cmd/link: avoid stamping runtime.defaultGOROOT when paths are being trimmed

Previously, runtime.GOROOT() would return the string "go" in a binary
build with -trimpath. This change stamps the empty string instead,
using a sentinel value passed from cmd/go that looks like the GOROOT
environment variable (either "$GOROOT" or "%GOROOT%", depending on the
platform).

Fixes #51461

Change-Id: I1f10ef2435016a7b6213bd8c547df911f7feeae7
Reviewed-on: https://go-review.googlesource.com/c/go/+/390024
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/work/gc.go

@@ -27,7 +27,7 @@ import (
 )
 
 // The 'path' used for GOROOT_FINAL when -trimpath is specified
-const trimPathGoRootFinal = "go"
+const trimPathGoRootFinal string = "$GOROOT"
 
 var runtimePackages = map[string]struct{}{
 	"internal/abi":            struct{}{},

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

@@ -0,0 +1,63 @@
+# Regression test for https://go.dev/issue/51461 and https://go.dev/issue/51483.
+#
+# When built with -trimpath, runtime.GOROOT() returned the bogus string "go"
+# if GOROOT was not set explicitly in the environment.
+# It should instead return the empty string, since we know that we don't
+# have a valid path to return.
+#
+# 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=
+
+go run .
+stdout '^GOROOT '$TESTGO_GOROOT'$'
+stdout '^runtime '$TESTGO_GOROOT${/}src${/}runtime'$'
+
+go test -v .
+stdout '^GOROOT '$TESTGO_GOROOT'$'
+stdout '^runtime '$TESTGO_GOROOT${/}src${/}runtime'$'
+
+! go run -trimpath .
+stdout '^GOROOT $'
+stderr '^package runtime is not in GOROOT \(src'${/}'runtime\)$'
+
+! go test -trimpath -v .
+stdout '^GOROOT $'
+stdout '^package runtime is not in GOROOT \(src'${/}'runtime\)$'
+
+-- go.mod --
+module example
+
+go 1.19
+-- main.go --
+package main
+
+import (
+	"fmt"
+	"go/build"
+	"os"
+	"runtime"
+)
+
+func main() {
+	fmt.Println("GOROOT", runtime.GOROOT())
+
+	p, err := build.Default.Import("runtime", "", build.FindOnly)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	fmt.Println("runtime", p.Dir)
+}
+-- main_test.go --
+package main
+
+import "testing"
+
+func TestMain(*testing.M) {
+	main()
+}

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

@@ -119,8 +119,13 @@ func Main(arch *sys.Arch, theArch Arch) {
 		}
 	}
 
-	final := gorootFinal()
-	addstrdata1(ctxt, "runtime.defaultGOROOT="+final)
+	if final := gorootFinal(); final == "$GOROOT" {
+		// cmd/go sets GOROOT_FINAL to the dummy value "$GOROOT" when -trimpath is set,
+		// but runtime.GOROOT() should return the empty string, not a bogus value.
+		// (See https://go.dev/issue/51461.)
+	} else {
+		addstrdata1(ctxt, "runtime.defaultGOROOT="+final)
+	}
 
 	buildVersion := buildcfg.Version
 	if goexperiment := buildcfg.Experiment.String(); goexperiment != "" {

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

@@ -804,7 +804,9 @@ func gorootFinal() string {
 func expandGoroot(s string) string {
 	const n = len("$GOROOT")
 	if len(s) >= n+1 && s[:n] == "$GOROOT" && (s[n] == '/' || s[n] == '\\') {
-		return filepath.ToSlash(filepath.Join(gorootFinal(), s[n:]))
+		if final := gorootFinal(); final != "" {
+			return filepath.ToSlash(filepath.Join(final, s[n:]))
+		}
 	}
 	return s
 }