add python 3.9

[busunkim96]

Modeled after #7009.

Fixes #7978

[komalali]

Looks like it needs a python label?

[mihaimaruseac]

There is a generic wheel that gets downloaded for python3.9. Is this very different than the one that would be generated solely for py39?

[busunkim96]

@mihaimaruseac I believe in many use cases the generic wheel is sufficient. As an example, the Google Cloud client libraries can run their tests with the generic wheel without problems.

However, the platform specific wheels are necessary to use the C++ Implementation.

There is also a C++ implementation for Python messages via a Python extension for better performance. Implementation type is controlled by an environment variable PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION (valid values: "cpp" and "python"). The default value is currently "python" but will be changed to "cpp" in future release.

#6214 explains this for 3.8.

[busunkim96]

@anandolee 👋 Could I have help with Kokoro? I looked at the logs for the new 3.9 builds and it fails with:

+ docker pull protobuftesting/python39_40b2c95dbd62f8d5856cf11b19cad04a8847f2f8
Using default tag: latest
Error response from daemon: pull access denied for protobuftesting/python39_40b2c95dbd62f8d5856cf11b19cad04a8847f2f8, repository does not exist or may require 'docker login'
+ sleep 10
...

Does a Docker image need to published manually?

[busunkim96]

It looks like https://github.com/protocolbuffers/protobuf/blob/523b1096669f64c3481c4f2a2cd45771bd68b351/kokoro/linux/dockerfile/push_testing_images.sh may need to be run?

[anandolee]

Python39 tests are now passing. Can you fix the distcheck @busunkim96 ? I can not open the error details for somehow...

[scotloach]

@busunkim96 what needs to happen to get the python39 wheel uploaded to PyPI?
https://pypi.org/project/protobuf/#files

[busunkim96]

@scotloach A release of protobuf needs to actually be made for the wheels to be published. I'm not on the protobuf team so I'm not sure what their plans are. @anandolee?

[ahtik]

https://pypi.org/project/protobuf/3.15.0rc1/#files has now the python 3.9 release candidate builds. Yet both python 3.8 and 3.9 are missing the Windows builds. I hope they will show up in the final release.

The Windows builds for python 3.8 were also missing in v3.14.0, so not entirely sure what's behind it, if there's a plan to build them?

[busunkim96]

@ahtik I don't think that was intended. I bumped #7967 which was filed for the missing Windows 3.8 wheels.