feat: allow populating binary's venv site-packages with symlinks (#2617)

This implements functionality to allow libraries to populate the
site-packages directory
of downstream binaries. The basic implementation is:
* Libraries provide tuples of `(runfile path, site packages path)` in
the
  `PyInfo.site_packages_symlinks` field.
* Binaries create symlinks (using declare_symlink) in their
site-packages directory
  pointing to the runfiles paths libraries provide.

The design was chosen because of the following properties:
* The site-packages directory is relocatable
* Populating site packages is cheap ( `O(number 3p dependencies)` )
* Dependencies are only created once in the runfiles, no matter
how many how many binaries there that use them. This minimizes disk
usage,
  file counts, inodes, etc.

The `site_packages_symlinks` field is a depset with topological
ordering. Using topological
ordering allows dependencies closer to the binary to have precedence,
which gives some
basic control over what entries are used.

Additionally, the runfiles path to link to can be None/empty, in which
case, the
directory in site-packages won't be created. This allows binaries to
prevent creation
of directories that might e.g. conflict.

For now, this functionality is disabled by default. The flag
`--venvs_site_packages=yes`
can be set to allow using it, which is automatically enable it for pypi
generated targets.

When enabled, it does basic detection of implicit namespace directories,
which
allows multiple distributions to "install" into the the same
site-packages directory.

Though this functionality is primarily useful for dependencies from pypi
(e.g. via
pip.parse), it is not yet activated for those targets, for two main
reasons:
1. The wheel extraction code creates pkgutil-style `__init__.py` shims
during the repo-phase.
The build phase can't distinguish these artifical rules_python generated
shims from
actual `__init__.py` files, which breaks the implicit namespace
detection logic.
2. A flag guard is needed before changing the behavior. Even though how
3p libraries are
added to sys.path is an implementation detail, the behavior has been
there for many
   years, so an escape hatch should be added.

Work towards #2156

Author: rickeylev

.bazelrc

@@ -4,8 +4,8 @@
 # (Note, we cannot use `common --deleted_packages` because the bazel version command doesn't support it)
 # To update these lines, execute
 # `bazel run @rules_bazel_integration_test//tools:update_deleted_packages`
-build --deleted_packages=examples/build_file_generation,examples/build_file_generation/random_number_generator,examples/bzlmod,examples/bzlmod_build_file_generation,examples/bzlmod_build_file_generation/other_module/other_module/pkg,examples/bzlmod_build_file_generation/runfiles,examples/bzlmod/entry_points,examples/bzlmod/entry_points/tests,examples/bzlmod/libs/my_lib,examples/bzlmod/other_module,examples/bzlmod/other_module/other_module/pkg,examples/bzlmod/patches,examples/bzlmod/py_proto_library,examples/bzlmod/py_proto_library/example.com/another_proto,examples/bzlmod/py_proto_library/example.com/proto,examples/bzlmod/runfiles,examples/bzlmod/tests,examples/bzlmod/tests/other_module,examples/bzlmod/whl_mods,examples/multi_python_versions/libs/my_lib,examples/multi_python_versions/requirements,examples/multi_python_versions/tests,examples/pip_parse,examples/pip_parse_vendored,examples/pip_repository_annotations,examples/py_proto_library,examples/py_proto_library/example.com/another_proto,examples/py_proto_library/example.com/proto,gazelle,gazelle/manifest,gazelle/manifest/generate,gazelle/manifest/hasher,gazelle/manifest/test,gazelle/modules_mapping,gazelle/python,gazelle/pythonconfig,gazelle/python/private,tests/integration/compile_pip_requirements,tests/integration/compile_pip_requirements_test_from_external_repo,tests/integration/custom_commands,tests/integration/ignore_root_user_error,tests/integration/ignore_root_user_error/submodule,tests/integration/local_toolchains,tests/integration/pip_parse,tests/integration/pip_parse/empty,tests/integration/py_cc_toolchain_registered
-query --deleted_packages=examples/build_file_generation,examples/build_file_generation/random_number_generator,examples/bzlmod,examples/bzlmod_build_file_generation,examples/bzlmod_build_file_generation/other_module/other_module/pkg,examples/bzlmod_build_file_generation/runfiles,examples/bzlmod/entry_points,examples/bzlmod/entry_points/tests,examples/bzlmod/libs/my_lib,examples/bzlmod/other_module,examples/bzlmod/other_module/other_module/pkg,examples/bzlmod/patches,examples/bzlmod/py_proto_library,examples/bzlmod/py_proto_library/example.com/another_proto,examples/bzlmod/py_proto_library/example.com/proto,examples/bzlmod/runfiles,examples/bzlmod/tests,examples/bzlmod/tests/other_module,examples/bzlmod/whl_mods,examples/multi_python_versions/libs/my_lib,examples/multi_python_versions/requirements,examples/multi_python_versions/tests,examples/pip_parse,examples/pip_parse_vendored,examples/pip_repository_annotations,examples/py_proto_library,examples/py_proto_library/example.com/another_proto,examples/py_proto_library/example.com/proto,gazelle,gazelle/manifest,gazelle/manifest/generate,gazelle/manifest/hasher,gazelle/manifest/test,gazelle/modules_mapping,gazelle/python,gazelle/pythonconfig,gazelle/python/private,tests/integration/compile_pip_requirements,tests/integration/compile_pip_requirements_test_from_external_repo,tests/integration/custom_commands,tests/integration/ignore_root_user_error,tests/integration/ignore_root_user_error/submodule,tests/integration/local_toolchains,tests/integration/pip_parse,tests/integration/pip_parse/empty,tests/integration/py_cc_toolchain_registered
+build --deleted_packages=examples/build_file_generation,examples/build_file_generation/random_number_generator,examples/bzlmod,examples/bzlmod_build_file_generation,examples/bzlmod_build_file_generation/other_module/other_module/pkg,examples/bzlmod_build_file_generation/runfiles,examples/bzlmod/entry_points,examples/bzlmod/entry_points/tests,examples/bzlmod/libs/my_lib,examples/bzlmod/other_module,examples/bzlmod/other_module/other_module/pkg,examples/bzlmod/patches,examples/bzlmod/py_proto_library,examples/bzlmod/py_proto_library/example.com/another_proto,examples/bzlmod/py_proto_library/example.com/proto,examples/bzlmod/runfiles,examples/bzlmod/tests,examples/bzlmod/tests/other_module,examples/bzlmod/whl_mods,examples/multi_python_versions/libs/my_lib,examples/multi_python_versions/requirements,examples/multi_python_versions/tests,examples/pip_parse,examples/pip_parse_vendored,examples/pip_repository_annotations,examples/py_proto_library,examples/py_proto_library/example.com/another_proto,examples/py_proto_library/example.com/proto,gazelle,gazelle/manifest,gazelle/manifest/generate,gazelle/manifest/hasher,gazelle/manifest/test,gazelle/modules_mapping,gazelle/python,gazelle/pythonconfig,gazelle/python/private,tests/integration/compile_pip_requirements,tests/integration/compile_pip_requirements_test_from_external_repo,tests/integration/custom_commands,tests/integration/ignore_root_user_error,tests/integration/ignore_root_user_error/submodule,tests/integration/local_toolchains,tests/integration/pip_parse,tests/integration/pip_parse/empty,tests/integration/py_cc_toolchain_registered,tests/modules/other,tests/modules/other/nspkg_delta,tests/modules/other/nspkg_gamma
+query --deleted_packages=examples/build_file_generation,examples/build_file_generation/random_number_generator,examples/bzlmod,examples/bzlmod_build_file_generation,examples/bzlmod_build_file_generation/other_module/other_module/pkg,examples/bzlmod_build_file_generation/runfiles,examples/bzlmod/entry_points,examples/bzlmod/entry_points/tests,examples/bzlmod/libs/my_lib,examples/bzlmod/other_module,examples/bzlmod/other_module/other_module/pkg,examples/bzlmod/patches,examples/bzlmod/py_proto_library,examples/bzlmod/py_proto_library/example.com/another_proto,examples/bzlmod/py_proto_library/example.com/proto,examples/bzlmod/runfiles,examples/bzlmod/tests,examples/bzlmod/tests/other_module,examples/bzlmod/whl_mods,examples/multi_python_versions/libs/my_lib,examples/multi_python_versions/requirements,examples/multi_python_versions/tests,examples/pip_parse,examples/pip_parse_vendored,examples/pip_repository_annotations,examples/py_proto_library,examples/py_proto_library/example.com/another_proto,examples/py_proto_library/example.com/proto,gazelle,gazelle/manifest,gazelle/manifest/generate,gazelle/manifest/hasher,gazelle/manifest/test,gazelle/modules_mapping,gazelle/python,gazelle/pythonconfig,gazelle/python/private,tests/integration/compile_pip_requirements,tests/integration/compile_pip_requirements_test_from_external_repo,tests/integration/custom_commands,tests/integration/ignore_root_user_error,tests/integration/ignore_root_user_error/submodule,tests/integration/local_toolchains,tests/integration/pip_parse,tests/integration/pip_parse/empty,tests/integration/py_cc_toolchain_registered,tests/modules/other,tests/modules/other/nspkg_delta,tests/modules/other/nspkg_gamma
 
 test --test_output=errors
 

CHANGELOG.md

@@ -107,6 +107,10 @@ Unreleased changes template.
   please check the {obj}`uv.configure` tag class.
 * Add support for riscv64 linux platform.
 * (toolchains) Add python 3.13.2 and 3.12.9 toolchains
+* (providers) (experimental) {obj}`PyInfo.site_packages_symlinks` field added to
+  allow specifying links to create within the venv site packages (only
+  applicable with {obj}`--bootstrap_impl=script`)
+  ([#2156](https://github.com/bazelbuild/rules_python/issues/2156)).
 
 {#v0-0-0-removed}
 ### Removed

MODULE.bazel

@@ -85,6 +85,7 @@ bazel_dep(name = "rules_shell", version = "0.3.0", dev_dependency = True)
 bazel_dep(name = "rules_multirun", version = "0.9.0", dev_dependency = True)
 bazel_dep(name = "bazel_ci_rules", version = "1.0.0", dev_dependency = True)
 bazel_dep(name = "rules_pkg", version = "1.0.1", dev_dependency = True)
+bazel_dep(name = "other", version = "0", dev_dependency = True)
 
 # Extra gazelle plugin deps so that WORKSPACE.bzlmod can continue including it for e2e tests.
 # We use `WORKSPACE.bzlmod` because it is impossible to have dev-only local overrides.
@@ -106,6 +107,11 @@ local_path_override(
     path = "gazelle",
 )
 
+local_path_override(
+    module_name = "other",
+    path = "tests/modules/other",
+)
+
 dev_python = use_extension(
     "//python/extensions:python.bzl",
     "python",

docs/BUILD.bazel

@@ -87,6 +87,7 @@ sphinx_stardocs(
     name = "bzl_api_docs",
     srcs = [
         "//python:defs_bzl",
+        "//python:features_bzl",
         "//python:packaging_bzl",
         "//python:pip_bzl",
         "//python:py_binary_bzl",

docs/_includes/experimental_api.md

@@ -0,0 +1,5 @@
+:::{warning}
+
+**Experimental API.** This API is still under development and may change or be
+removed without notice.
+:::

docs/api/rules_python/python/config_settings/index.md

@@ -213,6 +213,23 @@ Values:
 ::::
 
 
+::::
+
+:::{flag} venvs_site_packages
+
+Determines if libraries use a site-packages layout for their files.
+
+Note this flag only affects PyPI dependencies of `--bootstrap_impl=script` binaries
+
+:::{include} /_includes/experimental_api.md
+:::
+
+
+Values:
+* `no` (default): Make libraries importable by adding to `sys.path`
+* `yes`: Make libraries importable by creating paths in a binary's site-packages directory.
+::::
+
 ::::{bzl:flag} bootstrap_impl
 Determine how programs implement their startup process.
 

internal_dev_deps.bzl

@@ -15,6 +15,7 @@
 """Dependencies that are needed for development and testing of rules_python itself."""
 
 load("@bazel_tools//tools/build_defs/repo:http.bzl", _http_archive = "http_archive", _http_file = "http_file")
+load("@bazel_tools//tools/build_defs/repo:local.bzl", "local_repository")
 load("@bazel_tools//tools/build_defs/repo:utils.bzl", "maybe")
 load("//python/private:internal_config_repo.bzl", "internal_config_repo")  # buildifier: disable=bzl-visibility
 
@@ -42,6 +43,11 @@ def rules_python_internal_deps():
     """
     internal_config_repo(name = "rules_python_internal")
 
+    local_repository(
+        name = "other",
+        path = "tests/modules/other",
+    )
+
     http_archive(
         name = "bazel_skylib",
         sha256 = "bc283cdfcd526a52c3201279cda4bc298652efa898b10b4db0837dc51652756f",

python/BUILD.bazel

@@ -79,6 +79,9 @@ bzl_library(
 bzl_library(
     name = "features_bzl",
     srcs = ["features.bzl"],
+    deps = [
+        "@rules_python_internal//:rules_python_config_bzl",
+    ],
 )
 
 bzl_library(

python/config_settings/BUILD.bazel

@@ -9,6 +9,7 @@ load(
     "LibcFlag",
     "PrecompileFlag",
     "PrecompileSourceRetentionFlag",
+    "VenvsSitePackages",
     "VenvsUseDeclareSymlinkFlag",
 )
 load(
@@ -195,6 +196,13 @@ string_flag(
     visibility = ["//visibility:public"],
 )
 
+string_flag(
+    name = "venvs_site_packages",
+    build_setting_default = VenvsSitePackages.NO,
+    # NOTE: Only public because it is used in pip hub repos.
+    visibility = ["//visibility:public"],
+)
+
 define_pypi_internal_flags(
     name = "define_pypi_internal_flags",
 )

python/features.bzl

@@ -19,8 +19,49 @@ load("@rules_python_internal//:rules_python_config.bzl", "config")
 # See https://git-scm.com/docs/git-archive/2.29.0#Documentation/git-archive.txt-export-subst
 _VERSION_PRIVATE = "$Format:%(describe:tags=true)$"
 
+def _features_typedef():
+    """Information about features rules_python has implemented.
+
+    ::::{field} precompile
+    :type: bool
+
+    True if the precompile attributes are available.
+
+    :::{versionadded} 0.33.0
+    :::
+    ::::
+
+    ::::{field} py_info_site_packages_symlinks
+
+    True if the `PyInfo.site_packages_symlinks` field is available.
+
+    :::{versionadded} VERSION_NEXT_FEATURE
+    :::
+    ::::
+
+    ::::{field} uses_builtin_rules
+    :type: bool
+
+    True if the rules are using the Bazel-builtin implementation.
+
+    :::{versionadded} 1.1.0
+    :::
+    ::::
+
+    ::::{field} version
+    :type: str
+
+    The rules_python version. This is a semver format, e.g. `X.Y.Z` with
+    optional trailing `-rcN`. For unreleased versions, it is an empty string.
+    :::{versionadded} 0.38.0
+    ::::
+    """
+
 features = struct(
-    version = _VERSION_PRIVATE if "$Format" not in _VERSION_PRIVATE else "",
+    TYPEDEF = _features_typedef,
+    # keep sorted
     precompile = True,
+    py_info_site_packages_symlinks = True,
     uses_builtin_rules = not config.enable_pystar,
+    version = _VERSION_PRIVATE if "$Format" not in _VERSION_PRIVATE else "",
 )