Re: [Qemu-devel] [PATCH v6 1/5] util: Introduce error reporting functions with fatal/abort

[Lluís Vilanova]

Markus Armbruster writes:

> Lluís Vilanova <address@hidden> writes:
>> Markus Armbruster writes:
>> 
>>> Thomas Huth <address@hidden> writes:
>>>> On 03.02.2016 10:48, Markus Armbruster wrote:
>>>>> David Gibson <address@hidden> writes:
>>>>> 
>>>>>> On Tue, Feb 02, 2016 at 10:47:35PM +0100, Thomas Huth wrote:
>>>>>>> On 02.02.2016 19:53, Markus Armbruster wrote:
>>>>>>>> Lluís Vilanova <address@hidden> writes:
>>>>>>> ...
>>>>>>> 
>>>>>>>>> diff --git a/include/qemu/error-report.h b/include/qemu/error-report.h
>>>>>>>>> index 7ab2355..6c2f142 100644
>>>>>>>>> --- a/include/qemu/error-report.h
>>>>>>>>> +++ b/include/qemu/error-report.h
>>>>>>>>> @@ -43,4 +43,23 @@ void error_report(const char *fmt, ...) 
>>>>>>>>> GCC_FMT_ATTR(1, 2);
>>>>>>>>> const char *error_get_progname(void);
>>>>>>>>> extern bool enable_timestamp_msg;
>>>>>>>>> 
>>>>>>>>> +/* Report message and exit with error */
>>>>>>>>> +void QEMU_NORETURN error_vreport_fatal(const char *fmt, va_list ap) 
>>>>>>>>> GCC_FMT_ATTR(1, 0);
>>>>>>>>> +void QEMU_NORETURN error_report_fatal(const char *fmt, ...) 
>>>>>>>>> GCC_FMT_ATTR(1, 2);
>>>>>>>> 
>>>>>>>> This lets people write things like
>>>>>>>> 
>>>>>>>> error_report_fatal("The sky is falling");
>>>>>>>> 
>>>>>>>> instead of
>>>>>>>> 
>>>>>>>> error_report("The sky is falling");
>>>>>>>> exit(1);
>>>>>>>> 
>>>>>>>> or
>>>>>>>> 
>>>>>>>> fprintf(stderr, "The sky is falling\n");
>>>>>>>> exit(1);
>>>>>>>> 
>>>>>>>> I don't think that's an improvement in clarity.
>>>>>>> 
>>>>>>> The problem is not the existing code, but that in a couple of new
>>>>>>> patches, I've now already seen that people are trying to use
>>>>>>> 
>>>>>>> error_setg(&error_fatal, ... );
>>>>>> 
>>>>>> So, I don't actually see any real advantage to error_report_fatal(...)
>>>>>> over error_setg(&error_fatal, ...).
>>>>> 
>>>>> I do.  Compare:
>>>>> 
>>>>> (a) error_report(...);
>>>>> exit(1);
>>>>> 
>>>>> (b) error_report_fatal(...);
>>>>> 
>>>>> (c) error_setg(&error_fatal, ...);
>>>>> 
>>>>> In my opinion, (a) is clearest: even a relatively clueless reader will
>>>>> know what exit(1) does, can guess what error_report() approximately
>>>>> does, and doesn't need to know what it does exactly.  (b) is slightly
>>>>> less obvious, and (c) is positively opaque.
>>>>> 
>>>>> Let's stick to the obvious (a) and be done with it.
>>>> 
>>>> Ok, (a) is fine for me too, as long as we avoid (c). Lluís, could you
>>>> maybe add that information to your patch that updates the HACKING text?
>> 
>>> I feel such detailed advice belings into error.h.  Sketch appended.
>> 
>>> If that doesn't succeed in keeping (c) out, make checkpatch flag it.
>> 
>>>> (and sorry for the fuzz with error_report_fatal() ... I thought it would
>>>> be a good solution to avoid (c), but if (a) is preferred instead, then
>>>> we should go with that solution instead).
>> 
>> I can easily change that, no problem. I'm just happy consensus is landing on
>> this subject.
>> 
>> 
>>>> And, by the way, what about the spots that currently already use
>>>> error_setg(&error_abort, ....) ? Should they be turned into
>>>> error_report() + abort() instead? Or only abort(), without error
>>>> message, since abort() is only about programming errors?
>> 
>>> As I wrote in my first reply to this thread, I'd like them to be cleaned
>>> up to just abort() or assert().
>> 
>>> I like assert(), because it gives me exactly what I can use to debug the
>>> programming error: a core dump (if enabled) and a source location
>>> (useful when no core dump).  I never bought the argument that we should
>>> use abort() instead of assert(0) because "what if NDEBUG?!?".  If you
>>> define NDEBUG, our 600+ abort()s won't save you from our 4000+
>>> assert()s.
>> 
>> Sorry, but I don't buy the argument of, "I prefer assert() because there's
>> already lots of them". To me, there's a semantic difference between debug 
>> builds
>> and regular ones (aka, assert vs abort).

> That's not what I said :)

> In the past, people have argued in favor of abort() by pointing to
> NDEBUG.  I don't buy that argument, but me not buying it is not why I
> prefer assert().  I do because it prints additional information that's
> occasionally useful.

>> Also, I think it adds to the confusion
>> that assert and abort seem to be used interchangeably in the code.

> For better or worse, we overwhelmingly use abort() instead of assert(0),
> but don't use if (!good) abort() instead of assert(good).  Doesn't make
> sense to me, but my appetite for tree-wide changes and the debates that
> go with them has limits.

>> What about this definition?
>> 
>> * exit(): user-triggered errors
>> * abort(): general programming errors
>> * assert(): additional sanity/consistency checks against programming errors
>> 
>> Now, abort & assert have an overlap. Should we discourage one in favour of 
>> the
>> other?

> I can't see how to decide whether a programming error is "general" or
> "additional", or why an "additional" one error deserves a message
> pointing to source code, but a "general" one does not.

>> Also:
>> 
>> * error_report_fatal ensures the same exit code is always used (otherwise it 
>> can
>> fail with inconsistent error codes)

> What if you *want* to use a different exit code?

> But I grant you that we should almost always use exit(1) for fatal
> errors.  And in fact we do!  There are a bunch of misguided exit(-1) in
> the code, but git-log -S'exit(-1)' finds only half a dozen offending
> commits since 2013, and none since 2015, so preventing more seems to be
> a mostly solved problem.

>> * error_report_abort brings the code information of assert into abort

> If you want your crashes to print source location information, don't
> reinvent the wheel, just use assert().

> &error_abort can't because the interesting spot isn't where we decide to
> abort, but where the error got created.

Fair enough. I don't want a flame on style either, although I might look like
wanting one :)


>> But of course, I'm happy either way :)
>> 
>> 
>>> diff --git a/include/qapi/error.h b/include/qapi/error.h
>>> index 45d6c72..ea7e74f 100644
>>> --- a/include/qapi/error.h
>>> +++ b/include/qapi/error.h
>>> @@ -162,6 +162,9 @@ ErrorClass error_get_class(const Error *err);
>>> * human-readable error message is made from printf-style @fmt, ...
>>> * The resulting message should be a single phrase, with no newline or
>>> * trailing punctuation.
>>> + * Please don't error_setg(&error_fatal, ...), use error_report() and
>>> + * exit(), because that's more obvious.
>>> + * Likewise, don't error_setg(&error_abort, ...), use assert().
>>> */
>>> #define error_setg(errp, fmt, ...)                              \
>>> error_setg_internal((errp), __FILE__, __LINE__, __func__,   \
>>> @@ -213,6 +216,8 @@ void error_setg_win32_internal(Error **errp,
>>> * the error object.
>>> * Else, move the error object from @local_err to address@hidden
>>> * On return, @local_err is invalid.
>>> + * Please don't error_propagate(&error_fatal, ...), use
>>> + * error_report_err() and exit(), because that's more obvious.
>>> */
>>> void error_propagate(Error **dst_errp, Error *local_err);
>> 
>>> @@ -291,12 +296,14 @@ void error_set_internal(Error **errp,
>>> GCC_FMT_ATTR(6, 7);
>> 
>>> /*
>>> - * Pass to error_setg() & friends to abort() on error.
>>> + * Special error destination to abort on error.
>>> + * See error_setg() and error_propagate() for details.
>>> */
>>> extern Error *error_abort;
>> 
>>> /*
>>> - * Pass to error_setg() & friends to exit(1) on error.
>>> + * Special error destination to exit(1) on error.
>>> + * See error_setg() and error_propagate() for details.
>>> */
>>> extern Error *error_fatal;
>> 
>> I see, this will make it clearer for people looking for functions without
>> reading HACKING. I can add this and reference it from the document.

> If you like, I can post it as a formal patch you can then include in
> your series.

That'd be great. Please cc me when you send it.

Thanks,
  Lluis