Bazel toolchain uses system python on MacOS

[DoDoENT]

Instead of using the Python 3.13 from rules_python, it uses system python (not even the homebrew version).

Since MacOS still uses Python 3.9.6, this causes build failures when using Emscripten 4.0.22 or newer:

ERROR: /Users/dodo/Library/Caches/bazel/_bazel_dodo/d37a55ce2ed2abf9c807bae8c8d24add/external/googletest+/BUILD.bazel:102:11: Compiling googlemock/src/gmock.cc failed: (Exit 1): emcc.sh failed: error executing CppCompile command (from cc_library rule target @@googletest+//:gtest)
  (cd /Users/dodo/Library/Caches/bazel/_bazel_dodo/d37a55ce2ed2abf9c807bae8c8d24add/sandbox/darwin-sandbox/649/execroot/_main && \
  exec env - \
    EMCC_WASM_BACKEND=1 \
    EM_BIN_PATH=external/emsdk++emscripten_deps+emscripten_bin_mac_arm64 \
    EM_CONFIG_PATH=external/emsdk++emscripten_cache+emscripten_cache/emscripten_config \
    NODE_JS_PATH=external/rules_nodejs++node+nodejs_darwin_arm64/bin/node \
    PATH=/bin:/usr/bin:/usr/local/bin \
    PWD=/proc/self/cwd \
  external/emsdk+/emscripten_toolchain/emcc.sh @bazel-out/wasm-fastbuild-ST-f416d161dfb4/bin/external/googletest+/_objs/gtest/gmock.o.params)
# Configuration: e22466f04beb073adc43c23b7e8a6ea4ea7fa8665f522c3b4b8b22ce82e3bd2f
# Execution platform: //platforms/macos:arm64

Use --sandbox_debug to see verbose messages from the sandbox and retain the sandbox build root for debugging
Traceback (most recent call last):
  File "/Users/dodo/Library/Caches/bazel/_bazel_dodo/d37a55ce2ed2abf9c807bae8c8d24add/sandbox/darwin-sandbox/649/execroot/_main/external/emsdk++emscripten_deps+emscripten_bin_mac_arm64/emscripten/emcc.py", line 32, in <module>
    from tools import (
  File "/Users/dodo/.bazel-cache/repo-contents/8b2e39f167779b741c4b581a9adb66e852d1535514f04ad2ebdd5fb6db927553/296e22c2-11b5-447e-8084-6f41b84c7f8f/emscripten/tools/building.py", line 107
    match settings.SUPPORT_LONGJMP:
          ^
SyntaxError: invalid syntax

[DoDoENT]

This fixes it on MacOS, but breaks it on Linux - I'm still investigating this. It works with --spawn_strategy=local, which means that I need to figure out how to inject python interpreter's runfiles into emscripten toolchain.

[sbc100]

What do you mean by "MacOS still uses Python 3.9.6"? I thought macOS didn't supply python at the system level these days. I guess I must have missed something.

[DoDoENT]

What do you mean by "MacOS still uses Python 3.9.6"?

/usr/bin/python3 on my MacOS Tahoe is 3.9.6

For everyday use, I use homebrew python (3.14), but bazel's sandboxing on MacOS prevents the use of homebrew python.

This led me to investigate why does emsdk use system python in the first place, since it already depends on rules_python. I discovered the culprit in emcc.sh, emar.sh, and emcc_link.sh scripts in the toolchains subfolder - they simply invoke python3 and expect that it exists in the environment.

In the commit I linked above, I patched that to use python3 provided by the rules_python. However, on Linux the build fails because python cannot find encodings builtin module. Namely, on Linux, the sandboxing is much better than on MacOS and the lack of providing python interpreter's runfiles to the invocation is what bites me. It does work with --spawn_strategy=local, but this is not acceptable.

I'm still investigating how to correctly solve this. Namely, using the interpreter fails on Bazel 9 because rules_python is using old-school reference to bash runfiles library to inject runfiles into a python shell wrapper which no longer exists in Bazel 9.

[sbc100]

/usr/bin/python3 on my MacOS Tahoe is 3.9.6

Do you know where that is coming from? According to the information I can find online macOS Tahoe does not ship with python pre-installed.

[sbc100]

Regardless, I agree bazel rules should never be depending on the system python ..

[DoDoENT]

Do you know where that is coming from?

I think it comes from Xcode installation, as part of its "command line tools". Various Internet search queries confirm my suspicion.

[DoDoENT]

I've spent over 6 hours trying various tricks to get Python runfiles included in the toolchain invocation, but without any luck so far. I'm afraid this surpasses my bazel abilities.

I've asked for help on Bazel Slack - maybe someone more experienced can take a look.

In the meantime, I'll keep using emscripten older than 4.0.22 or find a way to let Bazel use python from Homebrew (I really dislike this not being hermetic)...

[DoDoENT]

This now fully works on Linux, but requires this change in rules_python.

[DoDoENT]

I also tested on macOS - it also works there.

[sbc100]

Sweet! Thanks for all the hard work on this!