feat(pypi): pip.defaults API for customizing pipstar 1/n (#2987)

Parse env markers in pip.parse using starlark

Summary:
- Allow switching to the Starlark implementation of the marker
  evaluation function.
- Add a way for users to modify the `env` for the marker evaluation when
  parsing the requirements. This can only be done by `rules_python` or
  the root module.
- Limit the platform selection when parsing the requirements files.

Work towards #2747
Work towards #2949
Split out from #2909

---------

Co-authored-by: Richard Levasseur <richardlev@gmail.com>

Author: aignas

CHANGELOG.md

@@ -62,7 +62,9 @@ END_UNRELEASED_TEMPLATE
 
 {#v0-0-0-added}
 ### Added
-* Nothing added.
+* (pypi) To configure the environment for `requirements.txt` evaluation, use the newly added
+  developer preview of the `pip.default` tag class. Only `rules_python` and root modules can use
+  this feature.
 
 {#v0-0-0-removed}
 ### Removed

python/private/pypi/BUILD.bazel

@@ -97,10 +97,10 @@ bzl_library(
     name = "evaluate_markers_bzl",
     srcs = ["evaluate_markers.bzl"],
     deps = [
-        ":pep508_env_bzl",
+        ":deps_bzl",
         ":pep508_evaluate_bzl",
-        ":pep508_platform_bzl",
         ":pep508_requirement_bzl",
+        ":pypi_repo_utils_bzl",
     ],
 )
 
@@ -113,6 +113,7 @@ bzl_library(
         ":hub_repository_bzl",
         ":parse_requirements_bzl",
         ":parse_whl_name_bzl",
+        ":pep508_env_bzl",
         ":pip_repository_attrs_bzl",
         ":simpleapi_download_bzl",
         ":whl_config_setting_bzl",

python/private/pypi/env_marker_info.bzl

@@ -17,7 +17,7 @@ The {obj}`--//python/config_settings:pip_env_marker_config` flag.
 The values to use for environment markers when evaluating an expression.
 
 The keys and values should be compatible with the [PyPA dependency specifiers
-specification](https://packaging.python.org/en/latest/specifications/dependency-specifiers/)
+specification](https://packaging.python.org/en/latest/specifications/dependency-specifiers/).
 
 Missing values will be set to the specification's defaults or computed using
 available toolchain information.

python/private/pypi/evaluate_markers.bzl

@@ -15,9 +15,7 @@
 """A simple function that evaluates markers using a python interpreter."""
 
 load(":deps.bzl", "record_files")
-load(":pep508_env.bzl", "env")
 load(":pep508_evaluate.bzl", "evaluate")
-load(":pep508_platform.bzl", "platform_from_str")
 load(":pep508_requirement.bzl", "requirement")
 load(":pypi_repo_utils.bzl", "pypi_repo_utils")
 
@@ -30,22 +28,27 @@ SRCS = [
     Label("//python/private/pypi/whl_installer:platform.py"),
 ]
 
-def evaluate_markers(requirements, python_version = None):
+def evaluate_markers(*, requirements, platforms):
     """Return the list of supported platforms per requirements line.
 
     Args:
         requirements: {type}`dict[str, list[str]]` of the requirement file lines to evaluate.
-        python_version: {type}`str | None` the version that can be used when evaluating the markers.
+        platforms: {type}`dict[str, dict[str, str]]` The environments that we for each requirement
+            file to evaluate. The keys between the platforms and requirements should be shared.
 
     Returns:
         dict of string lists with target platforms
     """
     ret = {}
-    for req_string, platforms in requirements.items():
+    for req_string, platform_strings in requirements.items():
         req = requirement(req_string)
-        for platform in platforms:
-            if evaluate(req.marker, env = env(platform_from_str(platform, python_version))):
-                ret.setdefault(req_string, []).append(platform)
+        for platform_str in platform_strings:
+            env = platforms.get(platform_str)
+            if not env:
+                fail("Please define platform: '{}'".format(platform_str))
+
+            if evaluate(req.marker, env = env):
+                ret.setdefault(req_string, []).append(platform_str)
 
     return ret