Re: thoughts on error passing, was Re: [PATCH 2/2] fsck: handle bad trees like other errors

[Michael Haggerty <mhagger@alum.mit.edu>]

On 09/27/2016 09:19 PM, Jeff King wrote:
> [...]
> I'm going to ramble for a minute, and I don't think it's worth exploring
> for this patch series in particular, so feel free to ignore me.
> 
> I think this error concept could be extended fairly elegantly with
> something like:
> 
>   typedef void (*err_fn)(void *, const char *fmt, va_list ap)
>   struct error_context {
>         err_fn fn;
>         void *data;
>   };
> 
>   int report_error(struct error_context *err, const char *fmt, ...)
>   {
>         if (err->fn) {
>                 va_list ap;
>                 va_start(ap, fmt);
>                 err->fn(err->data, fmt, ap);
>                 va_end(ap);
>         }
>         return -1;
>   }

I like this idea. It's nicely flexible (more so than the `struct strbuf
*err` that is currently used for reference transactions) without being
cumbersome.

> Then low-level functions just take a context and do:
> 
>   return report_error(&err, "some error: %s", foo);
> 
> And then the callers would pick one of a few generic error contexts:
> 
>   - passing NULL silences the errors
> 
>   - a global for chaining to error, like:
> 
>        struct error_context print_errors = {
>           error, /* actually a wrapper to handle va_list and NULL data */
>           NULL
>        };

There could also be a global for chaining to `warn()` or `die()`.

> [...]

Michael