cmd/go, testing: add TestMain support

Fixes #8202.

LGTM=r, bradfitz
R=r, josharian, bradfitz
CC=golang-codereviews
https://golang.org/cl/148770043

2 files changed, 69 insertions, 5 deletions

diff --git a/src/testing/testing.go b/src/testing/testing.go
index 731762cb1d..21460b0ed4 100644
--- a/src/testing/testing.go
+++ b/src/testing/testing.go
@@ -117,6 +117,26 @@
 // The entire test file is presented as the example when it contains a single
 // example function, at least one other function, type, variable, or constant
 // declaration, and no test or benchmark functions.
+//
+// Main
+//
+// It is sometimes necessary for a test program to do extra setup or teardown
+// before or after testing. It is also sometimes necessary for a test to control
+// which code runs on the main thread. To support these and other cases,
+// if a test file contains a function:
+//
+//	func TestMain(m *testing.M)
+//
+// then the generated test will call TestMain(m) instead of running the tests
+// directly. TestMain runs in the main goroutine and can do whatever setup
+// and teardown is necessary around a call to m.Run. It should then call
+// os.Exit with the result of m.Run.
+//
+// The minimal implementation of TestMain is:
+//
+//	func TestMain(m *testing.M) { os.Exit(m.Run()) }
+//
+// In effect, that is the implementation used when no TestMain is explicitly defined.
 package testing
 
 import (
@@ -431,23 +451,49 @@ func tRunner(t *T, test *InternalTest) {
 // An internal function but exported because it is cross-package; part of the implementation
 // of the "go test" command.
 func Main(matchString func(pat, str string) (bool, error), tests []InternalTest, benchmarks []InternalBenchmark, examples []InternalExample) {
+	os.Exit(MainStart(matchString, tests, benchmarks, examples).Run())
+}
+
+// M is a type passed to a TestMain function to run the actual tests.
+type M struct {
+	matchString func(pat, str string) (bool, error)
+	tests       []InternalTest
+	benchmarks  []InternalBenchmark
+	examples    []InternalExample
+}
+
+// MainStart is meant for use by tests generated by 'go test'.
+// It is not meant to be called directly and is not subject to the Go 1 compatibility document.
+// It may change signature from release to release.
+func MainStart(matchString func(pat, str string) (bool, error), tests []InternalTest, benchmarks []InternalBenchmark, examples []InternalExample) *M {
+	return &M{
+		matchString: matchString,
+		tests:       tests,
+		benchmarks:  benchmarks,
+		examples:    examples,
+	}
+}
+
+// Run runs the tests. It returns an exit code to pass to os.Exit.
+func (m *M) Run() int {
 	flag.Parse()
 	parseCpuList()
 
 	before()
 	startAlarm()
-	haveExamples = len(examples) > 0
-	testOk := RunTests(matchString, tests)
-	exampleOk := RunExamples(matchString, examples)
+	haveExamples = len(m.examples) > 0
+	testOk := RunTests(m.matchString, m.tests)
+	exampleOk := RunExamples(m.matchString, m.examples)
 	stopAlarm()
 	if !testOk || !exampleOk {
 		fmt.Println("FAIL")
 		after()
-		os.Exit(1)
+		return 1
 	}
 	fmt.Println("PASS")
-	RunBenchmarks(matchString, benchmarks)
+	RunBenchmarks(m.matchString, m.benchmarks)
 	after()
+	return 0
 }
 
 func (t *T) report() {
diff --git a/src/testing/testing_test.go b/src/testing/testing_test.go
new file mode 100644
index 0000000000..87a5c16d6e
--- /dev/null
+++ b/src/testing/testing_test.go
@@ -0,0 +1,18 @@
+// Copyright 2014 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package testing_test
+
+import (
+	"os"
+	"testing"
+)
+
+// This is exactly what a test would do without a TestMain.
+// It's here only so that there is at least one package in the
+// standard library with a TestMain, so that code is executed.
+
+func TestMain(m *testing.M) {
+	os.Exit(m.Run())
+}