add tests for relative_path

Author: chowder

python/private/py_executable_bazel.bzl

@@ -347,22 +347,26 @@ def _create_zip_main(ctx, *, stage2_bootstrap, runtime_details, venv):
 
 # Return a relative path from one path to another, where both paths are each
 # relative paths from a common root.
-def _relative_path(from_, to):
+def relative_path(from_, to):
     from_parts = from_.split("/")
     to_parts = to.split("/")
 
-    # Strip common "../" parts from both paths
+    # Strip common leading parts from both paths
     # (no while loops in starlark :( )
-    n = max(len(from_parts), len(to_parts))
+    n = min(len(from_parts), len(to_parts))
     for _ in range(n):
-        if from_parts[0] == ".." and to_parts[0] == "..":
+        if from_parts[0] == to_parts[0]:
             from_parts.pop(0)
             to_parts.pop(0)
         else:
             break
 
-    parent = "/".join([".."] * len(from_parts))
-    return "/".join([parent] + to_parts)
+    # Impossible to compute a relative path without knowing what ".." is
+    if from_parts and from_parts[0] == "..":
+        fail("cannot compute relative path from '%s' to '%s'", from_, to)
+
+    parts = ([".."] * len(from_parts)) + to_parts
+    return "/".join(parts)
 
 
 # Create a venv the executable can use.
@@ -391,7 +395,7 @@ def _create_venv(ctx, output_prefix, imports, runtime_details):
         interpreter = ctx.actions.declare_symlink("{}/bin/{}".format(venv, py_exe_basename))
         interpreter_actual_path = runtime.interpreter.short_path
         venv_bin_dir = paths.dirname(interpreter.short_path)
-        rel_path = _relative_path(from_=venv_bin_dir, to=interpreter_actual_path)
+        rel_path = relative_path(from_=venv_bin_dir, to=interpreter_actual_path)
         ctx.actions.symlink(output = interpreter, target_path = rel_path)
     else:
         py_exe_basename = paths.basename(runtime.interpreter_path)

tests/bootstrap_impls/BUILD.bazel

@@ -87,3 +87,7 @@ sh_py_run_test(
     sh_src = "sys_executable_inherits_sys_path_test.sh",
     target_compatible_with = _SUPPORTS_BOOTSTRAP_SCRIPT,
 )
+
+load(":venv_relative_path_tests.bzl", "relative_path_test_suite")
+
+relative_path_test_suite("relative_path_test")

tests/bootstrap_impls/venv_relative_path_tests.bzl

@@ -0,0 +1,121 @@
+# Copyright 2023 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.
+
+"Unit tests for yaml.bzl"
+
+load("@bazel_skylib//lib:unittest.bzl", "analysistest", "asserts", "unittest")
+load("//python/private:py_executable_bazel.bzl", "relative_path")  # buildifier: disable=bzl-visibility
+
+def _relative_path_test_impl(ctx):
+    env = unittest.begin(ctx)
+
+    # Basic test cases
+
+    asserts.equals(
+        env,
+        "../../c/d",
+        relative_path(
+            from_ = "a/b",
+            to = "c/d",
+        ),
+    )
+
+    asserts.equals(
+        env,
+        "../../c/d",
+        relative_path(
+            from_ = "../a/b",
+            to = "../c/d",
+        ),
+    )
+
+    asserts.equals(
+        env,
+        "../../../c/d",
+        relative_path(
+            from_ = "../a/b",
+            to = "../../c/d",
+        ),
+    )
+
+    asserts.equals(
+        env,
+        "../../d",
+        relative_path(
+            from_ = "a/b/c",
+            to = "a/d",
+        ),
+    )
+
+    asserts.equals(
+        env,
+        "d/e",
+        relative_path(
+            from_ = "a/b/c",
+            to = "a/b/c/d/e",
+        ),
+    )
+
+    # Real examples
+
+    # external py_binary uses external python runtime
+    asserts.equals(
+        env,
+        "../../../../../rules_python~~python~python_3_9_x86_64-unknown-linux-gnu/bin/python3",
+        relative_path(
+            from_ = "../rules_python~/python/private/_py_console_script_gen_py.venv/bin",
+            to = "../rules_python~~python~python_3_9_x86_64-unknown-linux-gnu/bin/python3",
+        ),
+    )
+
+    # internal py_binary uses external python runtime
+    asserts.equals(
+        env,
+        "../../../../rules_python~~python~python_3_9_x86_64-unknown-linux-gnu/bin/python3",
+        relative_path(
+            from_ = "test/version_default.venv/bin",
+            to = "../rules_python~~python~python_3_9_x86_64-unknown-linux-gnu/bin/python3",
+        ),
+    )
+
+    # external py_binary uses internal python runtime
+    # asserts.equals(
+    #    env,
+    #    "???",
+    #    relative_path(
+    #        from_ = "../rules_python~/python/private/_py_console_script_gen_py.venv/bin",
+    #        to = "python/python_3_9_x86_64-unknown-linux-gnu/bin/python3",
+    #    ),
+    #)
+    # ^ TODO: Technically we can infer ".." to be the workspace name?
+
+    # internal py_binary uses internal python runtime
+    asserts.equals(
+        env,
+        "../../../python/python_3_9_x86_64-unknown-linux-gnu/bin/python3",
+        relative_path(
+            from_ = "scratch/main.venv/bin",
+            to = "python/python_3_9_x86_64-unknown-linux-gnu/bin/python3",
+        ),
+    )
+
+    return unittest.end(env)
+
+relative_path_test = unittest.make(
+    _relative_path_test_impl,
+    attrs = {},
+)
+
+def relative_path_test_suite(name):
+    unittest.suite(name, relative_path_test)