. But if the new one can't run
>
> $VCS filter-branch --commit-filter ...
>
> as fast as git, I'd have serious doubts about the sanity of the
> proponents. Even on a 200,000 commit repo, that's just not going to
> take a ton of time, and only needs to be done once.
Yeah, that'd be nice, if a functional equivalent of filter-branch could
do the job at all by itself. No chance of that: see above about hash mixing.
> > At that time it would be a Really Good Thing if as few of our commit
> > refs as possible are opaque magic cookies
>
> Actually, I disagree. It would be a really good thing if they are
> precise. Do you really want to put anybody through the trouble of
> translating randomized format cookies, which may point to any of
> several commits, again? Then revising their scripts every time a new
> variant shows up?
It has yet to be demonstrated that this is a problem in a real use case.
And, actually, I already checked this; the Emacs history doesn't have
any version-stamp collisions in actually referenced revisions.
> Existence proof comes before characterization, please.
I don't understand that.
--
<a href="http://www.catb.org/~esr/">Eric S. Raymond</a>
- Re: resolving ambiguity in action stamps (was: Everyone, please stop making my life more difficult), (continued)
- Re: resolving ambiguity in action stamps (was: Everyone, please stop making my life more difficult), Stephen J. Turnbull, 2014/09/13
- Re: resolving ambiguity in action stamps, Harald Hanche-Olsen, 2014/09/13
- Re: resolving ambiguity in action stamps, Eric S. Raymond, 2014/09/13
- Re: resolving ambiguity in action stamps, Yuri Khan, 2014/09/13
- Re: resolving ambiguity in action stamps, Stephen J. Turnbull, 2014/09/14
- Re: resolving ambiguity in action stamps, Eric S. Raymond, 2014/09/14
- Re: resolving ambiguity in action stamps, David Kastrup, 2014/09/14
- Re: resolving ambiguity in action stamps, Stephen J. Turnbull, 2014/09/14
- Re: resolving ambiguity in action stamps, David Kastrup, 2014/09/14
- Re: resolving ambiguity in action stamps, Stephen J. Turnbull, 2014/09/14
- Re: resolving ambiguity in action stamps,
Eric S. Raymond <=
- Re: resolving ambiguity in action stamps, Stephen J. Turnbull, 2014/09/14
- Re: resolving ambiguity in action stamps, David Kastrup, 2014/09/14
- Re: resolving ambiguity in action stamps, Eric S. Raymond, 2014/09/14
- Re: resolving ambiguity in action stamps, Stephen J. Turnbull, 2014/09/14
- Re: resolving ambiguity in action stamps, Andreas Schwab, 2014/09/13
- Re: Everyone, please stop making my life more difficult, Sam Steingold, 2014/09/12
- Re: Everyone, please stop making my life more difficult, David Kastrup, 2014/09/12
- Re: Everyone, please stop making my life more difficult, Yuri Khan, 2014/09/12
- Re: Everyone, please stop making my life more difficult, Sam Steingold, 2014/09/12
- Re: Everyone, please stop making my life more difficult, David Kastrup, 2014/09/12