Fix the autodetecting Python toolchain on Mac

See bazelbuild/continuous-integration#578 for context. The gist of it is that when PATH is not set (as happens when a binary using this toolchain is used as a tool from Starlark), the shell comes up with its own PATH to run the pywrapper script. However, it does not necessarily export this PATH, causing "which" to misbehave and fail to locate a Python interpreter.

The fix is to add "export PATH" and a regression test.

Fixes bazelbuild/continuous-integration#578, work toward #7899.

RELNOTES: None
PiperOrigin-RevId: 249263849

src/test/shell/bazel/python_version_test.sh

@@ -52,18 +52,12 @@ case "$(uname -s | tr [:upper:] [:lower:])" in
 msys*)
   # As of 2018-08-14, Bazel on Windows only supports MSYS Bash.
   declare -r is_windows=true
-  # As of 2019-04-26, this test is disabled on Windows (via "no_windows" tag),
+  # As of 2019-05-18, this test is disabled on Windows (via "no_windows" tag),
   # so this code shouldn't even have run.
-  # TODO(bazelbuild/continuous-integration#578): Enable this test for Windows.
+  # TODO(#7844): Enable this test for Windows once our autodetecting toolchain
+  # works transparently for this platform.
   fail "This test does not run on Windows."
   ;;
-darwin*)
-  # As of 2019-04-26, this test is disabled on mac, but there's no "no_mac" tag
-  # so we just have to trivially succeed.
-  # TODO(bazelbuild/continuous-integration#578): Enable this test for Mac.
-  echo "This test does not run on Mac; exiting early." >&2
-  exit 0
-  ;;
 *)
   declare -r is_windows=false
   ;;
@@ -583,4 +577,129 @@ function test_default_output_root_is_bazel_bin() {
   expect_not_log "bazel-out/.*-py3.*/bin"
 }
 
+# Tests that we get a warning when host tools fail due to being built for the
+# wrong Python version. See #7899 for context.
+# TODO(#6443): Delete this once we no longer have the host configuration.
+function test_host_version_mismatch_warning() {
+  mkdir -p test
+
+  cat > test/BUILD << 'EOF'
+py_binary(
+    name = "success_tool",
+    srcs = ["success_tool.py"],
+    python_version = "PY2",
+)
+
+py_binary(
+    name = "fail_tool",
+    srcs = ["fail_tool.py"],
+    python_version = "PY2",
+)
+
+genrule(
+    name = "success_gen",
+    cmd = "$(location :success_tool) > $@",
+    tools = [":success_tool"],
+    outs = ["success_out.txt"],
+)
+
+genrule(
+    name = "fail_gen",
+    cmd = "$(location :fail_tool) > $@",
+    tools = [":fail_tool"],
+    outs = ["fail_out.txt"],
+)
+EOF
+  cat > test/success_tool.py << EOF
+print("Successfully did nothing.")
+EOF
+  cat > test/fail_tool.py << EOF
+import sys
+sys.exit(1)
+EOF
+
+  # The warning should be present if the tool
+  #   1) was built in the host config,
+  #   2) with the wrong version,
+  #   3) with toolchains enabled,
+  #   4) and it failed during execution.
+  bazel build //test:fail_gen \
+      --incompatible_use_python_toolchains=true --host_force_python=PY3 \
+      &> $TEST_log && fail "bazel build succeeded (expected failure)"
+  expect_log "it is a Python 2 program that was built in the host \
+configuration, which uses Python 3"
+
+  # No warning if there was no version mismatch.
+  bazel build //test:fail_gen \
+      --incompatible_use_python_toolchains=true --host_force_python=PY2 \
+      &> $TEST_log && fail "bazel build succeeded (expected failure)"
+  expect_not_log "program that was built in the host configuration"
+
+  # No warning if toolchains are disabled.
+  bazel build //test:fail_gen \
+      --incompatible_use_python_toolchains=false --host_force_python=PY2 \
+      &> $TEST_log && fail "bazel build succeeded (expected failure)"
+  expect_not_log "program that was built in the host configuration"
+
+  # No warning if it succeeded during execution.
+  bazel build //test:success_gen \
+      --incompatible_use_python_toolchains=true --host_force_python=PY2 \
+      &> $TEST_log || fail "bazel build failed (expected success)"
+  expect_not_log "program that was built in the host configuration"
+}
+
+# Regression test for (bazelbuild/continuous-integration#578): Ensure that a
+# py_binary built with the autodetecting toolchain works when used as a tool
+# from Starlark rule. In particular, the wrapper script that launches the real
+# second-stage interpreter must be able to tolerate PATH not being set.
+function test_py_binary_with_autodetecting_toolchain_usable_as_tool() {
+  mkdir -p test
+
+  cat > test/BUILD << 'EOF'
+load(":tooluser.bzl", "tooluser_rule")
+
+py_binary(
+    name = "tool",
+    srcs = ["tool.py"],
+)
+
+tooluser_rule(
+    name = "tooluser",
+    out = "out.txt",
+)
+EOF
+  cat > test/tooluser.bzl << EOF
+def _tooluser_rule_impl(ctx):
+    ctx.actions.run(
+        inputs = [],
+        outputs = [ctx.outputs.out],
+        executable = ctx.executable._tool,
+        arguments = [ctx.outputs.out.path],
+    )
+
+tooluser_rule = rule(
+    implementation = _tooluser_rule_impl,
+    attrs = {
+        "_tool": attr.label(
+            executable = True,
+            default = "//test:tool",
+            # cfg param is required but its value doesn't matter
+            cfg = "target"),
+        "out": attr.output(),
+    },
+)
+EOF
+  cat > test/tool.py << EOF
+import sys
+with open(sys.argv[1], 'wt') as out:
+    print("Tool output", file=out)
+EOF
+
+  bazel build //test:tooluser \
+      --incompatible_use_python_toolchains=true \
+      || fail "bazel build failed"
+  cat bazel-bin/test/out.txt &> $TEST_log
+  expect_log "Tool output"
+}
+
 run_suite "Tests for how the Python rules handle Python 2 vs Python 3"

tools/python/pywrapper_template.txt

@@ -18,6 +18,13 @@ die() {
     exit 1
 }
 
+# Make sure PATH is exported. If we're called with PATH unset, as happens when
+# we're invoked as a tool during the build, the shell will initialize its own
+# PATH but not necessarily export it. This would break our call to `which`. See
+# https://github.com/bazelbuild/continuous-integration/issues/578 for more
+# information.
+export PATH
+
 # Try the "python%VERSION%" command name first, then fall back on "python".
 PYTHON_BIN=`which python%VERSION% || echo ""`
 USED_FALLBACK=0