Re: [Qemu-devel] [PATCH v2] mirror: drop local_err in mirror_complete

[Max Reitz <mreitz@redhat.com>]

On 2013-10-19 10:05, Kevin Wolf wrote:
> Am 18.10.2013 um 19:59 hat Max Reitz geschrieben:
>> Then there's still the problem that I'm not the one who introduced
>> error_propagate. Someone obviously intended some purpose for it, so
>> even if it doesn't make a difference now (and my RFC is unneeded),
>> I'd still use it to propagate errors (instead of passing the error
>> pointer). My point being that there *is* a function for propagating
>> errors and I think we should therefore use it. The current qemu code
>> seems to alternate between the two alternatives, although using
>> error_propagate seems more common to me (at least, that was the
>> result when I looked through the code trying to decide whether to
>> use it or not).
>>
>> Generally, we need a proper discussion about whether error_propagate
>> is obsolete or not. Afterwards, we can adapt the current codebase to
>> the result of that discussion; but until then, I oppose changing
>> existing code without actual need to do so but personal preference.
>>
>> Max
>>
>>
>> PS: I wrote my error_propagate RFC in part because I was
>> disappointed about how much of a no-op error_propagate actually is
>> and was trying to change that. ;-)
> It's not a no-op.

That's why I wrote “how much of a” instead of “that error_propagate is a 
no-op”. ;-)

> The point is that the caller can pass NULL as errp if
> it isn't interested in the error message. If you do anything else with
> errp than just passing it to other functions - most commonly a
> error_is_set() check - you need to make sure that you use a local Error
> variable and propagate it. Otherwise, if the caller passed NULL, your
> error path would never get executed.

Okay, together with Paolo's comment I believe I now can see the meaning 
of error_propagate and its intended use; thank you both for explaining.

Max