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

[Eric Blake <eblake@redhat.com>]

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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.

> 
> 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?

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).

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.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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