Re: clarification of `rev-list --no-walk ^<rev>`?

From: Philip Oakley <philipoakley@iee.org>
Date: Sep 19, 2016, 19:31 UTC
Message-ID: <989F47918A374EEF8C7FECD3CFC6767E@PhilipOakley>
In-Reply-To: <xmqq8tunhns4.fsf@gitster.mtv.corp.google.com>

From: "Junio C Hamano" <gitster@pobox.com>

Show 28 quoted lines

> Michael J Gruber <git@drmicha.warpmail.net> writes:
>
>>> It can be read that
>>>
>>> $ git cherry-pick maint next
>>>
>>> would pick two single commits, while
>>>
>>> $ git cherry-pick maint next ^master
>>>
>>> could implicitly be read as
>>>
>>> $ git cherry-pick maint next --do-walk ^master
>
> You can read it as "master..next maint" that does force walking.
>
>>> Clearly that's not what is intended, which is
>>>
>>> $ git cherry-pick --do-walk maint next ^master
>
> I do not see the distinction betwee the above two you seem to be
> trying to make.  Care to explain?
>
>>> but it is open to interpretation as to where in the command line the 
>>> caret
>>> range prefix's --do-walk (to countermand the --no-walk) should applied.
>
> I do not think it can be position dependent.

OK. (background, long story) When I first read the man page, and in trying to explain a user confusion between cherry pick commits and am'ing the same commits via format-patch (where sometimes the patches had coverlapping context issues), I was trying to confirm for myself that 'git cherry-pick B C' and 'git cherry-pick C B' should get the same end result, and not be mistaken (e.g. user misunderstanding) for a range.

I first spotted the 'git help cherry-pick's line:

<commit> - no traversal is done by default, as if the --no-walk option was specified, see git-rev-list(1).

So off I goes to rev-list, and see that the options no-walk/do-walk are said (implied) to be position dependent.

--do-walk  - Overrides a previous --no-walk.

Meanwhile the --no-walk option says:

 --no-walk - This (option) has no effect if a range is specified.

So at this point I am wondering about the command line ordering and what comprises the range if the negative ref is given last, or at least just before a --do-walk.

Thus (at this point) it felt like one of those specification rabbit-holes that I often see at $dayjob. It was unclear as to the point at which the 'range' was to be applied in to the command line to get the expected examples.

Climbing back out of the rabbit-hole. I now see that after the end of the cherry-pick <commit> line I quoted, there is a secondary "Note that specifying a range will feed all <commit>. arguments to a single revision walk", which at the time did not register (a classic human error.. Three Mile Island et al.).

Similarly, in the rev-list --no-walk option, its says (mid-para) "This has no effect if a range is specified." so in some ways that confirms the cherry pick statement, but again a less obvious corollary.

However, there is still a small step missing, which is to confirm that using a negative ^ref anywhere(?) makes the whole list of refs into a range (i.e. it will look-back along the command line) to cancel any --no-walk options in place.

Given that a walk usually requires a range, I'm now having difficulty seeing how the --no-walk <revs> --do-walk can be combined anyway.

The bits I felt was missing (in the docs) was to say explicitly somewhere that a negative ref defined that we had a range (to link back to those walk-no-walk statements), and the extent of rev paramaters it applied to.

And after re-reading, that some of those corollary statements about ranges flipping the walk-no-walk condition should be brought forward to be more obvious within the primary rev-list (to avoid the typical reader error).

>     Philip probably has a
> confused notion that "rev-list A..B C..D" is somehow a union of set
> A..B and C..D?

That wasn't the issue. Though it does beg the question that it's the same as "rev-list D B ^A ^C" isn't it?

Show 10 quoted lines

>
>>> If the user did want just the single commit at the tip of maint, and 
>>> then
>>> the range master..next, what would be their command line, and also, how
>>> would the man page warn against false expectations?
>
> Yeah, this can show us that all of the have is coming from that
> exact confusion I suspected Philip has.  We need to clarify in the
> documentation that rev-list set operation does *NOT* have union of
> multiple sets to unconfuse the readers.

I'd say it was the walk - no walk range confusion. Inclusion of any range definition of any sort (in particular ^rev) causes the expectation that an ordered list of single revs can be included, to be broken. i.e. cherry-pick B D F G Q..T; isn't B D F G R S T, is it?

I've also have a quick browse of the test scripts and didn't see any tests that actually cover the example of `git cherry-pick maint next ^master` where both have multiple commits to pick, so couldn't see what the test would expect.

-- Philip