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Re: Implicit keep-alive after reintegrate merge

From: Mark Mielke <mark_at_mark.mielke.cc>
Date: Mon, 30 Jan 2012 10:01:23 -0500

Stefan: I believe you are agreeing that the merge in either direction is
the same complexity, and describing how --reintegrate moves the
responsibility for the complexity to the owner of the private branch,
and requires resolution before submission. I think you are saying this
is a good thing because diff3 isn't perfect.

In my experience:

No merge is perfect. The situation is either complex, or it is not
complex - and moving resolution to the private branch is a matter of
process - not a matter of algorithm. That is, it is the responsibility
of the team to decide that "we will always make sure our private branch
is up to date before submitting to the integration stream."

In particular, if I have a stream with 100 users working in parallel,
all submitting on a regular basis because this is their full time paid
job to work on a piece of software, it may be a race to actually get the
submission - depending on if the algorithm can detect whether the same
files are being changed or not.

The first thing the tool can do to be genuinely useful in this
situation, is to accept some of the responsibility of detecting whether
or not the race is one of these "diff3 is not idempotent" situations,
and providing automatic handling. If the case has been hit, then
--reintegrate could be used as a form of "special error checking" where
it does the same as "merge", except in the case that the merge has a
true conflict with any particular element of the change set (as opposed
to a potential conflict with the end result), where the results of diff3
would need to be "trusted", then it could bail and provide the user with
the information required to resolve the conflict locally before submission.

The second thing the tool can do to be genuinely useful in this
situation, is to allow for this check to be overridden. If I didn't
trust diff3 - I wouldn't use merges at all. Sometimes a source
management tool just needs to help me resolve conflicts. Especially with
merge tracking and intelligent designer workflows, many cases of so
called "conflicts" touch unrelated lines of code, and it *is* safe to
complete the merge, even to the integration stream. I should have the
ability to choose to do this, rather than race for submission with 100
other users.

The worst thing the tool can do is to declare that "diff3 is idempotent
therefore it should be disabled" during --reintegrate. Yuck. This is a
partial solution and at least as I understand it - it is even dangerous.
What happens if I use --reintegrate in a situation that actually does
require merge resolution? Will every situation be blocked? Or will it
take --reintegrate as a license to overwrite results, trusting that I
can do all the necessary conflict checking myself? I have seen nothing
so far that allows me to conclude that architecturally, Subversion
requires the --reintegrate behaviour. It's a short cut in providing a
complete branch merging solution for users of the system. Somebody
started work on the canvas, and then drafted in the last corner rather
than finish it. :-)


On 01/30/2012 08:23 AM, Stefan Sperling wrote:
> The same applies to "reintegrate", BTW. It is a Subversion-specific
> concept that might not be represented in CM theory because it is, as
> you point out, just a special case of the general merge (you didn't
> describe what "merge" means in your theory so I'm just going to make
> assumptions).

>> Just to make sure it's understood: When you create a branch, the origin
>> of the branch is an interesting bit of information. However, for
>> merging, it is entirely irrelevant if branch A was created from B or the
>> other way around. To illustrate:
>> (1)
>> +- b_at_r2 ---- b_at_r3 ----
>> (branch) / | (merge)
>> / v
>> --- a_at_r1 -------------+- a_at_r4 ----
>> (2)
>> --- a_at_r1 ----------- a_at_r3 ----
>> \ | (merge)
>> (branch) \ v
>> +- b_at_r2 ------+- b_at_r4 ----
>> Cases (1) and (2) are exactly equivalent as far as the merge algorithm
>> is concerned, but Subversion calls the first a reintegrate merge and the
>> second a sync merge, and treats them differently, as if branch (a) were
>> somehow special. It's not.
> If you always use the 2-URL merge syntax all the abstractions go away
> and you'll have symmetry.
> (1) svn co a_at_r4 wc; svn merge b_at_r2 b_at_r3 a
> (2) svn co b_at_r4 wc; svn merge a_at_r1 a_at_r3 b
> See? Perfectly symmetrical.
> Your example is too simple, though.
> You only have one change being merged either way, and no cycles.
> Generally, we want to avoid spurious conflicts from diff3 which happen
> when changes are applied twice because diff3 is not idempotent.
> I.e. we break the nice symmetry to work around a limitation of diff3.
> In the following case we can avoid spurious conflicts by picking
> our parameters carefully:
> (3)
> +-b_at_r2--+ b_at_r3--b@r4-b_at_r5 ----
> (branch) / ^ | (merge 2)
> / | (merge 1) v
> --- a_at_r1 ------a_at_r2-----------+- a_at_r6 ----
> Merge 1 brings a_at_r2 into b_at_r2.
> Merge 2 brings b_at_r4 into a_at_r5.
> (3.1) svn co b_at_r2 wc; svn merge a_at_r1 a_at_r2 b
> There are two ways of performing merge 2.
> The first is symmetrical and re-applies a_at_r2 to a_at_r6, via b_at_r3,
> with possible spurious conflicts from diff3:
> (3.2 a) svn co a_at_r5 wc; svn merge b_at_r2 b_at_r5 a
> The second does not re-apply a_at_r2, so there are no possible conflicts
> from diff3 because of a_at_r2/b_at_r3. Only b_at_r4 can conflict.
> (3.2 b) svn co a_at_r5 wc; svn merge b_at_r3 b_at_r5 a
> The result is the same, however.
> What we use during --reintegrate is (3.2 b).
> You can argue that this approach is broken and we should be using (3.2 a)
> for symmetry, and let users deal with spurious conflicts.
> But (3.2 b) is always correct and more convenient if diff3 fails to
> produce a conflict-free diff when b_at_r3 is applied to a_at_r5.
> So why not use it?
> Alternatively, do you know of a diff3 replacement that is idempotent?

Mark Mielke<mark_at_mielke.cc>
Received on 2012-01-30 16:01:58 CET

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