all: add -mod=vendor to GOFLAGS in tests that execute 'go' commands within std or cmd

Updates #30228
Updates #30240
Updates #30241

Change-Id: Idc311ba77e99909318b5b86f8ef82d4878f73e47
Reviewed-on: https://go-review.googlesource.com/c/go/+/165378
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/addr2line/addr2line_test.go

@@ -115,3 +115,5 @@ func TestAddr2Line(t *testing.T) {
 	testAddr2Line(t, exepath, syms[symName])
 	testAddr2Line(t, exepath, "0x"+syms[symName])
 }
+
+func init() { testenv.SetModVendor() }

src/cmd/compile/internal/gc/scope_test.go

@@ -202,6 +202,8 @@ var testfile = []testline{
 
 const detailOutput = false
 
+func init() { testenv.SetModVendor() }
+
 // Compiles testfile checks that the description of lexical blocks emitted
 // by the linker in debug_info, for each function in the main package,
 // corresponds to what we expect it to be.

src/cmd/cover/cover_test.go

@@ -77,6 +77,8 @@ var debug = flag.Bool("debug", false, "keep rewritten files for debugging")
 // We use TestMain to set up a temporary directory and remove it when
 // the tests are done.
 func TestMain(m *testing.M) {
+	testenv.SetModVendor()
+
 	dir, err := ioutil.TempDir("", "gotestcover")
 	if err != nil {
 		fmt.Fprintln(os.Stderr, err)

src/cmd/go/go_test.go

@@ -212,7 +212,9 @@ func TestMain(m *testing.M) {
 			return
 		}
 
-		out, err := exec.Command(gotool, args...).CombinedOutput()
+		buildCmd := exec.Command(gotool, args...)
+		buildCmd.Env = append(os.Environ(), "GOFLAGS=-mod=vendor")
+		out, err := buildCmd.CombinedOutput()
 		if err != nil {
 			fmt.Fprintf(os.Stderr, "building testgo failed: %v\n%s", err, out)
 			os.Exit(2)

src/cmd/internal/obj/x86/obj6_test.go

@@ -134,6 +134,8 @@ func parseOutput(t *testing.T, td *ParsedTestData, asmout []byte) {
 	}
 }
 
+func init() { testenv.SetModVendor() }
+
 func TestDynlink(t *testing.T) {
 	testenv.MustHaveGoBuild(t)
 

src/cmd/link/internal/ld/ld_test.go

@@ -13,6 +13,8 @@ import (
 	"testing"
 )
 
+func init() { testenv.SetModVendor() }
+
 func TestUndefinedRelocErrors(t *testing.T) {
 	t.Parallel()
 	testenv.MustHaveGoBuild(t)

src/cmd/link/link_test.go

@@ -38,6 +38,8 @@ func TestLargeSymName(t *testing.T) {
 	_ = AuthorPaidByTheColumnInch
 }
 
+func init() { testenv.SetModVendor() }
+
 func TestIssue21703(t *testing.T) {
 	t.Parallel()
 

src/cmd/nm/nm_test.go

@@ -30,6 +30,8 @@ func testMain(m *testing.M) int {
 		return 0
 	}
 
+	testenv.SetModVendor()
+
 	tmpDir, err := ioutil.TempDir("", "TestNM")
 	if err != nil {
 		fmt.Println("TempDir failed:", err)

src/cmd/objdump/objdump_test.go

@@ -24,6 +24,8 @@ func TestMain(m *testing.M) {
 	if !testenv.HasGoBuild() {
 		return
 	}
+	testenv.SetModVendor()
+
 	var exitcode int
 	if err := buildObjdump(); err == nil {
 		exitcode = m.Run()

src/cmd/vet/all/main.go

@@ -44,6 +44,7 @@ func main() {
 	log.SetPrefix("vet/all: ")
 	log.SetFlags(0)
 
+	testenv.SetModVendor()
 	var err error
 	cmdGoPath, err = testenv.GoTool()
 	if err != nil {

src/cmd/vet/vet_test.go

@@ -32,6 +32,8 @@ func TestMain(m *testing.M) {
 }
 
 func testMain(m *testing.M) int {
+	testenv.SetModVendor()
+
 	dir, err := ioutil.TempDir("", "vet_test")
 	if err != nil {
 		fmt.Fprintln(os.Stderr, err)

src/crypto/x509/x509_test.go

@@ -1146,6 +1146,8 @@ func TestParsePEMCRL(t *testing.T) {
 	// Can't check the signature here without a package cycle.
 }
 
+func init() { testenv.SetModVendor() }
+
 func TestImports(t *testing.T) {
 	testenv.MustHaveGoRun(t)
 

src/debug/gosym/pclntab_test.go

@@ -21,6 +21,8 @@ var (
 	pclinetestBinary string
 )
 
+func init() { testenv.SetModVendor() }
+
 func dotest(t *testing.T) {
 	testenv.MustHaveGoBuild(t)
 	// For now, only works on amd64 platforms.

src/go/importer/importer_test.go

@@ -16,6 +16,8 @@ import (
 	"testing"
 )
 
+func init() { testenv.SetModVendor() }
+
 func TestForCompiler(t *testing.T) {
 	testenv.MustHaveGoBuild(t)
 

src/go/internal/srcimporter/srcimporter_test.go

@@ -10,7 +10,6 @@ import (
 	"go/types"
 	"internal/testenv"
 	"io/ioutil"
-	"os"
 	"path"
 	"path/filepath"
 	"runtime"
@@ -19,22 +18,7 @@ import (
 	"time"
 )
 
-func TestMain(m *testing.M) {
-	// Add -mod=vendor to GOFLAGS to ensure that we don't fetch modules while importing std or cmd.
-	//
-	// TODO(golang.org/issue/30240): If we load go.mod files from vendor/
-	// automatically, this will probably no longer be necessary.
-	var goflags []string
-	for _, f := range strings.Fields(os.Getenv("GOFLAGS")) {
-		if !strings.HasPrefix(f, "-mod=") && !strings.HasPrefix(f, "--mod=") {
-			goflags = append(goflags, f)
-		}
-	}
-	goflags = append(goflags, "-mod=vendor")
-	os.Setenv("GOFLAGS", strings.Join(goflags, " "))
-
-	os.Exit(m.Run())
-}
+func init() { testenv.SetModVendor() }
 
 const maxTime = 2 * time.Second
 

src/internal/testenv/testenv.go

@@ -19,6 +19,7 @@ import (
 	"runtime"
 	"strconv"
 	"strings"
+	"sync"
 	"testing"
 )
 
@@ -77,6 +78,31 @@ func MustHaveGoRun(t testing.TB) {
 	}
 }
 
+var modVendorOnce sync.Once
+
+// SetModVendor adds the "-mod=vendor" flag to the GOFLAGS environment variable.
+// This allows tests whose working directories are within the cmd and std
+// modules to run ``go'' commands without accessing the network to load
+// dependencies modules.
+//
+// SetModVendor must be called before any test may read the GOFLAGS environment
+// variable.
+//
+// TODO(golang.org/issue/30240): If we load go.mod files from vendor/
+// automatically, this will probably no longer be necessary.
+func SetModVendor() {
+	modVendorOnce.Do(func() {
+		var goflags []string
+		for _, f := range strings.Fields(os.Getenv("GOFLAGS")) {
+			if !strings.HasPrefix(f, "-mod=") && !strings.HasPrefix(f, "--mod=") {
+				goflags = append(goflags, f)
+			}
+		}
+		goflags = append(goflags, "-mod=vendor")
+		os.Setenv("GOFLAGS", strings.Join(goflags, " "))
+	})
+}
+
 // GoToolPath reports the path to the Go tool.
 // It is a convenience wrapper around GoTool.
 // If the tool is unavailable GoToolPath calls t.Skip.

src/runtime/crash_test.go

@@ -25,6 +25,8 @@ import (
 var toRemove []string
 
 func TestMain(m *testing.M) {
+	testenv.SetModVendor()
+
 	status := m.Run()
 	for _, file := range toRemove {
 		os.RemoveAll(file)

src/runtime/pprof/proto_test.go

@@ -301,6 +301,8 @@ func TestProcSelfMaps(t *testing.T) {
 	})
 }
 
+func init() { testenv.SetModVendor() }
+
 // TestMapping checkes the mapping section of CPU profiles
 // has the HasFunctions field set correctly. If all PCs included
 // in the samples are successfully symbolized, the corresponding