feat: updates to the py_extension rule for building C extensionsPy extension

[rsartor-cmd]

Per feedback from #3851, this PR re-works the platform detection logic to rely on platform constraints instead of the Python runtime/toolchain.

Key Changes

1. Introduced abi_tag to PyCcToolchainInfo

• Modified py_cc_toolchain_info.bzl to add the abi_tag field to the PyCcToolchainInfo provider.
• Modified py_cc_toolchain_rule.bzl to add an optional abi_tag attribute to the py_cc_toolchain rule.
• If abi_tag is not explicitly specified, the rule automatically derives a default tag (e.g., cpython-311 from python_version = "3.11" ), ensuring 100% backward compatibility with existing toolchains.

2. Refactored py_extension Toolchain Dependency

• Modified py_extension_rule.bzl to switch the rule's toolchain dependency from TARGET_TOOLCHAIN_TYPE to PY_CC_TOOLCHAIN_TYPE .
• Updated _py_extension_impl to retrieve the ABI tag from py_cc_toolchain.abi_tag instead of py_runtime.pyc_tag and py_runtime.abi_flags .
• Simplified the output filename formatting and fixed a bug to use the dynamic extension (e.g., .pyd on Windows) instead of hardcoding .so .

3. Enforced PLATFORMS Registry

• Removed the legacy fallback path in _get_platform that parsed cc_toolchain CPU and target names.
• The rule now strictly requires the target platform to be registered in rules_python 's central PLATFORMS registry, failing the build with a clear error message if it is not found. This aligns the rule
with the modern platform constraints model and cleans up legacy parsing code.

4. Updated and Added Tests

• Updated py_extension_test.py to reflect the correct platform/version-specific extension filename.
• Implemented a new test suite in py_limited_api_tests.bzl to verify that the py_limited_api attribute correctly configures the .abi3.so filename suffix across various dependency configurations.
• Note on Validation: An initial design for compile-time dependency compatibility validation was discarded due to performance concerns regarding depset flattening at analysis time.
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Verification

Ran the entire test suite using bazel-8.7.0 :

• 9/9 Limited API Analysis Tests: PASSED
• 2/2 Pre-existing Analysis Tests: PASSED
• Python Integration Unit Test: PASSED
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TAG=agy
CONV=23f8a5e8-2d99-401a-9903-1256b7d42b0e

[gemini-code-assist]

Code Review

This pull request refactors the py_extension rule to use constraint-based platform tag derivation instead of C++ toolchain-based derivation, introduces abi_tag to PyCcToolchainInfo, and removes the _check_limited_api_compatibility validation logic. The review feedback highlights several critical issues: first, the new constraint-based platform detection is broken for musl Linux platforms because they share the same constraints as gnu platforms; second, changing the limited API check to bool(ctx.attr.py_limited_api) breaks backward compatibility for targets explicitly setting "none"; third, the hardcoded _constraints list should be dynamically derived from PLATFORMS to prevent future maintenance issues; and finally, the documentation for py_limited_api needs to be updated since the dependency validation logic was removed.

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[rsartor-cmd]

The mypy CI failure appears to be caused by the check action using Python 3.9, but the newest mypy only supporting 3.10+

Run docker build /home/runner/work/_actions/jpetrucciani/mypy-check/master --build-arg "pyver=3.9" -t mypy

#11 3.773 ERROR: Ignored the following versions that require a different python version: 1.20.0 Requires-Python >=3.10; 1.20.1 Requires-Python >=3.10; 1.20.2 Requires-Python >=3.10; 2.0.0 Requires-Python >=3.10; 2.1.0 Requires-Python >=3.10
#11 3.774 ERROR: Could not find a version that satisfies the requirement mypy==2.1.0 (from versions: 0.1, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.20, 0.21, 0.221, 0.222, 0.223, 0.224, 0.225, 0.226, 0.227, 0.228, 0.229, 0.230, 0.231, 0.232, 0.233, 0.234, 0.235, 0.236, 0.237, 0.238, 0.239, 0.240, 0.241, 0.250, 0.251, 0.252, 0.253, 0.254, 0.255, 0.256, 0.470, 0.471, 0.501, 0.510, 0.511, 0.520, 0.521, 0.530, 0.540, 0.550, 0.560, 0.570, 0.580, 0.590, 0.600, 0.610, 0.620, 0.630, 0.641, 0.650, 0.660, 0.670, 0.700, 0.701, 0.710, 0.711, 0.720, 0.730, 0.740, 0.750, 0.760, 0.761, 0.770, 0.780, 0.781, 0.782, 0.790, 0.800, 0.812, 0.900, 0.901, 0.902, 0.910, 0.920, 0.921, 0.930, 0.931, 0.940, 0.941, 0.942, 0.950, 0.960, 0.961, 0.971, 0.981, 0.982, 0.990, 0.991, 1.0.0, 1.0.1, 1.1.1, 1.2.0, 1.3.0, 1.4.0, 1.4.1, 1.5.0, 1.5.1, 1.6.0, 1.6.1, 1.7.0, 1.7.1, 1.8.0, 1.9.0, 1.10.0, 1.10.1, 1.11.0, 1.11.1, 1.11.2, 1.12.0, 1.12.1, 1.13.0, 1.14.0, 1.14.1, 1.15.0, 1.16.0, 1.16.1, 1.17.0, 1.17.1, 1.18.1, 1.18.2, 1.19.0, 1.19.1)
#11 3.775 ERROR: No matching distribution found for mypy==2.1.0
#11 ERROR: process "/bin/sh -c pip install -U pip &&     pip install -r requirements.txt" did not complete successfully: exit code: 1

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For what it's worth, I ran mypy --show-column-numbers --hide-error-context python/runfiles and mypy --show-column-numbers --hide-error-context tests/runfiles locally and it found no issues with 3 files.