Re: [PATCH v5 1/5] setup: add gentle version of read_gitfile

From: Jonathan Nieder <jrnieder@gmail.com>
Date: Apr 28, 2015, 21:06 UTC
Message-ID: <20150428210647.GP5467@google.com>
In-Reply-To: <20150428204853.GA12153@peff.net>

Jeff King wrote:

> On Tue, Apr 28, 2015 at 01:42:13PM -0700, Jonathan Nieder wrote:
>> Jeff King wrote:

Show 10 quoted lines

>>> But the NULL does not carry the information about _which_ error, and
>>> Erik is suggesting that the caller may need to change behavior based on
>>> that information. IOW, his current patch (return NULL and set the
>>> specific integer code in a variable) allows this, but switching the
>>> integer code out for a human-readable strbuf does not.
>>
>> Right.  Two ways to handle that are:
>
> Sure, but "this system" that I was referring to one was not one of
> those ways. :)

That means I wasn't communicating well.

I never advocated *not* providing an integer return code when you want to affect control flow.

An example of a function that provides information both to the caller and user and doesn't conflate the two is ref_transaction_commit. It returns TRANSACTION_NAME_CONFLICT for a certain kind of recoverable error and TRANSACTION_GENERIC_ERROR for all other errors. In either case, the 'err' parameter is populated with information intended for the user.

Sometimes you might want to affect control flow without providing information for the user. Then there's no need for a string output parameter and you can just provide the int.

At other times you might want to provide information for the user without affecting control flow. Then you can provide a string without the int.

Providing error codes when you also want to return a string or struct pointer is an unrelated problem. One runs into that problem even if only intending to provide a return code. Possible approaches, like mentioned before, are

 - an int * parameter to pass back the return code
 - using ERR_PTR to embed error codes in the pointer space

There is another approach which is sometimes nicer:

 - return int and pass the string or struct pointer through an output
   paramter

Thanks and sorry for the lack of clarity, Jonathan