cmd/go/internal/test: ensure that build.ToolDir is accurate in tests

This fixes a build failure due to inability to locate the "vet" tool
when the test binary is built with -trimpath.

Updates #51461

Change-Id: I81838cc8842e4ff7900cab81af60501ebba86ff1
Reviewed-on: https://go-review.googlesource.com/c/go/+/391808
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/go/internal/cfg/cfg.go

@@ -87,16 +87,6 @@ func defaultContext() build.Context {
 
 	ctxt.JoinPath = filepath.Join // back door to say "do not use go command"
 
-	ctxt.GOROOT = findGOROOT()
-	if runtime.Compiler != "gccgo" {
-		// Note that we must use runtime.GOOS and runtime.GOARCH here,
-		// as the tool directory does not move based on environment
-		// variables. This matches the initialization of ToolDir in
-		// go/build, except for using ctxt.GOROOT rather than
-		// runtime.GOROOT.
-		build.ToolDir = filepath.Join(ctxt.GOROOT, "pkg/tool/"+runtime.GOOS+"_"+runtime.GOARCH)
-	}
-
 	// Override defaults computed in go/build with defaults
 	// from go environment configuration file, if known.
 	ctxt.GOPATH = envOr("GOPATH", gopath(ctxt))
@@ -146,10 +136,36 @@ func defaultContext() build.Context {
 }
 
 func init() {
+	SetGOROOT(findGOROOT())
 	BuildToolchainCompiler = func() string { return "missing-compiler" }
 	BuildToolchainLinker = func() string { return "missing-linker" }
 }
 
+func SetGOROOT(goroot string) {
+	BuildContext.GOROOT = goroot
+
+	GOROOT = goroot
+	if goroot == "" {
+		GOROOTbin = ""
+		GOROOTpkg = ""
+		GOROOTsrc = ""
+	} else {
+		GOROOTbin = filepath.Join(goroot, "bin")
+		GOROOTpkg = filepath.Join(goroot, "pkg")
+		GOROOTsrc = filepath.Join(goroot, "src")
+	}
+	GOROOT_FINAL = findGOROOT_FINAL(goroot)
+
+	if runtime.Compiler != "gccgo" && goroot != "" {
+		// Note that we must use runtime.GOOS and runtime.GOARCH here,
+		// as the tool directory does not move based on environment
+		// variables. This matches the initialization of ToolDir in
+		// go/build, except for using BuildContext.GOROOT rather than
+		// runtime.GOROOT.
+		build.ToolDir = filepath.Join(goroot, "pkg/tool/"+runtime.GOOS+"_"+runtime.GOARCH)
+	}
+}
+
 // Experiment configuration.
 var (
 	// RawGOEXPERIMENT is the GOEXPERIMENT value set by the user.
@@ -279,12 +295,12 @@ func CanGetenv(key string) bool {
 }
 
 var (
-	GOROOT       = BuildContext.GOROOT
+	GOROOT       string
+	GOROOTbin    string
+	GOROOTpkg    string
+	GOROOTsrc    string
+	GOROOT_FINAL string
 	GOBIN        = Getenv("GOBIN")
-	GOROOTbin    = filepath.Join(GOROOT, "bin")
-	GOROOTpkg    = filepath.Join(GOROOT, "pkg")
-	GOROOTsrc    = filepath.Join(GOROOT, "src")
-	GOROOT_FINAL = findGOROOT_FINAL()
 	GOMODCACHE   = envOr("GOMODCACHE", gopathDir("pkg/mod"))
 
 	// Used in envcmd.MkEnv and build ID computations.
@@ -386,10 +402,10 @@ func findGOROOT() string {
 	return def
 }
 
-func findGOROOT_FINAL() string {
+func findGOROOT_FINAL(goroot string) string {
 	// $GOROOT_FINAL is only for use during make.bash
 	// so it is not settable using go/env, so we use os.Getenv here.
-	def := GOROOT
+	def := goroot
 	if env := os.Getenv("GOROOT_FINAL"); env != "" {
 		def = filepath.Clean(env)
 	}

src/cmd/go/internal/test/flagdefs_test.go

@@ -5,13 +5,19 @@
 package test
 
 import (
+	"cmd/go/internal/cfg"
 	"cmd/go/internal/test/internal/genflags"
 	"flag"
+	"internal/testenv"
 	"reflect"
 	"strings"
 	"testing"
 )
 
+func TestMain(m *testing.M) {
+	cfg.SetGOROOT(testenv.GOROOT(nil))
+}
+
 func TestPassFlagToTestIncludesAllTestFlags(t *testing.T) {
 	flag.VisitAll(func(f *flag.Flag) {
 		if !strings.HasPrefix(f.Name, "test.") {