testing: add Testing function

The Testing function reports whether the program is a test created
by "go test".

Fixes #52600

Change-Id: Ie0fff7c7dfdfdf997c18b4b6112632600b327cc8
Reviewed-on: https://go-review.googlesource.com/c/go/+/475496
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

api/next/52600.txt

@@ -0,0 +1 @@
+pkg testing, func Testing() bool #52600

src/cmd/go/internal/load/test.go

@@ -253,6 +253,10 @@ func TestPackagesAndErrors(ctx context.Context, opts PackageOpts, p *Package, co
 		pxtest.collectDeps()
 	}
 
+	// Arrange for testing.Testing to report true.
+	ldflags := append(p.Internal.Ldflags, "-X", "testing.testBinary=1")
+	gccgoflags := append(p.Internal.Gccgoflags, "-Wl,--defsym,testing.gccgoTestBinary=1")
+
 	// Build main package.
 	pmain = &Package{
 		PackagePublic: PackagePublic{
@@ -269,8 +273,8 @@ func TestPackagesAndErrors(ctx context.Context, opts PackageOpts, p *Package, co
 			BuildInfo:      p.Internal.BuildInfo,
 			Asmflags:       p.Internal.Asmflags,
 			Gcflags:        p.Internal.Gcflags,
-			Ldflags:        p.Internal.Ldflags,
+			Ldflags:        ldflags,
-			Gccgoflags:     p.Internal.Gccgoflags,
+			Gccgoflags:     gccgoflags,
 			OrigImportPath: p.Internal.OrigImportPath,
 			PGOProfile:     p.Internal.PGOProfile,
 		},

src/testing/testing.go

@@ -644,6 +644,22 @@ func Short() bool {
 	return *short
 }
 
+// testBinary is set by cmd/go to "1" if this is a binary built by "go test".
+// The value is set to "1" by a -X option to cmd/link. We assume that
+// because this is possible, the compiler will not optimize testBinary
+// into a constant on the basis that it is an unexported package-scope
+// variable that is never changed. If the compiler ever starts implementing
+// such an optimization, we will need some technique to mark this variable
+// as "changed by a cmd/link -X option".
+var testBinary = "0"
+
+// Testing reports whether the current code is being run in a test.
+// This will report true in programs created by "go test",
+// false in programs created by "go build".
+func Testing() bool {
+	return testBinary == "1"
+}
+
 // CoverMode reports what the test coverage mode is set to. The
 // values are "set", "count", or "atomic". The return value will be
 // empty if test coverage is not enabled.

src/testing/testing_test.go

@@ -5,6 +5,8 @@
 package testing_test
 
 import (
+	"bytes"
+	"internal/testenv"
 	"os"
 	"path/filepath"
 	"testing"
@@ -232,3 +234,62 @@ func TestSetenvWithParallelGrandParentBeforeSetenv(t *testing.T) {
 		})
 	})
 }
+
+// testingTrueInInit is part of TestTesting.
+var testingTrueInInit = false
+
+// testingTrueInPackageVarInit is part of TestTesting.
+var testingTrueInPackageVarInit = testing.Testing()
+
+// init is part of TestTesting.
+func init() {
+	if testing.Testing() {
+		testingTrueInInit = true
+	}
+}
+
+var testingProg = `
+package main
+
+import (
+	"fmt"
+	"testing"
+)
+
+func main() {
+	fmt.Println(testing.Testing())
+}
+`
+
+func TestTesting(t *testing.T) {
+	if !testing.Testing() {
+		t.Errorf("testing.Testing() == %t, want %t", testing.Testing(), true)
+	}
+	if !testingTrueInInit {
+		t.Errorf("testing.Testing() called by init function == %t, want %t", testingTrueInInit, true)
+	}
+	if !testingTrueInPackageVarInit {
+		t.Errorf("testing.Testing() variable initialized as %t, want %t", testingTrueInPackageVarInit, true)
+	}
+
+	if testing.Short() {
+		t.Skip("skipping building a binary in short mode")
+	}
+	testenv.MustHaveGoRun(t)
+
+	fn := filepath.Join(t.TempDir(), "x.go")
+	if err := os.WriteFile(fn, []byte(testingProg), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	cmd := testenv.Command(t, testenv.GoToolPath(t), "run", fn)
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("%v failed: %v\n%s", cmd, err, out)
+	}
+
+	s := string(bytes.TrimSpace(out))
+	if s != "false" {
+		t.Errorf("in non-test testing.Test() returned %q, want %q", s, "false")
+	}
+}