Ceph

gcc 14 has introduced a change which results in ceph build failures:

[  270s] /home/abuild/rpmbuild/BUILD/ceph-18.0.0-4135-g87cd54281c8/src/osd/osd_types.h: In lambda function:
[  270s] /home/abuild/rpmbuild/BUILD/ceph-18.0.0-4135-g87cd54281c8/src/common/dout.h:184:73: error: call to non-'constexpr' function 'virtual unsigned int DoutPrefixProvider::get_subsys() const'
[  270s]   184 |     dout_impl(pdpp->get_cct(), ceph::dout::need_dynamic(pdpp->get_subsys()), v) \
[  270s]       |                                                         ~~~~~~~~~~~~~~~~^~
[  270s] /home/abuild/rpmbuild/BUILD/ceph-18.0.0-4135-g87cd54281c8/src/common/dout.h:155:58: note: in definition of macro 'dout_impl'
[  270s]   155 |       return (cctX->_conf->subsys.template should_gather<sub, v>());    \
[  270s]       |                                                          ^~~
[  270s] /home/abuild/rpmbuild/BUILD/ceph-18.0.0-4135-g87cd54281c8/src/osd/osd_types.h:3618:3: note: in expansion of macro 'ldpp_dout'
[  270s]  3618 |   ldpp_dout(dpp, 10) << "build_prior all_probe " << all_probe << dendl;
[  270s]       |   ^~~~~~~~~
[  270s] /home/abuild/rpmbuild/BUILD/ceph-18.0.0-4135-g87cd54281c8/src/common/dout.h:51:20: note: 'virtual unsigned int DoutPrefixProvider::get_subsys() const' declared here
[  270s]    51 |   virtual unsigned get_subsys() const = 0;
[  270s]       |                    ^~~~~~~~~~

The gcc change is described at https://gcc.gnu.org/pipermail/gcc-patches/2023-May/617196.html.

The ceph FTBFS was mentioned in a followup post at https://gcc.gnu.org/pipermail/gcc-patches/2023-May/618384.html, and apparently this failure is now expected, as DoutPrefixProvider::get_subsys() isn't declared constexpr but really should be.

I tried to fix this experimentally by simply declaring constexpr get_subsys(), e.g.:

diff --git a/src/common/dout.h b/src/common/dout.h
index a1375fbb910..6e91750708a 100644
--- a/src/common/dout.h
+++ b/src/common/dout.h
@@ -61,7 +61,7 @@ class NoDoutPrefix : public DoutPrefixProvider {

   std::ostream& gen_prefix(std::ostream& out) const override { return out; }
   CephContext *get_cct() const override { return cct; }
-  unsigned get_subsys() const override { return subsys; }
+  constexpr unsigned get_subsys() const override { return subsys; }
 };

 // a prefix provider with static (const char*) prefix
@@ -88,7 +88,7 @@ class DoutPrefixPipe : public DoutPrefixProvider {
     return out;
   }
   CephContext *get_cct() const override { return dpp.get_cct(); }
-  unsigned get_subsys() const override { return dpp.get_subsys(); }
+  constexpr unsigned get_subsys() const override { return dpp.get_subsys(); }

   virtual void add_prefix(std::ostream& out) const = 0;
 };

...but that has some problems:

1) Instead of an outright build failure, I get warning: virtual functions cannot be 'constexpr' before C++20 [-Winvalid-constexpr]. I imaging this is undesirable.
2) Even if 1 is desirable, there's plenty of other subclasses of DoutPrefixProvider which would all also need to have their get_subsys() methods declared conxtexpr for the build to complete.

TBH the whole dout thing is black magic to me, so I could really use some assistance with how best to fix this.