[libvirt] [PATCH] util: return generic error in virCopyLastError if error is not set

[Erik Skultety]

On Thu, May 17, 2018 at 01:42:36PM +0300, Nikolay Shirokovskiy wrote:
>
>
> On 17.05.2018 13:11, Nikolay Shirokovskiy wrote:
> > virCopyLastError is intended to be used after last error is set.
> > However due to virLastErrorObject failures (very unlikely through
> > as thread local error is allocated on first use) we can have zero
> > fields in a copy as a result. In particular code field can be set
> > to VIR_ERR_OK.
> >
> > In some places (qemu monitor, qemu agent and qemu migaration code
> > for example) we use copy result as a flag and this leads to bugs.
> >
> > Let's set copy result to generic error ("cause unknown") in this case.
> > Approach is based on John Ferlan comments in [1] and [2] to initial
> > attempt which fixes issue in much less generic way.
> >
> > [1] https://www.redhat.com/archives/libvir-list/2018-April/msg02700.html
> > [2] https://www.redhat.com/archives/libvir-list/2018-May/msg00124.html
> > ---
> >  src/util/virerror.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/src/util/virerror.c b/src/util/virerror.c
> > index c000b00..9f158af 100644
> > --- a/src/util/virerror.c
> > +++ b/src/util/virerror.c
> > @@ -343,7 +343,7 @@ virCopyLastError(virErrorPtr to)
> >      if (err)
> >          virCopyError(err, to);
> >      else
> > -        virResetError(to);
> > +        virErrorGenericFailure(err);
>
> virErrorGenericFailure(to) of course

Yeah, I just prepared a mail to respond to the original patch, nevermind :)...
Although I still don't see how this solves anything. First of all, the whole
else branch is useless, since we "memset'd" the caller-passed object right
before we could potentially hit the 'else' branch. As you've noted in the
commit message, virLastErrorObject() can only fail with NULL on OOM, yet one of
the things virErrorGenericFailure will try to do is strdup(), which I highly
doubt would succeed after a previous OOM, IOW if libvirt encounters OOM it's
pretty much stuck in a loop of failed attempts to report an OOM error
encountering some more. If you really want to return something, I'd say we
should return VIR_ERR_NO_MEMORY instead, probably checking the validity of the
caller-passed object, since we just blindly trust it's been allocated properly
and access it right away.

Erik