Re: [Qemu-devel] [PATCH 7/7] error: On abort, report where the error was created

[Markus Armbruster]

Eric Blake <address@hidden> writes:

> On 06/22/2015 01:26 PM, Markus Armbruster wrote:
>> This is particularly useful when we abort in error_propagate(),
>> because there the stack backtrace doesn't lead to where the error was
>> created.  Looks like this:
>> 
>>     Unexpected error at /work/armbru/qemu/blockdev.c:322:
>>     qemu-system-x86_64: -drive if=none,werror=foo: 'foo' invalid
>> write error action
>>     Aborted (core dumped)
>>     [Exit 134 (SIGABRT)]
>> 
>> Note: to get this example output, I monkey-patched drive_new() to pass
>> &error_abort to blockdev_init().
>> 
>> To keep the error handling boiler plate from growing even more, all
>> error_setFOO() become macros expanding into error_setFOO_internal()
>> with additional __FILE__, __LINE__ arguments.  Not exactly pretty, but
>> it works.
>
> I agree with Laszlo that adding __func__ to the mix also helps.

Okay, I'll give it a try.

>> The macro trickery breaks down when you take the address of an
>> error_setFOO().  Fortunately, we do that in just one place: qemu-ga's
>> Windows VSS provider and requester DLL wants to call
>> error_setg_win32() through a function pointer "to avoid linking glib
>> to the DLL".  Use error_setg_win32_internal() there.  The use of the
>> function pointer is already wrapped in a macro, so the churn isn't
>> bad.
>> 
>> Code size increases by some 14KiB for me (0.3%).  Tolerable.  Could be
>> less if we passed relative rather than absolute source file names to
>> the compiler.
>
> I also like it.
>
>> +#define error_setg(errp, fmt, ...) \
>> +    error_setg_internal((errp), __FILE__, __LINE__, (fmt), ## __VA_ARGS__)
>> +void error_setg_internal(Error **errp, const char *src, int line,
>> +                         const char *fmt, ...) GCC_FMT_ATTR(4, 5);
>>  
>
>> +#define error_setg_errno(errp, os_error, fmt, ...)                      \
>> +    error_setg_errno_internal((errp), __FILE__, __LINE__, (os_error),   \
>> +                              (fmt), ## __VA_ARGS__)
>
> Nit - why the difference in \ alignment?

I'm dense today... difference between where and where?

> Nit - as used here, 'errp', 'fmt', and 'os_error' can be used
> unambiguously; you don't need '(errp)' given the context of a
> parenthesized comma-separated list (even if someone DID want to unusual
> by passing in '(a,b)' with a comma operator for their 'errp' argument,
> they'd have to supply the () because of the semantics of making the
> macro call).

I put parenthesis around macro parameters in the expansion pretty much
unthinkingly, because thought is expensive :)

> Nit - '## __VA_ARGS__' is a gcc-ism and not portable C99; but I think
> clang supports it, and we don't really care about other compilers at the
> moment. At any rate, we already use it elsewhere in qemu.git.

Correct.  For what it's worth, ./HACKING recommends it.