cmd/go: add goversion environment variable to testing script language

Updates #28221

Change-Id: I8a1e352cd9122bce200d45c6b19955cb50308d71
Reviewed-on: https://go-review.googlesource.com/c/147280
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/script_test.go

@@ -11,6 +11,7 @@ import (
 	"bytes"
 	"context"
 	"fmt"
+	"go/build"
 	"internal/testenv"
 	"io/ioutil"
 	"os"
@@ -104,6 +105,7 @@ func (ts *testScript) setup() {
 		"GOROOT=" + testGOROOT,
 		tempEnvName() + "=" + filepath.Join(ts.workdir, "tmp"),
 		"devnull=" + os.DevNull,
+		"goversion=" + goVersion(ts),
 		":=" + string(os.PathListSeparator),
 	}
 
@@ -130,6 +132,16 @@ func (ts *testScript) setup() {
 	}
 }
 
+// goVersion returns the current Go version.
+func goVersion(ts *testScript) string {
+	tags := build.Default.ReleaseTags
+	version := tags[len(tags)-1]
+	if !regexp.MustCompile(`^go([1-9][0-9]*)\.(0|[1-9][0-9]*)$`).MatchString(version) {
+		ts.fatalf("invalid go version %q", version)
+	}
+	return version[2:]
+}
+
 var execCache par.Cache
 
 // run runs the test script.

src/cmd/go/testdata/script/README

@@ -36,6 +36,7 @@ Scripts also have access to these other environment variables:
 	PATH=<actual PATH>
 	TMPDIR=$WORK/tmp
 	devnull=<value of os.DevNull>
+	goversion=<current Go version; for example, 1.12>
 
 The environment variable $exe (lowercase) is an empty string on most systems, ".exe" on Windows.