Re: [PATCH 06/19] block/stream: fix -Werror=maybe-uninitialized false-positives

[Vladimir Sementsov-Ogievskiy]

On 02.04.24 18:34, Eric Blake wrote:
> On Tue, Apr 02, 2024 at 12:58:43PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> > > > Again, same false-positives, because of WITH_GRAPH_RDLOCK_GUARD()..
> > > >
> > > > Didn't you try to change WITH_ macros somehow, so that compiler believe in our 
> > > > good intentions?
> > >
> > >
> > > #define WITH_QEMU_LOCK_GUARD_(x, var) \
> > >       for (g_autoptr(QemuLockable) var = \
> > >                   qemu_lockable_auto_lock(QEMU_MAKE_LOCKABLE_NONNULL((x))); \
> > >            var; \
> > >            qemu_lockable_auto_unlock(var), var = NULL)
> > >
> > > I can't think of a clever way to rewrite this. The compiler probably
> > > thinks the loop may not run, due to the "var" condition. But how to
> > > convince it otherwise? it's hard to introduce another variable too..
> >
> >
> > hmm. maybe like this?
> >
> > #define WITH_QEMU_LOCK_GUARD_(x, var) \
> >      for (g_autoptr(QemuLockable) var = \
> >                  qemu_lockable_auto_lock(QEMU_MAKE_LOCKABLE_NONNULL((x))), \
> >           var2 = (void *)(true); \
> >           var2; \
> >           qemu_lockable_auto_unlock(var), var2 = NULL)
> >
> >
> > probably, it would be simpler for compiler to understand the logic this way. 
> > Could you check?
>
> Wouldn't that attach __attribute__((cleanup(xxx))) to var2, at which
> point we could cause the compiler to call xxx((void*)(true)) if the
> user does an early return inside the lock guard, with disastrous
> consequences?  Or is the __attribute__ applied only to the first out
> of two declarations in a list?

Oh, most probably you are right, seems g_autoptr apply it to both variables. 
Also, we don't need qemu_lockable_auto_unlock(var) separate call, if we 
zero-out another variable. So, me fixing:

#define WITH_QEMU_LOCK_GUARD_(x, var) \
    for (QemuLockable *var __attribute__((cleanup(qemu_lockable_auto_unlock))) 
= qemu_lockable_auto_lock(QEMU_MAKE_LOCKABLE_NONNULL((x))), \
         *var2 = (void *)(true); \
         var2; \
         var2 = NULL)

(and we'll need to modify qemu_lockable_auto_unlock() to take "QemuLockable 
**x" argument)

--
Best regards,
Vladimir