On Mon, Jul 9, 2012 at 4:16 PM, Julian Foad <[email protected]> wrote:
> To move forward and decide what behaviour is right, we need to be able to
> compare the 1.7 behaviour with the proposed behaviour in *specific*
> scenarios. So we need to be able to enumerate the specific scenarios that we
> mean by the general term "merging with subtree mergeinfo". This is what I am
> doing currently.
>
> The twist is that the best way to enumerate all the possibilities for 1.7
> merges, and the best way to enumerate all the possibilities for an ideal
> symmetric merge, are different. For example, the 1.7 non-reintegrate merge
> (let's say from branch A to branch B) doesn't look at any mergeinfo
> indicating merges from B to A, so the primary way to categorize those cases
> is by their last complete A->B merge (of the root of the merge, and of a
> subtree). B->A merges can also affect the result if performed after the last
> A->B merge, so we categorize secondarily by their B->A mergeinfo. By
> contrast, an ideal symmetric merge only cares about the last complete merge
> (A->B or B->A); any earlier merges in the other direction make no difference
> at all.
>
> After separately enumerating the 1.7 cases and the (ideal) symmetric merge
> cases, we can split or combine categories as necessary to merge these cases
> into a single list. Then we can decide what constitutes "same" or
> "backward-compatible" behaviour in each case. Don't be suspicious -- I'm not
> trying to twist the word "compatible" to mean something else -- rather, what
> I mean is that not all of the possible scenarios are ones in which the 1.7
> behaviour is "good". For example, we know that in 1.7 if the last complete
> merge was A->B (in r10, say) and then you reintegrate a subtree (B/foo ->
> A/foo in r20), then try to sync A->B again, Subversion will not notice that
> r20 merge and will attempt to re-merge r19:20 of A/foo into B/foo, which
> "works" only in rather trivial cases (due to auto-resolving of some types of
> duplicate change) and I trust we can agree it is in general wrong.
>
> So far, I've only been compiling this categorization from my head; the next
> step is to write tests to confirm or correct this.
>
>
> CATEGORIZING SUBTREE MERGES
>
> For the purposes of this categorization, we consider:
>
> * The merging history of the root 'R' (the root node of the requested merge
> source and target trees), and of a subtree 'S' (a single node or subtree
Minor point: I've always considered a single node as a subtree as far
as mergetracking goes; so file merges are "subtree merges". I mention
this only because most of the code and testing comments I've written
over the years assume this. Again, it's a minor point, but AFAICT we
can largely ignore the node type for this level of conversation.
> whose merging history differs from that of the root node in a significant
> way).
You might want to define this a bit more explicitly. I'm pretty sure
I know what you mean, but it might not be clear to others. Something
like: "Mergeinfo[1] differences between the root and the subtree, if
they exist, describe revisions that are inoperative on both the root
and the subtree".
[1] "Mergeinfo" interpreted here in the most general way, including
explicit mergeinfo, inherited mergeinfo, and implicit mergeinfo (i.e.
natural history).
> Only one subtree is considered; multiple subtrees are assumed to be handled
> independently, even if they are nested (such as root 'A', subtree 'A/D' whose
> history differs from 'A', and subtree 'A/D/foo' whose history differs from
> 'A/D').
>
> * The "last complete merge" of the Root (in one or both directions), and
> the "last complete merge" of the Subtree (in one or both directions). The
> "last complete merge" in direction A->B means the last revision R for which
> all revisions on A, up to and including R, are (currently) recorded as having
> been merged from A to B. This state could have been reached through any kind
> of merge or sequence of merges; all that matters is what the current
> mergeinfo says has been merged.
>
> * There is an assumption that the actual content changes were in fact
> merged in accordance with what the mergeinfo says, subject to any editing
> that was required to resolve any conflicts detected by the merge process and
> any semantic conflicts.
>
>
> CATEGORIZING SUBTREE MERGES: 1.7 Non-Reintegrate
>
> These cases are for a reintegrate merge A->B.
A bit confused, the category is "1.7 Non-Reintegrate" but the cases
are for a reintegrate merge A->B? Could you clarify? For example,
given row 3 below, are we talking about the third merge here:
A--------------------------->
\ ^ \
subtree / \
merge subtree ?
\ merge \
V / V
B--------------------------->
or here?
A--------------------------->
\ ^ ^
subtree / /
merge subtree ?
\ merge /
V / /
B--------------------------->
> In each row of this table, up to two Root merges are indicated, and their
> relative ordering is significant; similarly for Subtree merges. The ordering
> of R merges relative to S merges is not significant.
>
> Root | Subtree | Behaviour
> ---------+-----------+-----------------------------------
> 1. never | same | OK (not a subtree scenario)
> +-----------+-----------------------------------
> 2. | [S<] S> | Merge all needed changes
> +-----------+-----------------------------------
> 3. | [S>] S< | All needed; & some duplicates in S
> ---------+-----------+-----------------------------------
> 4. [R<] R> | same | OK (not a subtree scenario)
> +-----------+-----------------------------------
> 5. | never | Merge all needed changes
> 6. | [S<] S> * |
> +-----------+-----------------------------------
> 7. | [S>] S< | All needed; & some duplicates in S
> ---------+-----------+-----------------------------------
> 8. [R>] R< | same | All needed; & some duplicates in R
> | | (not a subtree scenario)
> +-----------+-----------------------------------
> 9. | none | All needed; & some duplicates in R
"none" is the same as "never" in the key right?
> 10. | [S<] S> |
> +-----------+-----------------------------------
> 11. | [S>] S< * | All needed; & some duplicates in R and S
> ---------+-----------+-----------------------------------
>
> Key:
> * -- S> not at same revision as R>, or S< not same as R<.
> R> -- last complete Root merge in direction A->B.
> R< -- last complete Root merge in direction B->A.
> never -- never been merged in either direction since the YCA of A and B.
> [S<] -- shorthand for both of the cases: no 'S<' merge and an 'S<' merge.
To clarify, do you mean "shorthand for *EITHER* of these cases: no
'S<' merge *OR* an 'S<' merge." right? Either the last complete merge
of the subtree was in the A->B direction or it was in the B->A
direction.
> duplicates -- changes that are already present in the target.
>
> Example: Row 9 represents the case where the Root's last complete merge was
> in the B->A direction, and its last complete A->B merge was earlier or never;
> and the Subtree likewise. The root's last complete merge was before or after
> but not the same as the subtree's.
But the last subtree in row 9 is listed as "none"? Did you mean row 11?
I'll hold off on further comments/questions until I get the above straight.
--
Paul T. Burba
CollabNet, Inc. -- www.collab.net -- Enterprise Cloud Development
Skype: ptburba
P.S. Home Toronto is treating you right -- I assume the move was
successful since you're back on this thread ;-)
> CATEGORIZING SUBTREE MERGES: 1.7 Reintegrate
>
> These cases are for a reintegrate merge B->A.
>
> Treat this table as just a rough first draft for now; I'm not sure if this is
> the best way to categorize the reintegrate cases, and I need to investigate
> this more thoroughly and test it.
>
> Root | Subtree | Behaviour
> ---------+-----------+-----------------------------------
> 1. never | same | OK (not a subtree scenario)
> +-----------+-----------------------------------
> 2. | [S<] S> | Rejected
> 3. | S< | Rejected??
> 4. | S> S< | Rejected
> ---------+-----------+-----------------------------------
> 5. [R<] R> | same | OK (not a subtree scenario)
> +-----------+-----------------------------------
> 6. | never | Rejected
> 7. | [S<] S> * | Rejected
> 8. | S< | Rejected??
> 9. | S> S< | Rejected
> ---------+-----------+-----------------------------------
> 10. [R>] R< | same | ?? (not a subtree scenario)
> +-----------+-----------------------------------
> 11. | never | Rejected?? )
> 12. | [S<] S> | Rejected > non-reint. cases
> 13. | [S>] S< | Rejected )
> ---------+-----------+-----------------------------------
>
>
> CATEGORIZING SUBTREE MERGES: Ideal Symmetric Merge
>
> Now we consider the ideal symmetric merge (not what's currently implemented).
>
> This is primarily concerned with the last complete merge of the root, in
> whichever direction that was, and similarly the last complete merge of the
> subtree. The (earlier) last complete merge of the root in the other
> direction is not significant, nor is that of the subtree.
>
> Root | Subtree | Ideal behaviour
> ------+---------------+----------------------------------
> 1. | same | OK (not a subtree scenario)
> R> or +---------------+----------------------------------
> 2. never | S> later | Merge all needed changes
> 3. | S< later |
> ------+---------------+
> 4. R> | never |
> 5. | S> earlier |
> 6. | S< earlier |
> ------+---------------+----------------------------------
> 7. | same | OK (not a subtree scenario)
> R< +---------------+----------------------------------
> 8. | S> later | Merge all needed changes
> 9. | S< later |
> 10. | never |
> 11. | S> earlier |
> 12. | S< earlier |
> ------+---------------+----------------------------------
>
> Key:
> R> The last complete merge of Root was in the A->B direction.
> R< The last complete merge of Root was in the B->A direction.
> never Has never been merged between A & B in either direction.
> same Same time and same direction as last complete merge of Root.
> earlier Earlier than the last complete merge of the Root.
> later Later than the last complete merge of the Root.
>
>
> I'm working on integarting these tables, and also on writing tests (or rather
> scenarios using the test suite framework).
>
> Please let me know if this seems like the approach we need, or any other
> thoughts.
>
> - Julian
> --
> Certified & Supported Apache Subversion Downloads:
> http://www.wandisco.com/subversion/download
>
>
>
> I (Julian Foad) wrote:
>> [...] I'm not at all demanding we break backward
>> compatibility. Sorry if it sounded like it. I'm just saying that
>> we're proposing to change the behaviour of the plain merge command,
>> and in doing that we need to work out what the details of the new
>> behaviour will be, and this thread is helping us to do just that.
>> I ended up with a bias towards trying to move toward a more rename-
>> friendly approach, but I recognise we can't get there yet so the
>> "follow each node's own ancestry" idea is just an idea for the
>> future. We need a simpler approach for now.
>>
>> Stefan Sperling wrote:
> [...]
>>> Agreed. Ideally, the symmetric merge will support all currently supported
>>> use cases, without throwing errors at users or requiring new command-line
>>> switches.
>>>
>>> I haven't yet made up my mind about interim measures for 1.8 though.
>>> I suppose if symmetric merge won't support all currently supported use cases
>>> in 1.8, we could keep the --symmetric option in place for 1.8, and drop it
>>> in 1.9 or later once the symmetric merge code can handle all use cases?
