Add go_bazel_test for transition hermeticity on go_proto_library

The test verifies that Go Starlark settings such as bootstrap_nogo and
pure are reset before they reach protoc, which is a dependency of
go_proto_library in multiple ways.

tests/core/transition/BUILD.bazel

@@ -5,3 +5,9 @@ go_bazel_test(
     size = "medium",
     srcs = ["cmdline_test.go"],
 )
+
+go_bazel_test(
+    name = "hermeticity_test",
+    size = "medium",
+    srcs = ["hermeticity_test.go"],
+)

tests/core/transition/hermeticity_test.go

@@ -0,0 +1,175 @@
+// Copyright 2021 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.
+
+package hermeticity_test
+
+import (
+	"bytes"
+	"fmt"
+	"strings"
+	"testing"
+
+	"github.com/bazelbuild/rules_go/go/tools/bazel_testing"
+)
+
+func TestMain(m *testing.M) {
+	bazel_testing.TestMain(m, bazel_testing.Args{
+		Main: `
+-- BUILD.bazel --
+load("@io_bazel_rules_go//proto:def.bzl", "go_proto_library")
+load("@rules_proto//proto:defs.bzl", "proto_library")
+
+proto_library(
+    name = "foo_proto",
+    srcs = ["foo.proto"],
+)
+
+go_proto_library(
+    name = "foo_go_proto",
+    importpath = "github.com/bazelbuild/rules_go/tests/core/transition/foo",
+    proto = ":foo_proto",
+)
+-- foo.proto --
+syntax = "proto3";
+
+package tests.core.transition.foo;
+option go_package = "github.com/bazelbuild/rules_go/tests/core/transition/foo";
+
+message Foo {
+  int64 value = 1;
+}
+`,
+		WorkspaceSuffix: `
+load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
+
+http_archive(
+    name = "com_google_protobuf",
+    sha256 = "a79d19dcdf9139fa4b81206e318e33d245c4c9da1ffed21c87288ed4380426f9",
+    strip_prefix = "protobuf-3.11.4",
+    # latest, as of 2020-02-21
+    urls = [
+        "https://mirror.bazel.build/github.com/protocolbuffers/protobuf/archive/v3.11.4.tar.gz",
+        "https://github.com/protocolbuffers/protobuf/archive/v3.11.4.tar.gz",
+    ],
+)
+
+load("@com_google_protobuf//:protobuf_deps.bzl", "protobuf_deps")
+
+protobuf_deps()
+
+http_archive(
+    name = "rules_proto",
+    sha256 = "4d421d51f9ecfe9bf96ab23b55c6f2b809cbaf0eea24952683e397decfbd0dd0",
+    strip_prefix = "rules_proto-f6b8d89b90a7956f6782a4a3609b2f0eee3ce965",
+    # master, as of 2020-01-06
+    urls = [
+        "https://mirror.bazel.build/github.com/bazelbuild/rules_proto/archive/f6b8d89b90a7956f6782a4a3609b2f0eee3ce965.tar.gz",
+        "https://github.com/bazelbuild/rules_proto/archive/f6b8d89b90a7956f6782a4a3609b2f0eee3ce965.tar.gz",
+    ],
+)
+`,
+	})
+}
+
+func TestGoProtoLibraryToolAttrsAreReset(t *testing.T) {
+	assertDependsCleanlyOn(t, "//:foo_go_proto", "@com_google_protobuf//:protoc")
+}
+
+func assertDependsCleanlyOn(t *testing.T, targetA, targetB string) {
+	assertDependsCleanlyOnWithFlags(
+		t,
+		targetA,
+		targetB,
+		"--@io_bazel_rules_go//go/config:static",
+		"--@io_bazel_rules_go//go/config:msan",
+		"--@io_bazel_rules_go//go/config:race",
+		"--@io_bazel_rules_go//go/config:debug",
+		"--@io_bazel_rules_go//go/config:linkmode=c-archive",
+		"--@io_bazel_rules_go//go/config:tags=fake_tag",
+	)
+	assertDependsCleanlyOnWithFlags(
+		t,
+		targetA,
+		targetB,
+		"--@io_bazel_rules_go//go/config:pure",
+	)
+}
+
+func assertDependsCleanlyOnWithFlags(t *testing.T, targetA, targetB string, flags ...string) {
+	query := fmt.Sprintf("deps(%s) intersect %s", targetA, targetB)
+	out, err := bazel_testing.BazelOutput(append(
+		[]string{
+			"cquery",
+			query,
+		},
+		flags...,
+	)...,
+	)
+	if err != nil {
+		t.Fatalf("bazel cquery '%s': %v", query, err)
+	}
+	cqueryOut := string(bytes.TrimSpace(out))
+	configHashes := extractConfigHashes(t, cqueryOut)
+	if len(configHashes) != 1 {
+		t.Fatalf(
+			"%s depends on %s in multiple configs with these differences in rules_go options: %s",
+			targetA,
+			targetB,
+			strings.Join(getGoOptions(t, configHashes...), "\n"),
+		)
+	}
+	goOptions := getGoOptions(t, configHashes[0])
+	if len(goOptions) != 0 {
+		t.Fatalf(
+			"%s depends on %s in a config with rules_go options: %s",
+			targetA,
+			targetB,
+			strings.Join(goOptions, "\n"),
+		)
+	}
+}
+
+func extractConfigHashes(t *testing.T, cqueryOut string) []string {
+	lines := strings.Split(cqueryOut, "\n")
+	var hashes []string
+	for _, line := range lines {
+		openingParens := strings.Index(line, "(")
+		closingParens := strings.Index(line, ")")
+		if openingParens == -1 || closingParens <= openingParens {
+			t.Fatalf("failed to find config hash in cquery out line: %s", line)
+		}
+		hashes = append(hashes, line[openingParens+1:closingParens])
+	}
+	return hashes
+}
+
+func getGoOptions(t *testing.T, hashes ...string) []string {
+	out, err := bazel_testing.BazelOutput(append([]string{"config"}, hashes...)...)
+	if err != nil {
+		t.Fatalf("bazel config %s: %v", strings.Join(hashes, " "), err)
+	}
+	lines := strings.Split(string(bytes.TrimSpace(out)), "\n")
+	differingGoOptions := make([]string, 0)
+	for _, line := range lines {
+		// Lines with configuration options are indented
+		if !strings.HasPrefix(line, "  ") {
+			continue
+		}
+		optionAndValue := strings.TrimLeft(line, " ")
+		if strings.HasPrefix(optionAndValue, "@io_bazel_rules_go//") {
+			differingGoOptions = append(differingGoOptions, optionAndValue)
+		}
+	}
+	return differingGoOptions
+}