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Re: [Qemu-devel] [PATCH 1/2] blockjob: assert(cb) in the entry functions

From: Changlong Xie
Subject: Re: [Qemu-devel] [PATCH 1/2] blockjob: assert(cb) in the entry functions of blockjob
Date: Thu, 23 Jun 2016 09:04:08 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0

On 06/23/2016 01:31 AM, Eric Blake wrote:
On 06/22/2016 04:19 AM, Paolo Bonzini wrote:

On 22/06/2016 12:12, Changlong Xie wrote:

commit/mirror/stream/backup use block_job_create(..., cb,..) to create
relevant blockjob. When they finished, these jobs will invoke
block_job_completed, then invoke job->cb() unconditionally. So i think
we need this to avoid segment fault. Actually backup has implemented this.

So this suggests that the right place to put the assertion would be
block_job_create.  But it's even better to add a

#define QEMU_NONNULL __attribute__((__nonnull__))

to include/qemu/compiler.h and declare the arguments as non-null.

Or alternatively fix things to only invoke job->cb() if it is non-NULL,
so that callers don't have to pass in a no-op callback just to appease a
non-NULL attribute.

Is there any reason, that we should invoke job->cb() unconditionally without nonnull check? There is no relevant clue in the historical commit messages. If yes, i prefer paolo's suggestion; otherwise eric's solution is better.


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