libmirclient.so.8 ABI broken but not incremented in Mir 0.6.0

[Daniel van Vugt]

The libmirclient.so.8 ABI was broken in Mir 0.6.0, kind of indirectly.

mirclient.pc links to:
<=0.5 Libs: -L@LIBDIR@ -lmirclient -lmirprotobuf -lprotobuf
>=0.6 Libs: -L@LIBDIR@ -lmirclient

This means any existing users of libmirclient.so.8 (like /usr/lib/x86_64-linux-gnu/libgdk-3.so.0) also link to libmirprotobuf.so.0. However that file does not exist any more in Mir 0.6.

[Daniel van Vugt (vanvugt)]

The old protobuf package might linger and resolve the unused dependency safely. I'll test it...

[Daniel van Vugt (vanvugt)]

Verified, the gtk-3 in utopic proposed still works without a rebuild after upgrading to libmirclient8 v0.6.0

There are issues with our -dev package dependencies but this bug seems to be benign while the old libmirprotobuf0 package stays installed.

[Daniel van Vugt (vanvugt)]

Oh, even better. Since no symbols were used from libmirprotobuf.so.0, it was actually optimized out by the linker. Once you upgrade to libmirclient.so.8 0.6+ you won't have any indirect linkage to the deprecated library from gdk. And GTK apps continue to work with the existing gtk+3.0 packages.

[Daniel van Vugt (vanvugt)]

All is OK. Our deps are actually trivial, as they should be:

$ objdump -p /usr/lib/x86_64-linux-gnu/libgdk-3.so.0 | grep libmir
  NEEDED libmirclient.so.8