go_test: don't require TestMain to call os.Exit (#2716)

Based on golang/go#34129 and https://golang.org/cl/219639

go/tools/builders/generate_test_main.go

@@ -69,6 +69,9 @@ import (
 	"os/exec"
 	"path/filepath"
 	"runtime"
+{{if .TestMain}}
+	"reflect"
+{{end}}
 	"strconv"
 	"testing"
 	"testing/internal/testdeps"
@@ -169,6 +172,8 @@ func main() {
 	os.Exit(m.Run())
 	{{else}}
 	{{.TestMain}}(m)
+	{{/* See golang.org/issue/34129 and golang.org/cl/219639 */}}
+	os.Exit(int(reflect.ValueOf(m).Elem().FieldByName("exitCode").Int()))
 	{{end}}
 }
 `

tests/core/go_test/BUILD.bazel

@@ -189,3 +189,9 @@ go_library(
     importpath = "github.com/bazelbuild/rules_go/tests/core/go_test/indirect_import_dep",
     deps = [":indirect_import_lib"],
 )
+
+go_bazel_test(
+    name = "testmain_without_exit_test",
+    srcs = ["testmain_without_exit_test.go"],
+)
+

tests/core/go_test/README.rst

@@ -3,6 +3,7 @@ Basic go_test functionality
 
 .. _go_test: /go/core.rst#_go_test
 .. _#1877: https://github.com/bazelbuild/rules_go/issues/1877
+.. _#34129: https:////github.com/golang/go/issues/34129
 
 Tests to ensure that basic features of `go_test`_ are working as expected.
 
@@ -97,3 +98,9 @@ indirect_import_test
 Checks that an external test can import another package that imports the library
 under test. The other package should be compiled against the internal test
 package, not the library under test. Verifies `#1877`_.
+
+testmain_without_exit
+---------------------
+
+Checks that TestMain without calling os.Exit directly works.
+Verifies `#34129`_ from Go 1.15.

tests/core/go_test/testmain_without_exit_test.go

@@ -0,0 +1,59 @@
+// Copyright 2020 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 testmain_without_exit
+
+import (
+	"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//go:def.bzl", "go_test")
+
+go_test(
+    name = "main_without_exit_test",
+    srcs = ["main_without_exit_test.go"],
+)
+
+-- main_without_exit_test.go --
+package test_main_without_exit
+
+import "testing"
+
+func TestMain(m *testing.M) {
+	m.Run()
+}
+
+func TestShouldFail(t *testing.T) {
+	t.Fail()
+}
+`,
+	})
+}
+
+func Test(t *testing.T) {
+	err := bazel_testing.RunBazel("test", "//:main_without_exit_test")
+	if err == nil {
+		t.Fatal("expected bazel test to have failed")
+	}
+
+	if xerr, ok := err.(*bazel_testing.StderrExitError); !ok || xerr.Err.ExitCode() != 3 {
+		t.Fatalf("expected bazel tests to fail with exit code 3 (TESTS_FAILED), got: %s", err)
+	}
+}