Re: [libvirt PATCH] Do not disable incompatible-pointer-types-discards-qualifiers

On Mon, Nov 16, 2020 at 10:48:20AM +0000, Daniel P. Berrangé wrote: >On Mon, Nov 16, 2020 at 11:26:45AM +0100, Michal Privoznik wrote: >> On 11/16/20 10:59 AM, Daniel P. Berrangé wrote: >> > On Mon, Nov 16, 2020 at 10:52:54AM +0100, Martin Kletzander wrote: >> > > On Mon, Nov 16, 2020 at 09:24:22AM +0000, Daniel P. Berrangé wrote: >> > > > On Sun, Nov 15, 2020 at 09:42:40PM +0100, Martin Kletzander wrote: >> > > > > This reverts commit b3710e9a2af402a2b620de570b062294e11190eb. >> > > > > >> > > > > That check is very valuable for our code, but it causes issue with glib >= >> > > > > 2.67.0 when building with clang. >> > > > > >> > > > > The reason is a combination of two commits in glib, firstly fdda405b6b1b which >> > > > > adds a g_atomic_pointer_{set,get} variants that enforce stricter type >> > > > > checking (by removing an extra cast) for compilers that support __typeof__, and >> > > > > commit dce24dc4492d which effectively enabled the new variant of glib's atomic >> > > > > code for clang. This will not be necessary when glib's issue #600 [0] (8 years >> > > > > old) is fixed. Thankfully, MR #1719 [1], which is supposed to deal with this >> > > > > issue was opened 3 weeks ago, so there is a slight sliver of hope. >> > > > > >> > > > > [0] https://gitlab.gnome.org/GNOME/glib/-/issues/600 >> > > > > [1] https://gitlab.gnome.org/GNOME/glib/-/merge_requests/1719 >> > > > > >> > > > > Signed-off-by: Martin Kletzander <mkletzan(a)redhat.com&gt; >> > > > > --- >> > > > > meson.build | 3 --- >> > > > > src/qemu/qemu_domain.c | 7 +++++++ >> > > > > src/util/vireventthread.c | 6 ++++++ >> > > > > src/util/viridentity.c | 6 ++++++ >> > > > > src/util/virobject.c | 6 ++++++ >> > > > > 5 files changed, 25 insertions(+), 3 deletions(-) >> > > > > >> > > > > diff --git a/meson.build b/meson.build >> > > > > index cecaad199d4c..04646e3a078c 100644 >> > > > > --- a/meson.build >> > > > > +++ b/meson.build >> > > > > @@ -405,9 +405,6 @@ cc_flags += [ >> > > > > # so use this Clang-specific arg to keep it quiet >> > > > > '-Wno-typedef-redefinition', >> > > > > >> > > > > - # Clang complains about casts in G_DEFINE_TYPE(...) >> > > > > - '-Wno-incompatible-pointer-types-discards-qualifiers', >> > > > > - >> > > > > # We don't use -Wc++-compat so we have to enable it explicitly >> > > > > '-Wjump-misses-init', >> > > > > >> > > > > diff --git a/src/qemu/qemu_domain.c b/src/qemu/qemu_domain.c >> > > > > index 2158080a56ad..3349476cceae 100644 >> > > > > --- a/src/qemu/qemu_domain.c >> > > > > +++ b/src/qemu/qemu_domain.c >> > > > > @@ -330,7 +330,14 @@ struct _qemuDomainLogContext { >> > > > > virLogManagerPtr manager; >> > > > > }; >> > > > > >> > > > > +#pragma clang diagnostic push >> > > > > +/* Workaround for glib's issue #600 on clang */ >> > > > > +#pragma clang diagnostic ignored "-Wincompatible-pointer-types-discards-qualifiers" >> > > > > + >> > > > > G_DEFINE_TYPE(qemuDomainLogContext, qemu_domain_log_context, G_TYPE_OBJECT); >> > > > > + >> > > > > +#pragma clang diagnostic pop >> > > > >> > > > I really don't want to see this pattern used. We may only have a handful >> > > > of G_DEFINE_TYPE today, but as we convert all 200 virClass over to GObject >> > > > this is going to be way to tedious. This has to be done as a global >> > > > thing IMHO. >> > > > >> > > >> > > Even when the warning we're disabling actually catches errors we make every now >> > > and then? Really? >> > >> > You can disable it globall, but only when compiling against the glib versions >> > affected. We'll still get coverage of the warning flag when building against >> > other versions. >> > >> > > >> > > I know the issue is very old and it only showed up now. And that the MR might >> > > not go through for quite some time. But how about a middle ground like the one >> > > I described in reply to Pavel? Looks like this: >> > > >> > > #define G_DEFINE_TYPE_WITH_WORKAROUND(a, b, c) \ >> > > _Pragma ("clang diagnostic push") \ >> > > _Pragma ("clang diagnostic ignored \"-Wincompatible-pointer-types-discards-qualifiers\"") \ >> > > G_DEFINE_TYPE(a, b, c) \ >> > > _Pragma ("clang diagnostic pop") >> > > >> > > If someone wants to make sure it does not break and we really want to get rid of >> > > this ASAP, then we can even syntax-check for it, describe it and only use it >> > > with glib that has this issue. >> > >> > That can't be done selectively - we would have to use that wrapper macro >> > unconditionally, so it'd be present for years until we bump the min glib >> > to a version that has the fix. >> >> So what about something like this: >> >> #if glib is broken >> # undefine G_DEFINE_TYPE >> # define G_DEFINE_TYPE \ >> # some pragma magic (basically what martin has above) \ >> #endif >> >> This way we can continue using G_DEFINE_TYPE and ditch this redefine once >> glib is updated to fixed version. > >If you can make that work, that'd be ok as it isolates the workaround in >a single place. > OK, thanks Michal for following up because I was kind of misunderstanding the intention. If we #undef the original G_DEFINE_TYPE we would have to open code it, being prone to errors if the original definition changes. If we change it to e.g. VIR_G_DEFINE_TYPE() (and disable G_DEFINE_TYPE usage out of sr/util/glibcompat.c) then we can just do what I wrote above and whenever it is removed it is easy to see what places in the code need changing. I know you are against that, but I don't see how different that is compared to, for example vir_g_canonicalize_filename(). And other functions in gcompat.h also, although they don't even have the version check.

>Regards, >Daniel >-- >|: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| >|: https://libvirt.org -o- https://fstop138.berrange.com :| >|: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|