Add CurrentRepository() to Python runfiles library

`runfiles.CurrentRepository()` can be used to get the canonical name of
the Bazel repository containing the caller at runtime. This information
is required to look up runfiles while taking repository mappings into
account.

src/test/py/bazel/py_test.py

@@ -207,6 +207,116 @@ def testPyTestWithStdlibCollisionRunsRemotely(self):
     self.AssertExitCode(exit_code, 0, stderr, stdout)
     self.assertIn('Test ran', stdout)
 
+class PyRunfilesLibraryTest(test_base.TestBase):
+  def testPyRunfilesLibraryCurrentRepository(self):
+    self.CreateWorkspaceWithDefaultRepos('WORKSPACE', [
+        'local_repository(',
+        '  name = "other_repo",',
+        '  path = "other_repo_path",',
+        ')'
+    ])
+
+    self.ScratchFile('pkg/BUILD.bazel', [
+        'py_library(',
+        '  name = "library",',
+        '  srcs = ["library.py"],',
+        '  visibility = ["//visibility:public"],',
+        '  deps = ["@bazel_tools//tools/python/runfiles"],',
+        ')',
+        '',
+        'py_binary(',
+        '  name = "binary",',
+        '  srcs = ["binary.py"],',
+        '  deps = [',
+        '    ":library",',
+        '    "@bazel_tools//tools/python/runfiles",'
+        '  ],',
+        ')',
+        '',
+        'py_test(',
+        '  name = "test",',
+        '  srcs = ["test.py"],',
+        '  deps = [',
+        '    ":library",',
+        '    "@bazel_tools//tools/python/runfiles",',
+        '  ],',
+        ')',
+    ])
+    self.ScratchFile('pkg/library.py', [
+        'from bazel_tools.tools.python.runfiles import runfiles',
+        'def print_repo_name():',
+        '  print("in pkg/library.py: \'%s\'" % runfiles.Create().CurrentRepository())',
+    ])
+    self.ScratchFile('pkg/binary.py', [
+        'from bazel_tools.tools.python.runfiles import runfiles',
+        'from pkg import library',
+        'library.print_repo_name()',
+        'print("in pkg/binary.py: \'%s\'" % runfiles.Create().CurrentRepository())',
+    ])
+    self.ScratchFile('pkg/test.py', [
+        'from bazel_tools.tools.python.runfiles import runfiles',
+        'from pkg import library',
+        'library.print_repo_name()',
+        'print("in pkg/test.py: \'%s\'" % runfiles.Create().CurrentRepository())',
+    ])
+
+    self.ScratchFile('other_repo_path/WORKSPACE')
+    self.ScratchFile('other_repo_path/pkg/BUILD.bazel', [
+        'py_binary(',
+        '  name = "binary",',
+        '  srcs = ["binary.py"],',
+        '  deps = [',
+        '    "@//pkg:library",',
+        '    "@bazel_tools//tools/python/runfiles",'
+        '  ],',
+        ')',
+        '',
+        'py_test(',
+        '  name = "test",',
+        '  srcs = ["test.py"],',
+        '  deps = [',
+        '    "@//pkg:library",',
+        '    "@bazel_tools//tools/python/runfiles",',
+        '  ],',
+        ')',
+    ])
+    self.ScratchFile('other_repo_path/pkg/binary.py', [
+        'from bazel_tools.tools.python.runfiles import runfiles',
+        'from pkg import library',
+        'library.print_repo_name()',
+        'print("in external/other_repo/pkg/binary.py: \'%s\'" % runfiles.Create().CurrentRepository())',
+    ])
+    self.ScratchFile('other_repo_path/pkg/test.py', [
+        'from bazel_tools.tools.python.runfiles import runfiles',
+        'from pkg import library',
+        'library.print_repo_name()',
+        'print("in external/other_repo/pkg/test.py: \'%s\'" % runfiles.Create().CurrentRepository())',
+    ])
+
+    exit_code, stdout, stderr = self.RunBazel(
+        ['run', '//pkg:binary'])
+    self.AssertExitCode(exit_code, 0, stderr, stdout)
+    self.assertIn('in pkg/binary.py: \'\'', stdout)
+    self.assertIn('in pkg/library.py: \'\'', stdout)
+
+    exit_code, stdout, stderr = self.RunBazel(
+        ['test', '//pkg:test', '--test_output=streamed'])
+    self.AssertExitCode(exit_code, 0, stderr, stdout)
+    self.assertIn('in pkg/test.py: \'\'', stdout)
+    self.assertIn('in pkg/library.py: \'\'', stdout)
+
+    exit_code, stdout, stderr = self.RunBazel(
+        ['run', '@other_repo//pkg:binary'])
+    self.AssertExitCode(exit_code, 0, stderr, stdout)
+    self.assertIn('in external/other_repo/pkg/binary.py: \'other_repo\'', stdout)
+    self.assertIn('in pkg/library.py: \'\'', stdout)
+
+    exit_code, stdout, stderr = self.RunBazel(
+        ['test', '@other_repo//pkg:test', '--test_output=streamed'])
+    self.AssertExitCode(exit_code, 0, stderr, stdout)
+    self.assertIn('in external/other_repo/pkg/test.py: \'other_repo\'', stdout)
+    self.assertIn('in pkg/library.py: \'\'', stdout)
+
 
 if __name__ == '__main__':
   unittest.main()

tools/python/gen_runfiles_constants.bzl

@@ -0,0 +1,32 @@
+# Copyright 2022 The Bazel Authors. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+_RUNFILES_CONSTANTS_TEMPLATE = """# Generated by gen_runfiles_constants.bzl
+# Internal-only; do no use.
+# The name of the runfiles directory corresponding to the main repository.
+MAIN_REPOSITORY_RUNFILES_DIRECTORY = '%s'
+"""
+
+def _gen_runfiles_constants_impl(ctx):
+    out = ctx.actions.declare_file(ctx.attr.name + ".py")
+    ctx.actions.write(out, _RUNFILES_CONSTANTS_TEMPLATE % ctx.workspace_name)
+
+    return DefaultInfo(
+        files = depset([out]),
+        runfiles = ctx.runfiles([out]),
+    )
+
+gen_runfiles_constants = rule(
+    implementation = _gen_runfiles_constants_impl,
+)

tools/python/runfiles/BUILD

@@ -1,3 +1,4 @@
+load("//tools/python:gen_runfiles_constants.bzl", "gen_runfiles_constants")
 load("//tools/python:private/defs.bzl", "py_library", "py_test")
 
 package(default_visibility = ["//visibility:private"])
@@ -20,7 +21,14 @@ filegroup(
 py_library(
     name = "runfiles",
     testonly = 1,
-    srcs = ["runfiles.py"],
+    srcs = [
+        "runfiles.py",
+        ":_runfiles_constants",
+    ],
+)
+
+gen_runfiles_constants(
+    name = "_runfiles_constants",
 )
 
 py_test(

tools/python/runfiles/BUILD.tools

@@ -1,7 +1,15 @@
+load("//tools/python:gen_runfiles_constants.bzl", "gen_runfiles_constants")
 load("//tools/python:private/defs.bzl", "py_library")
 
 py_library(
     name = "runfiles",
-    srcs = ["runfiles.py"],
+    srcs = [
+        "runfiles.py",
+        ":_runfiles_constants",
+    ],
     visibility = ["//visibility:public"],
 )
+
+gen_runfiles_constants(
+    name = "_runfiles_constants",
+)

tools/python/runfiles/runfiles.py

@@ -58,9 +58,12 @@
       p = subprocess.Popen([r.Rlocation("path/to/binary")], env, ...)
 """
 
+import inspect
 import os
 import posixpath
+import sys
 
+from ._runfiles_constants import MAIN_REPOSITORY_RUNFILES_DIRECTORY
 
 def CreateManifestBased(manifest_path):
   return _Runfiles(_ManifestBased(manifest_path))
@@ -114,6 +117,7 @@ class _Runfiles(object):
 
   def __init__(self, strategy):
     self._strategy = strategy
+    self._python_runfiles_root = _FindPythonRunfilesRoot()
 
   def Rlocation(self, path):
     """Returns the runtime path of a runfile.
@@ -161,6 +165,61 @@ def EnvVars(self):
     """
     return self._strategy.EnvVars()
 
+  def CurrentRepository(self, frame = 1):
+    """Returns the canonical name of the caller's Bazel repository.
+
+    For example, this function returns '' (the empty string) when called from
+    the main repository and a string of the form 'rules_python~0.13.0` when
+    called from code in the repository corresponding to the rules_python Bazel
+    module.
+
+    More information about the difference between canonical repository names and
+    the `@repo` part of labels is available at:
+    https://bazel.build/build/bzlmod#repository-names
+
+    NOTE: This function inspects the callstack to determine where in the
+    runfiles the caller is located to determine which repository it came from.
+    This may fail or produce incorrect results depending on who the caller is,
+    for example if it is not represented by a Python source file. Use the
+    `frame` argument to control the stack lookup.
+
+    Args:
+      frame: int; the stack frame to return the repository name for. Defaults to
+          1, the caller of the CurrentRepository function.
+    Returns:
+      The canonical name of the Bazel repository containing the file containing
+      the frame-th caller of this function
+    Raises:
+      ValueError: if the caller cannot be determined or the caller's file path
+          is not contained in the Python runfiles tree
+    """
+    try:
+      caller_path = inspect.getfile(sys._getframe(frame))
+    except (TypeError, ValueError):
+      raise ValueError("failed to determine caller's file path")
+    caller_runfiles_path = os.path.relpath(caller_path, self._python_runfiles_root)
+    if caller_runfiles_path.startswith(".." + os.path.sep):
+      raise ValueError('{} does not lie under the runfiles root {}'.format(caller_path, self._python_runfiles_root))
+
+    caller_runfiles_directory = caller_runfiles_path[:caller_runfiles_path.find(os.path.sep)]
+    if caller_runfiles_directory == MAIN_REPOSITORY_RUNFILES_DIRECTORY:
+      # The canonical name of the main repository (also known as the workspace)
+      # is the empty string.
+      return ''
+    # For all other repositories, the name of the runfiles directory is the
+    # canonical name.
+    return caller_runfiles_directory
+
+def _FindPythonRunfilesRoot():
+  root = __file__
+  # Walk up our own runfiles path to the root of the runfiles tree from which
+  # the current file is being run. This path coincides with what the Bazel
+  # Python stub sets up as sys.path[0]. Since that entry can be changed at
+  # runtime, we rederive it here.
+  for _ in range("bazel_tools/tools/python/runfiles/runfiles.py".count("/") + 1):
+    root = os.path.dirname(root)
+  return root
+
 
 class _ManifestBased(object):
   """`Runfiles` strategy that parses a runfiles-manifest to look up runfiles."""

tools/python/runfiles/runfiles_test.py

@@ -262,6 +262,9 @@ def testPathsFromEnvvars(self):
     self.assertEqual(mf, "")
     self.assertEqual(dr, "")
 
+  def testCurrentRepository(self):
+    self.assertEqual(runfiles.Create().CurrentRepository(), "")
+
   @staticmethod
   def IsWindows():
     return os.name == "nt"