Re: [GSoC] [PATCH] test: avoid pipes in git related commands for test suite

[Eric Sunshine <sunshine@sunshineco.com>]

On Wed, Mar 14, 2018 at 5:57 AM, Ævar Arnfjörð Bjarmason
<avarab@gmail.com> wrote:
> On Wed, Mar 14 2018, Eric Sunshine jotted:
>> On Tue, Mar 13, 2018 at 4:19 PM, Pratik Karki <predatoramigo@gmail.com> wrote:
>>> -    'git diff-tree -r -M --name-status  HEAD^ HEAD | \
>>> -     grep "^R100..*path0/COPYING..*path2/COPYING" &&
>>> -     git diff-tree -r -M --name-status  HEAD^ HEAD | \
>>> -     grep "^R100..*path0/README..*path2/README"'
>>> +    'git diff-tree -r -M --name-status  HEAD^ HEAD >actual &&
>>> +     grep "^R100..*path0/COPYING..*path2/COPYING" actual &&
>>> +     git diff-tree -r -M --name-status  HEAD^ HEAD >actual &&
>>> +     grep "^R100..*path0/README..*path2/README" actual'
>>
>> Although this "mechanical" transformation is technically correct, it
>> is nevertheless wasteful. The exact same "git diff-tree ..." command
>> is run twice, and both times output is captured to file 'actual',
>> which makes the second invocation superfluous. Instead, a better
>> transformation would be:
>>
>>     git diff-tree ... >actual &&
>>     grep ... actual &&
>>     grep ... actual
>>
> I think we have to be careful to not be overly picky with rejecting
> mechanical transformations that fix bugs on the basis that while we're
> at it the test could also be rewritten.
>
> I.e. this bug was there before, maybe we should purely focus on just
> replacing the harmful pipe pattern that hides errors in this series and
> leave rewriting the actual test logic for a later patch.

Thanks for presenting an opposing opinion. While I understand your
position, the reason for my suggested transformation is that if the
patch already transformed the code in the way suggested, it would
increase my confidence, as a reviewer, that the patch author had
_studied_ and _understood_ the code. Increased confidence is
especially important for mechanical transformations since -- as seen
in the unsnipped review comment below -- blindly-applied mechanical
transformations can be suboptimal or outright incorrect.

It's also the sort of review comment I would make even to very
seasoned project participants[1].

[1]: https://public-inbox.org/git/CAPig+cQLmYQeRhPxvZHmY7gApnbE25H_KoSWs-ZjuBo4BruimQ@mail.gmail.com/

>>> -       test $(git cat-file commit refs/remotes/glob | \
>>> -              grep "^parent " | wc -l) -eq 2
>>> +       test $(git cat-file commit refs/remotes/glob >actual &&
>>> +              grep "^parent " actual | wc -l) -eq 2
>>
>> This is not a great transformation. If "git cat-file" fails, then
>> neither 'grep' nor 'wc' will run, and the result will be as if 'test'
>> was called without an argument before "-eq". For example:
>>
>>     % test $(false >actual && grep "^parent " actual | wc -l) -eq 2
>>     test: -eq: unary operator expected
>>
>> It would be better to run "git cat-file" outside of "test $(...)". For instance:
>>
>>     git cat-file ... >actual &&
>>     test $(grep ... actual | wc -l) -eq 2
>>
>> Alternately, you could take advantage of the test_line_count() helper function:
>>
>>     git cat-file ... >actual &&
>>     grep ... actual >actual2 &&
>>     test_line_count = 2 actual2
>
> In this case though as you rightly point out the rewrite is introducing
> a regression, which should definitely be fixed.