[PATCH 2 of 2] rollback: avoid unsafe rollback when not at tip (issue2998)

[Matt Mackall]

On Fri, 2011-09-30 at 13:32 +0200, Laurens Holst wrote:
> Op 30-09-11 04:55, Greg Ward schreef:
> > # HG changeset patch
> > # User Greg Ward<greg at gerg.ca>
> > # Date 1317350673 14400
> > # Node ID c10af212a55cbfa84b5d5182ee41028c4baa33c6
> > # Parent  f295542f482d4b567fa6229b93c1854aca78792c
> > rollback: avoid unsafe rollback when not at tip (issue2998)
> >
> > You can get into trouble if you commit, update back to an older
> > changeset, and then rollback. The update removes your valuable changes
> > from the working dir, then rollback removes them history. Oops: you've
> > just irretrievably lost data running nothing but core Mercurial
> > commands.  (Or, more subtly, rollback from a shared clone that was
> > already at an older changeset -- no update required, just rollback
> > from the wrong directory.)
> >
> > The fix assumes that rollbacks can be considered "safe" or "unsafe". A
> > safe rollback is one that aborts a transaction whose data already
> > exists somewhere else: pull, unbundle, or push. Don't be so picky
> > about rolling back those transactions, since the repo or bundle from
> > which they came presumably still exists. Every other rollback, most
> > typically of a commit transaction, is considered unsafe. Finally, you
> > can get back the existing dangerous behaviour with --force.
> 
> Why is rolling back a commit you just did unsafe? As long as you haven’t 
> updated to a different revision, all your code will still remain in the 
> working copy, your dirstate is also kept intact, and even the commit 
> message is still available in tools like TortoiseHg or the command line 
> history. So I don’t think any information is lost there, and this is for 
> me the no. 1 case where I use rollback, it would suck if I had to start 
> habitually adding a --force flag. Then there would be no protection 
> anymore from the cases where there is actual data loss.

Greg's description doesn't actually match his code:

+        if not force and not safe and nottip:
+            raise util.Abort(_('attempt to rollback a commit when not
at tip '
+                               '(use -f to force and lose data)'))
+

In practice, 'safe' includes 'being at tip'.

-- 
Mathematics is the supreme nostalgia of our time.