build: isolate symbols of Python extensions on Linux

[alalek]

to avoid conflicts with other loaded libprotobuf.so libraries.

resolves #3899

[grpc-kokoro]

Thanks for your pull request. The automated tests will run as soon as one of the admins verifies this change is ok for us to run on our infrastructure.

[grpc-kokoro]

Thanks for your pull request. The automated tests will run as soon as one of the admins verifies this change is ok for us to run on our infrastructure.

[haberman]

The risk of this approach is: if someone gets a google::protobuf::Message* from Python somehow, then tries to pass it to another protobuf API, we could get a crash then because the two protobuf library versions could have different internal layouts for objects.

Mixing libprotobuf.so versions in the same process seems inherently dangerous. If we go down this path, we should ensure as strictly as possible that you can't get a google::protobuf::Message*, proto2::protobuf::Descriptor*, etc. from the Python library.

[alalek]

Thank you for taking a look on this!

Protobuf is very popular library (even more than we want really), so avoiding mixed usage of different libprotobuf versions is very hard sometimes:

• for example, using proto3 formats with system libprotobuf.so 2.x
• or Python extension case: it is build on another platform (manylinux) without any version of libprotobuf.so and after that this extension is reused on other multiple Linux systems (with or without own libprotobuf.so).

Main assumption here is that protobuf implementations are isolated via API interfaces (according this: #435 (comment)) and don't share libprotobuf entries at all.
Second assumption is that non-system protobuf library is linked statically (case of Python extension from Pypi).

I agree, that some detection of this on compile / link time should be better, but this is out of scope of this patch.

This patch is attempt to isolate/hide symbols of statically linked protobuf from Linux loader (to avoid linking them with external .so with similar symbols of libprotobuf including different version).

[xfxyjwf]

Closing old issues.