Figure out a good strategy for managing python version/s

[phlax]

description

some parts of tooling/ci require python3.8 and more generally recent python 3.x versions have syntax incompatibility

for those reasons the envoy-build-image sets python3.8 as the default interpreter

some OS dont have support for this out of the box so we could do with a good strategy for handling this

a couple of ideas in this respect are:

• use a python_runtime bazel rule to pull in the required py version
• explicitly set python3.8 in the shebang lines so devs can keep lower versions without having to update-alternatives or similar
• use a venv runtime (i think this is the least prefered option for various reasons)

[jpeach]

May be related to bazelbuild/bazel#8685

[jpeach]

The build requires a program named python. Using update-alternatives to point /usr/bin/python to python3 works until the thrift test, which explicitly check for python 2. IIUC most of the rest of the build and tooling wants python 3 (though it doesn't ever invoke anything with /usr/bin/python3).

I would be happy to work on improving this if there's a committer with Bazel expertise who can shepherd.

[ RUN      ] TransportsAndProtocols/ThriftTranslationIntegrationTest.Translates/FromFramedBinaryToFramedBinary
Using address /tmp/envoy_test_uds.OLGfdl/fixture.sock
Error occurred while attempting to use the default Python toolchain (@rules_python//python:autodetecting_toolchain).
According to '/usr/bin/python -V', version is 'Python 3.9.4', but we need version 2. PATH is:

.:/home/jpeach/.cache/bazelisk/downloads/bazelbuild/bazel-3.7.2-linux-x86_64/bin:/home/jpeach/bin:/home/jpeach/go/bin:/home/linuxbrew/.linuxbrew/opt/*/libexec/gnubin:/home/linuxbrew/.linuxbrew/bin:/home/linuxbrew/.linuxbrew/sbin:/home/jpeach/bin:/home/jpeach/go/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin:/snap/bin

Please ensure an interpreter with version 2 is available on this platform as 'python2' or 'python', or else register an appropriate Python toolchain as per the documentation for py_runtime_pair (https://github.com/bazelbuild/rules_python/blob/master/docs/python.md#py_runtime_pair).

[phlax]

IIUC most of the rest of the build and tooling wants python 3 (though it doesn't ever invoke anything with /usr/bin/python3

there is a specific dependency on python3.8 in one of the tools - so the build image has python3.8 installed and set as the default interpreter - and the general advice is to use 3.8 or above

regarding the python2 requirement - i would hope we can update that to use a more recent version

regarding this ticket i think the best way would be to use a py_runtime bazel rule (https://docs.bazel.build/versions/master/be/python.html#py_runtime) - the more we put into bazel the less assumptions we make about the end user/devs environment and the more likely everything is to work out of the box

if you were up for taking this on @jpeach - i am happy to review etc

[jpeach]

if you were up for taking this on @jpeach - i am happy to review etc

Great! I'll try to dig in to this over the next few weeks.