Re: [PATCH v4 2/3] diff: Let "git diff -O" read orderfile from any file, fail properly

From: Antoine Pelisse <apelisse@gmail.com>
Date: Dec 17, 2013, 20:37 UTC
Message-ID: <CALWbr2zXNF-aJHHnBnW1q1yaCmWt-rmMWypBWFanTBAK1pMWiQ@mail.gmail.com>
In-Reply-To: <xmqqsitrmkhe.fsf@gitster.dls.corp.google.com>

On Tue, Dec 17, 2013 at 6:54 PM, Junio C Hamano <gitster@pobox.com> wrote:

Show 41 quoted lines

> Samuel Bronson <naesten@gmail.com> writes:
>
>> On Mon, Dec 16, 2013 at 4:32 PM, Junio C Hamano <gitster@pobox.com> wrote:
>>> Samuel Bronson <naesten@gmail.com> writes:
>>>
>>>>  for i in 1 2
>>>>  do
>>>>       test_expect_success "orderfile using option ($i)" '
>>>>       git diff -Oorder_file_$i --name-only HEAD^..HEAD >actual &&
>>>>       test_cmp expect_$i actual
>>>>  '
>>>
>>> This funny indentation in the previous step needs to be fixed, and
>>> the added block below should match.
>>
>> Even though this results in oddly-indented --verbose output?
>>
>>>> +     rm -f order_fifo &&
>>>> +     mkfifo order_fifo &&
>>>> +     cat order_file_$i >order_fifo &
>>>> +     git diff -O order_fifo --name-only HEAD^..HEAD >actual &&
>>>
>>> I think this part can be racy depending on which between cat and
>>> "git diff" are scheduled first, no?  Try running this test under
>>> load and I think you will see it deadlocked.
>>>
>>> Besides, the above breaks && chain; even if mkfifo breaks (hence not
>>> allowing cat to run), "git diff" will go ahead and run, no?
>>
>> Hmm.  Well, what I really wanted to put here was a "process substitution":
>>
>>     git diff -O <(cat order_file_$i) --name-only HEAD^..HEAD >actual &&
>>
>> but I did not see this feature listed in the dash(1) manpage, so I
>> assumed it wasn't allowed by POSIX.  And, having looked, I indeed
>> don't see it mentioned in POSIX either.
>>
>> I'm not terribly surprised that I screwed up the translation to FIFOs;
>> how would I really want to do it?
>
> How about not doing a fifo?

That would certainly defeat the purpose of the test, which is to test against a fifo :-) I'm not sure about the deadlock though. Both read and write will wait for each other to start operating on the fifo.

You can probably fix the &&-chain by doing something like:

    mkfifo order_fifo && {
        cat order_file_$i >order_fifo &
        git diff -O order_fifo --name-only HEAD^..HEAD >actual
    } && ...

Also, "rm -f order_fifo" should probably be done in test_when_finished rather than at the beginning of the test.

Antoine,