Re: [Qemu-arm] [RFC] error: auto propagated local_err

[Kevin Wolf <kwolf@redhat.com>]

Am 19.09.2019 um 14:00 hat Vladimir Sementsov-Ogievskiy geschrieben:
> 19.09.2019 12:17, Kevin Wolf wrote:
> > Am 18.09.2019 um 19:10 hat Eric Blake geschrieben:
> >> On 9/18/19 8:02 AM, Vladimir Sementsov-Ogievskiy wrote:
> >>> + */
> >>> +#define MAKE_ERRP_SAFE(errp) \
> >>> +g_auto(ErrorPropagationStruct) (__auto_errp_prop) = {.errp = (errp)}; \
> >>> +if ((errp) == NULL || *(errp) == error_abort || *(errp) == error_fatal) { \
> >>> +    (errp) = &__auto_errp_prop.local_err; \
> >>> +}
> >>
> >> Not written to take a trailing semicolon in the caller.
> >>
> >> You could even set __auto_errp_prop unconditionally rather than trying
> >> to reuse incoming errp (the difference being that error_propagate() gets
> >> called more frequently).
> > 
> > I think this difference is actually a problem.
> > 
> > When debugging things, I hate error_propagate(). It means that the Error
> > (specifically its fields src/func/line) points to the outermost
> > error_propagate() rather than the place where the error really happened.
> > It also makes error_abort completely useless because at the point where
> > the process gets aborted, the interesting information is already lost.
> > 
> > So I'd really like to restrict the use of error_propagate() to places
> > where it's absolutely necessary. Unless, of course, you can fix these
> > practical problems that error_propagate() causes for debugging.
> > 
> > In fact, in the context of Greg's series, I think we really only need to
> > support hints for error_fatal, which are cases that users are supposed
> > to see. We should exclude error_abort in MAKE_ERRP_SAFE() because these
> > are things that are never supposed to happen. A good stack trace is more
> > important there than adding a hint to the message.
> > 
> 
> Interesting, that to handle error_append_hint problem, we don't need to
> create local_err in case of errp==NULL either..
> 
> So, possibly, we need the following steps:
> 
> 1. implement MAKE_ERRP_SAFE_FOR_HINT (which only leave "*(errp) == error_fatal" in the if condition
> 2. rebase Greg's series on it, to fix hints for fatal errors
> 3. implement MAKE_ERRP_SAFE_FOR_DEREFERENCE (which only leave "(errp) == NULL" in the if condition
> 4. convert all (almost all) local_err usage to use MAKE_ERRP_SAFE_FOR_DEREFERENCE, which will
>     fix problem with error_abort (and also drop a lot of calls of error_propagate)
> 5. merely convert "void func(.., errp)" to "int func(.., errp)" and drop MAKE_ERRP_SAFE_FOR_DEREFERENCE()
>     magic, together with dereferencing.

Long macro names, but as the parameter will always only be "errp", it
fits easily on a line, so this is fine.

I think I like this plan.

Kevin