build fails with protobuf 3.8.0

[ollieparanoid]

Alpine Linux edge has just updated their protobuf from 3.6.1 to 3.8.0. postmarketOS builds on top of Alpine edge and it has mir 1.1.2 packaged. Due to the soname change from protobuf, mir needs to be rebuilt. But it does not build against protobuf 3.8.0:

[ 20%] Building CXX object src/server/scene/CMakeFiles/mirscene.dir/broadcasting_session_event_sink.cpp.o
In file included from /home/pmos/build/src/mir-1.1.2/src/server/frontend/session_mediator.cpp:19:
In file included from /home/pmos/build/src/mir-1.1.2/src/server/frontend/session_mediator.h:22:
In file included from /home/pmos/build/src/mir-1.1.2/src/server/frontend/display_server.h:22:
/home/pmos/build/src/mir-1.1.2/src/include/common/mir/protobuf/display_server.h:37:27: error: no type named 'Closure' in namespace 'google::protobuf'
        google::protobuf::Closure* done) = 0;
        ~~~~~~~~~~~~~~~~~~^
/home/pmos/build/src/mir-1.1.2/src/include/common/mir/protobuf/display_server.h:41:27: error: no type named 'Closure' in namespace 'google::protobuf'
        google::protobuf::Closure* done) = 0;
        ~~~~~~~~~~~~~~~~~~^
...

Mir's display_server.h has not been changed since the last two years, so I expect the current version of mir to behave the same way.

I've been reading through the protobuf source, and it seems that google::protobuf::Closure was removed between versions. Unfortunately the git log of protobuf is unreadable and I couldn't find anything useful online. Closure used to be defined in src/google/protobuf/stubs/common.h.

(CC: @z3ntu)

[ollieparanoid]

@MartijnBraam figured out, that the definition of the class has been moved from common.h to callback.h, so with this patch I can get past that particular compilation error:

--- a/src/include/common/mir/protobuf/display_server.h
+++ b/src/include/common/mir/protobuf/display_server.h
@@ -19,6 +19,7 @@
 #ifndef MIR_PROTOBUF_DISPLAY_SERVER_H_
 #define MIR_PROTOBUF_DISPLAY_SERVER_H_
 
+#include <google/protobuf/stubs/callback.h>
 #include "mir_protobuf.pb.h"
 
 namespace mir
   

[ollieparanoid]

...and with similar patching in other files, I can get past all protobuf 3.8 related errors. Here is the full patch file: https://gitlab.com/postmarketOS/pmaports/blob/8f6a331ffd350897c4fc8c13fe65a59520534d2f/unity8/mir/0001-Fix-google-protobuf-closure-related-errors.patch

[wmww]

Thanks @ollieparanoid! Would you mind opening a pull request here on GitHub? That way our CI runs and the team can more easily review and merge it.