Gated Merge?

Gated Merge?

[Junio C Hamano]

I wish there were a way to let me annotate a commit with a statement
like this:

 * When merging a branch that does not have me in it into a trunk
   that already has me, do this extra check on the result of the
   merge before allowing it to be recorded.

For example, suppose there is a topic that changes all instances of
this

	__attribute__((format(printf, X, Y)))

into a new macro FORMAT_PRINTF(X, Y).  With such a facility, I can
annotate the commit at the tip of that series to define an extra
check:

   ! git diff HEAD | grep '^+.*__attribute__((format(printf'

which would prevent me from merging a topic that introduces a new
instance of the printf-format attribute that is spelled in an
old-fashioned way.

In this particular example, it might be even better if the facility
allowed me to declare something like this instead:

 * When merging a branch that does not have me in it into a trunk
   that already has me, do this extra processing on the result of
   the merge before recording it.

The difference between them is the "extra processing" part.  For
this example, it would be something that is built around:

    perl -p -e '
        s{__attribute__\(\(format\(\s*printf,\s*(\d+),\s*(\d+)\)\)\)}
         {FORMAT_PRINTF($1, $2)}gx
    '

to update the old-fashioned one to use the new macro, and the result
would become an evil merge.

To realize this, there are low-hanging-fruit building blocks that
are trivial:

 - Such an annotation can be made as a note attached to the commit
   in question;

 - Such a check can be done in pre-commit hook;

 - Such a pre-commit hook can iterate over this set of commits

    DIFF=$(git rev-list --left-right HEAD...MERGE_HEAD)
    
   collect these Merge Gate annotations from the commit appears on
   the left hand side (e.g. only exists on the trunk that a side
   branch is about to be merged in), and run them.

But the last one feels very costly, and I suspect that there must be
a better way to do this.  I do not think we want the "what to do"
part (i.e. checking new lines for __attribute__ in the first
example, or rewriting new lines that has __attribute_ in the second
example) to be in git-core; that clearly is outside the scope of the
plumbing.  But the part that finds these "annotations", especially
if we can come up with a fast implementation that may be hard to
script, may be a good addition to the plumbing command set.

Thoughts?

Re: Gated Merge?

[Andrew Ardill]

On 12 February 2016 at 09:06, Junio C Hamano <gitster@pobox.com> wrote:
>
> To realize this, there are low-hanging-fruit building blocks that
> are trivial:
>
>  - Such an annotation can be made as a note attached to the commit
>    in question;
>
>  - Such a check can be done in pre-commit hook;
>
>  - Such a pre-commit hook can iterate over this set of commits
>
>     DIFF=$(git rev-list --left-right HEAD...MERGE_HEAD)
>
>    collect these Merge Gate annotations from the commit appears on
>    the left hand side (e.g. only exists on the trunk that a side
>    branch is about to be merged in), and run them.
>
> But the last one feels very costly, and I suspect that there must be
> a better way to do this.  I do not think we want the "what to do"
> part (i.e. checking new lines for __attribute__ in the first
> example, or rewriting new lines that has __attribute_ in the second
> example) to be in git-core; that clearly is outside the scope of the
> plumbing.  But the part that finds these "annotations", especially
> if we can come up with a fast implementation that may be hard to
> script, may be a good addition to the plumbing command set.
>
> Thoughts?

What is the benefit in doing this in notes vs having the tests in the
working tree?

Pros:

 - merge-gates can be added after the commit, but will stick with the
commit if it moves around (as opposed to creating a second commit to
add the merge-gate to the working tree)

 - cross repository standards can be established that allow
merge-gates to be detected and run automatically (arguably could be
done with a standardised folder structure too, but that is more
disruptive)

 - can view the relevant merge-gates in git log against each commit
that they are protecting

Cons:

 - difficult to see the current complete set of merge-gates

 - difficult to make changes to a number of merge-gates at the same time

 - poorly defined behaviour when multiple merge-gates overlap in
functionality. Which gates execute first? What if I reorder the
commits?


My practical knowledge of notes is severely lacking so excuse me if I
missed anything obvious.

Regards,

Andrew Ardill

(sorry for the double post Junio, gmail ate my plain text encoding at
some point...)

Re: Gated Merge?

[Stefan Beller]

On Thu, Feb 11, 2016 at 2:42 PM, Andrew Ardill <andrew.ardill@gmail.com> wrote:
> On 12 February 2016 at 09:06, Junio C Hamano <gitster@pobox.com> wrote:
>>
>> To realize this, there are low-hanging-fruit building blocks that
>> are trivial:
>>
>>  - Such an annotation can be made as a note attached to the commit
>>    in question;
>>
>>  - Such a check can be done in pre-commit hook;
>>
>>  - Such a pre-commit hook can iterate over this set of commits
>>
>>     DIFF=$(git rev-list --left-right HEAD...MERGE_HEAD)
>>
>>    collect these Merge Gate annotations from the commit appears on
>>    the left hand side (e.g. only exists on the trunk that a side
>>    branch is about to be merged in), and run them.
>>
>> But the last one feels very costly, and I suspect that there must be
>> a better way to do this.  I do not think we want the "what to do"
>> part (i.e. checking new lines for __attribute__ in the first
>> example, or rewriting new lines that has __attribute_ in the second
>> example) to be in git-core; that clearly is outside the scope of the
>> plumbing.  But the part that finds these "annotations", especially
>> if we can come up with a fast implementation that may be hard to
>> script, may be a good addition to the plumbing command set.
>>
>> Thoughts?
>
> What is the benefit in doing this in notes vs having the tests in the
> working tree?
>
> Pros:
>
>  - merge-gates can be added after the commit, but will stick with the
> commit if it moves around (as opposed to creating a second commit to
> add the merge-gate to the working tree)
>
>  - cross repository standards can be established that allow
> merge-gates to be detected and run automatically (arguably could be
> done with a standardised folder structure too, but that is more
> disruptive)
>
>  - can view the relevant merge-gates in git log against each commit
> that they are protecting
>
> Cons:
>
>  - difficult to see the current complete set of merge-gates
>
>  - difficult to make changes to a number of merge-gates at the same time
>
>  - poorly defined behaviour when multiple merge-gates overlap in
> functionality. Which gates execute first? What if I reorder the
> commits?

The way I could imagine "merge gates" currently:

 * they are some kind of invariant to be checked, or an action
performed, an event
 * they can be introduced by a commit X.
 * they can be turned off by a commit Y
 * an offspring commit inherits all "merge gates" from its parents (in
order of the parents

So the event for one merge gate is in effect iff you merge one commit from X..Y

Now we want to know for a given commit C, which merge gates are there?
To find all events we would need to walk the whole history of C and record
all found events, which are not turned off.

So I'd think that could be accelerated with event bitmaps (which work
similar to the
reachability bitmaps, but having a different goal and record which
events are alive
for certain commits, such that you only need to walk a bounded set of commits to
find out.


When first reading this email I thought this is a typical maintainers problem,
so it should happen all the time in larger projects, such as linux, how do they
handle system wide refactorings?

>
>
> My practical knowledge of notes is severely lacking so excuse me if I
> missed anything obvious.
>
> Regards,
>
> Andrew Ardill
>
> (sorry for the double post Junio, gmail ate my plain text encoding at
> some point...)
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Re: Gated Merge?

[Junio C Hamano]

Andrew Ardill <andrew.ardill@gmail.com> writes:

> What is the benefit in doing this in notes vs having the tests in the
> working tree?

Interesting.  I have never thought of adding this information to the
project history proper---I've viewed this as primarily an aid for
keeping track of topics in-flight by an individual, i.e. something
that the rest of the project do not want to even see.

> Pros:
>
>  - merge-gates can be added after the commit, but will stick with the
> commit if it moves around (as opposed to creating a second commit to
> add the merge-gate to the working tree)

I think that this will not be a problem in practice if we took your
alternative approach to cap the topic with an extra commit that adds
the Merge Gate test; because the workflow this targets treat a topic
as a single unit, the extra commit will be rebased together with the
real commits in the topic.

>  - cross repository standards can be established that allow
> merge-gates to be detected and run automatically (arguably could be
> done with a standardised folder structure too, but that is more
> disruptive)

Yeah, I think that this is very similar to "something that the rest
of the project do not want to even see" problem, if you use an in-tree
approach.

> Cons:
>
>  - difficult to see the current complete set of merge-gates

Yes, we would need to have a quick way to enumerate commits with
these notes in the rev-list output.

>  - difficult to make changes to a number of merge-gates at the same time

Hmm, I am not sure if/how that will be an issue.

>  - poorly defined behaviour when multiple merge-gates overlap in
> functionality. Which gates execute first? What if I reorder the
> commits?

True. With an in-tree approach, you can define the order with the
filenames, for example, and the above will be clearer. Instead, you
would need to worry about name clashes, though.

Re: Gated Merge?

[Jeff King]

On Fri, Feb 12, 2016 at 09:44:13AM -0800, Junio C Hamano wrote:

> Andrew Ardill <andrew.ardill@gmail.com> writes:
> 
> > What is the benefit in doing this in notes vs having the tests in the
> > working tree?
> 
> Interesting.  I have never thought of adding this information to the
> project history proper---I've viewed this as primarily an aid for
> keeping track of topics in-flight by an individual, i.e. something
> that the rest of the project do not want to even see.

After reading Andrew's message, I wondered if these are really any
different than regular tests at all.

Let's say I implement feature X on a topic branch, and I add a
regression test for it. Once that is merged, now we have that regression
test forever[1], and any future topic branches that get merged from
master must pass that test or be rejected.

If I update the interface for foo(), it is the same thing. We do not
write a specific test for it, but we expect that the compiler will catch
any callers of the old foo, because the new tree carries the updated
definition that will not work with them (and we often structure our
interface refactoring exactly to catch such things).

So let's go back to your FORMAT_PRINT example. The topic changes all of
the format-attributes into FORMAT_PRINTF, but we never add a "test" that
says "now there are no more bare format-attributes, and if there are, it
is a regression".

But I don't think this needs to have anything to do with merges in
particular, or rules like "when merging a branch that does not have me
in it". It is about saying "from here on out, the tree state should
match this property, and we can test it by running this script". And
then running "make code-lint-tests" becomes part of the acceptance
testing for a proposed topic merge, just like "make test" is already
(and likewise, people forking _new_ branches from master after the topic
is merged would make sure they do not fail the code-lint tests before
even submitting the topic).

-Peff

[1] Of course it doesn't _have_ to be forever. We sometimes modify or
    back out tests as new situations come up, and we could do the same for
    these sorts of code-lint tests.

Re: Gated Merge?

[Junio C Hamano]

Jeff King <peff@peff.net> writes:

> But I don't think this needs to have anything to do with merges in
> particular, or rules like "when merging a branch that does not have me
> in it". It is about saying "from here on out, the tree state should
> match this property, and we can test it by running this script". And
> then running "make code-lint-tests" becomes part of the acceptance
> testing for a proposed topic merge, just like "make test" is already
> (and likewise, people forking _new_ branches from master after the topic
> is merged would make sure they do not fail the code-lint tests before
> even submitting the topic).

That certainly is true, but this strays more and more from my
original motive of implementing an automated evil-merge scheme that
is better than what I currently have.

We try to do our tree-wide refactoring in such a way that it would
break the compilation (by changing function signature) when we can.
Catching with "make test" would certainly generalize it, but the
endgame result I was shooting for was to come up with a solution for
each topic in-flight just once and keep replaying it until it is
merged.

Re: Gated Merge?

[Jeff King]

On Fri, Feb 12, 2016 at 11:06:04AM -0800, Junio C Hamano wrote:

> Jeff King <peff@peff.net> writes:
> 
> > But I don't think this needs to have anything to do with merges in
> > particular, or rules like "when merging a branch that does not have me
> > in it". It is about saying "from here on out, the tree state should
> > match this property, and we can test it by running this script". And
> > then running "make code-lint-tests" becomes part of the acceptance
> > testing for a proposed topic merge, just like "make test" is already
> > (and likewise, people forking _new_ branches from master after the topic
> > is merged would make sure they do not fail the code-lint tests before
> > even submitting the topic).
> 
> That certainly is true, but this strays more and more from my
> original motive of implementing an automated evil-merge scheme that
> is better than what I currently have.
> 
> We try to do our tree-wide refactoring in such a way that it would
> break the compilation (by changing function signature) when we can.
> Catching with "make test" would certainly generalize it, but the
> endgame result I was shooting for was to come up with a solution for
> each topic in-flight just once and keep replaying it until it is
> merged.

Yeah, that makes sense. I do repeated merges myself, and it is a pain.
I do not usually use your Reintegrate scripts, though when I have done
so, they work pretty well. And I don't think you're going to come up
with anything much better for the general case.

Let's split the problem into "detect a problem" from "apply the fix for
a topic".

You do not want to stop detecting once the original topic is merged. The
new rule is "we spell this as FORMAT_PRINTF" and you want to detect it
in simultaneous topics, _and_ in future topics. So if it is worth
checking in an auomated way, that should go into the tree.

So once we've detected a problem, how do we fix it?

For the specific case of FORMAT_PRINTF, you showed a possible "extra
processing" snippet to fix the problem in an automated way. But if we
realize that these gated attributes are really just another form of
test, it should be obvious that we cannot do so automatically in the
general case. You do not expect to fix a failed "make test" on a merge
in an automated way; there are too many possible resolutions. The best
we can do is detect the problem, create a patch, and then apply that
patch as appropriate.

So I'm somewhat doubtful that it would be worth the effort to create
infrastructure for "automate this fix" that is anything except "apply
this patch and tell me if we now pass the test".

-Peff