cmd/go: diagnose unset GOROOT when built with -trimpath

For #51483

Change-Id: I4546c20cf968b595020a1eba888fe1d9a1c6cfc5
Reviewed-on: https://go-review.googlesource.com/c/go/+/391811
Reviewed-by: Russ Cox <rsc@golang.org>
Trust: Bryan Mills <bcmills@google.com>

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

@@ -370,7 +370,10 @@ func findGOROOT() string {
 	if env := Getenv("GOROOT"); env != "" {
 		return filepath.Clean(env)
 	}
-	def := filepath.Clean(runtime.GOROOT())
+	def := ""
+	if r := runtime.GOROOT(); r != "" {
+		def = filepath.Clean(r)
+	}
 	if runtime.Compiler == "gccgo" {
 		// gccgo has no real GOROOT, and it certainly doesn't
 		// depend on the executable's location.

src/cmd/go/main.go

@@ -142,6 +142,10 @@ func main() {
 		}
 	}
 
+	if cfg.GOROOT == "" {
+		fmt.Fprintf(os.Stderr, "go: cannot find GOROOT directory: 'go' binary is trimmed and GOROOT is not set\n")
+		os.Exit(2)
+	}
 	if fi, err := os.Stat(cfg.GOROOT); err != nil || !fi.IsDir() {
 		fmt.Fprintf(os.Stderr, "go: cannot find GOROOT directory: %v\n", cfg.GOROOT)
 		os.Exit(2)

src/cmd/go/script_test.go

@@ -22,6 +22,7 @@ import (
 	"path/filepath"
 	"regexp"
 	"runtime"
+	"runtime/debug"
 	"strconv"
 	"strings"
 	"sync"
@@ -373,6 +374,17 @@ Script:
 				ok = testenv.HasSymlink()
 			case "case-sensitive":
 				ok = isCaseSensitive(ts.t)
+			case "trimpath":
+				if info, _ := debug.ReadBuildInfo(); info == nil {
+					ts.fatalf("missing build info")
+				} else {
+					for _, s := range info.Settings {
+						if s.Key == "-trimpath" && s.Value == "true" {
+							ok = true
+							break
+						}
+					}
+				}
 			default:
 				if strings.HasPrefix(cond.tag, "exec:") {
 					prog := cond.tag[len("exec:"):]

src/cmd/go/testdata/script/build_trimpath_goroot.txt

@@ -8,11 +8,16 @@
 # TODO(#51483): when runtime.GOROOT() returns the empty string,
 # go/build should default to 'go env GOROOT' instead.
 
-[short] skip
-
 env GOROOT=
 env GOROOT_FINAL=
 
+[trimpath] ! go env GOROOT
+[trimpath] stderr '^go: cannot find GOROOT directory: ''go'' binary is trimmed and GOROOT is not set$'
+[trimpath] stop
+
+
+[short] skip
+
 go run .
 stdout '^GOROOT '$TESTGO_GOROOT'$'
 stdout '^runtime '$TESTGO_GOROOT${/}src${/}runtime'$'