cmd/go: refuse -w with an invalid GOPATH

Fixes #35338

Change-Id: Ic2a3a446ef56b1e5723d6192c8aeec32ae0bbeac
Reviewed-on: https://go-review.googlesource.com/c/go/+/205779
Run-TryBot: Baokun Lee <nototon@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/envcmd/env.go

@@ -353,6 +353,13 @@ func checkEnvWrite(key, val string) error {
 		default:
 			return fmt.Errorf("invalid %s value %q", key, val)
 		}
+	case "GOPATH":
+		if strings.HasPrefix(val, "~") {
+			return fmt.Errorf("GOPATH entry cannot start with shell metacharacter '~': %q", val)
+		}
+		if !filepath.IsAbs(val) && val != "" {
+			return fmt.Errorf("GOPATH entry is relative; must be absolute path: %q", val)
+		}
 	}
 
 	if !utf8.ValidString(val) {

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

@@ -89,3 +89,10 @@ stderr 'arguments must be KEY=VALUE: invalid argument: GOOS'
 # go env -w rejects invalid GO111MODULE values, as otherwise cmd/go would break
 ! go env -w GO111MODULE=badvalue
 stderr 'invalid GO111MODULE value "badvalue"'
+
+# go env -w rejects invalid GOPATH values
+! go env -w GOPATH=~/go
+stderr 'GOPATH entry cannot start with shell metacharacter'
+
+! go env -w GOPATH=./go
+stderr 'GOPATH entry is relative; must be absolute path'