Re: New SCM and commit list

[Daniel Barkalow]

On Tue, 12 Apr 2005, Adam J. Richter wrote:

> On 2005-04-11, Daniel Barkalow wrote:
> >If merge took trees instead of single files, and had some way of detecting
> >renames (or it got additional information about the differences between
> >files), would that give BK-quality performance? Or does BK also support
> >cases like:
> >
> >orig ---> first ---> first-merge -
> > |                /               \
> > |------> second -                 -> final
> > |                \               /
> > |------> third ---> third-merge -
> >
> >where the final merge requires, for complete cleanliness, a comparison of
> >more than 3 states (since some changes will have orig as the common
> >ancestor and some will have second).
> 
> 	With 3-way merge and the ability to regenerate the relevant
> files from each step, this should be easy to handle as long
> as you have a list of which patches are considered to have been
> duplicated.  Let's detail your example:
> 
> orig ---> first 1a 1b 1c ---> first-merge - 1d 1e
>  |                          /                    \
>  |------> second 2a 2b 2c -                       -> final
>  |                          \                    /
>  |------> third 3a 3b 3c ---> third-merge - 3d 3e
> 
> Here, 1a, 1b, etc. refer to specific states of the source tree.
> I will refer to differences by a notation like "1a->1b", which
> is the difference to go from snapshot 1a to 1b.  All that the
> merge algorithm for the final merge needs to know is that the
> ends of the branches (that is, 1e and 3e) both contain the
> following diffs:
> 
> 		orig->2a
> 		2a->2b
> 		2b->2c
> 
> 	The function merge(orig, ver1, ver2) can try to reverse
> the duplicate merges in one of the branches:
> 
> 		1e' = merge( 1e, 2c->2b);
> 		1e'' = merge(1e', 2b->2a);
> 		1e''' = merge(1e'', 2a->orig);
> 		return merge(1e''', 2c->3e)

If 1d->1e depends on something in the 2 series, which is why I would
expect 1e to be pushing something containing the 2 series, there must be
conflicts. Likewise on the 3 series.

> 	Of course, conflicts can happen, but that can happen
> in any merge.  There are also other ways to calculate the
> merge and because there are different ways one can write a
> merge function, it is possible that merging in a different
> order might produce slightly different results.  For example,
> it would be possible to reverse the dpulicates in your "third merge"
> branch instead of your "first merge" branch, or one could
> reconstruct a branch without the duplicated merges by executing
> the other changes forward from a common ancestor, like so:
> 
> 		1e''' = merge(orig, 3d->3e);
> 
> 	...regardless, the point is that all the information
> that is absolutely needed is a list of instance of diffs
> to be skipped.  It is not even necessary that the changes
> have such a clearly explainable ancestory as that you have
> described.  All the merge program needs to know are the changes
> to be skipped, although information like changes the skipped
> patches are duplicating may be useful for things like trying
> to reverse a patch in your "third-merge" branch in your
> example if reverseing the patch in "first-merge" fails.

Right, an extended primitive solves the problem, certainly, and much more
effectively than sticking with 3-way merge.

> 	I believe that at least bitkeeper, darcs, a free python-based
> system that I can't remember at the moment, and possibly arch do this
> sort of machination already.
> 
> 
> >Does this happen in real life? [...]
> 
> 	Yes.  Both individual users and Linux distributions incorporate
> patches that they think are useful to them and then futher patches
> that they develop.  The time costs of rejecting such patches would
> likely be paid for by other integration or development work not being
> done.

It seems to me that users who use extra patches keep these separate from
their own patches (which they often keep in multiple series):

orig ---> other-people ---> local use, distribution
 |                       /
 |------> mine ----------
 |                       \
 |------> etc    ---------> mainline

If mainline is going to get the third-party patches in a distro tree, it
should get them from the original authors, not as part of a miscellaneous
patch set from the distro. If one patch series depends on another patch
series, it should hold off until the other one goes in, not include it in
the submission.

> >It seems like sane development processes
>                ^^^^
> >wouldn't have multiple mainline-candidate patch sets including the same
> >patches, if for no other reason than that, should the merge fail, nobody
> >with any clue about the original patches would be anywhere nearby.
> 
> 	If you could avoid prejudicial subjective adjectives, it
> it would make it easier for the saneness or insaneness of an
> approach to be apparent just by discussing your more objective criteria,
> like the remainder of your sentence, which is where the focus should
> be.
> 
> 	(1) Does allowing duplicate patches really mean that
> 	   "nobody with any clue about the original patches would be
> 	   anywhere near by?"  What attracts these clueful people
> 	   just by third parties having to rebase their patches?

The clueful people are the original authors (first, second, and
third); 1d-1e and 3d-3e would be rebased by their authors against a new
orig that's the merge of the 1c, 2c, and 3c (which all have a good common
ancestor).

Actually, the best 3-way merge path might be:

merge(merge(merge(3d,orig->1c),3d->3e),1d->1e)

That is, generate a complete merge at the point where people each merged
in the second line, and then continue forward from there.

> 	(2) Does this supposed benefit outweigh the cost of rejecting
> 	    many patches unnecessarily?  I know from my own experience
> 	    that I have either given up on or had to put into a very low
> 	    priority mode at least 66% of the patches that I haven't
> 	    gotten integrated, but which I am confident the kernel
> 	    would be better having (e.g.: devfs shrink, lookup()
> 	    trapping, ipv4 as a loadable (not not yet removable) module,
> 	    sysfs memory shrink, factoring much of the DMA mapping to
> 	    the common bus code from individual drivers, fewer kmap's
> 	    in crypto, I could go on).

This is unfortunate, certainly, but the alternative under discussion would
be to get those patches into as many other trees as possible until
Andrew/Linus picks them up and then finds that he's gotten multiple
copies of them via different routes. If each of these went in through the
respective maintainer, there would be no problem, and if any of them went
in without the respective maintainer's sign-off, that would upset
people.

	-Daniel
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