Jeff King <> writes:

> On Thu, Aug 08, 2013 at 08:31:44PM +0200, Stefan Beller wrote:
>> The next occurrences are at:
>>      /* Never use a non-valid filename anywhere if at all possible */
>>      name_a = DIFF_FILE_VALID(one) ? name_a : name_b;
>>      name_b = DIFF_FILE_VALID(two) ? name_b : name_a;
>>      a_one = quote_two(a_prefix, name_a + (*name_a == '/'));
>>      b_two = quote_two(b_prefix, name_b + (*name_b == '/'));
>> In the last line of this block 'name_b' is dereferenced and compared
>> to '/'. This would crash if name_b was NULL. Hence in the following code
>> we can assume name_b being non-null.
> I think your change is correct, but I find the reasoning above a little
> suspect. It assumes that the second chunk of code (accessing name_a and
> name_b) is correct, and pins the correctness of the code you are
> changing to it. If the second chunk is buggy, then you are actually
> making the code worse.

True.  I think the original code structure design is name_a should
always exist but name_b may not (the caller of run_diff_cmd() that
eventually calls this call these "name" and "other", and the intent
is renaming filepair is what needs "other").

> I wonder if the implicit expectation of the function to take at least
> one non-NULL name would be more obvious if the first few lines were
> written as:
>   if (DIFF_FILE_VALID(one)) {
>           if (!DIFF_FILE_VALID(two))
>                   name_b = name_a;
>   } else if (DIFF_FILE_VALID(two))
>           name_a = name_b;
>   else
>           die("BUG: two invalid files to diff");
> That covers all of the cases explicitly, though it is IMHO uglier to
> read (and there is still an implicit assumption that the name is
> non-NULL if DIFF_FILE_VALID() is true).

I think that is an overall improvement, especially if we also update
the checks of {one,two}->mode made for the block that deals with
submodules to use DIFF_FILE_VALID().

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