cmd/go: add cmpenv command to testing script language

This will be used by later commits in this sequence.

Updates #28221

Change-Id: I2b22b9f88a0183636cde9509606f03f079eb33f1
Reviewed-on: https://go-review.googlesource.com/c/147277
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/script_test.go

@@ -331,6 +331,7 @@ var scriptCmds = map[string]func(*testScript, bool, []string){
 	"addcrlf": (*testScript).cmdAddcrlf,
 	"cd":      (*testScript).cmdCd,
 	"cmp":     (*testScript).cmdCmp,
+	"cmpenv":  (*testScript).cmdCmpenv,
 	"cp":      (*testScript).cmdCp,
 	"env":     (*testScript).cmdEnv,
 	"exec":    (*testScript).cmdExec,
@@ -396,7 +397,21 @@ func (ts *testScript) cmdCmp(neg bool, args []string) {
 	if len(args) != 2 {
 		ts.fatalf("usage: cmp file1 file2")
 	}
+	ts.doCmdCmp(args, false)
+}
 
+// cmpenv compares two files with environment variable substitution.
+func (ts *testScript) cmdCmpenv(neg bool, args []string) {
+	if neg {
+		ts.fatalf("unsupported: ! cmpenv")
+	}
+	if len(args) != 2 {
+		ts.fatalf("usage: cmpenv file1 file2")
+	}
+	ts.doCmdCmp(args, true)
+}
+
+func (ts *testScript) doCmdCmp(args []string, env bool) {
 	name1, name2 := args[0], args[1]
 	var text1, text2 string
 	if name1 == "stdout" {
@@ -413,6 +428,11 @@ func (ts *testScript) cmdCmp(neg bool, args []string) {
 	ts.check(err)
 	text2 = string(data)
 
+	if env {
+		text1 = ts.expand(text1)
+		text2 = ts.expand(text2)
+	}
+
 	if text1 == text2 {
 		return
 	}

src/cmd/go/testdata/script/README

@@ -92,6 +92,10 @@ The commands are:
   from the most recent exec or go command.
   (If the files have differing content, the failure prints a diff.)
 
+- cmpenv file1 file2
+  Like cmp, but environment variables are substituted in the file contents
+  before the comparison. For example, $GOOS is replaced by the target GOOS.
+
 - cp src... dst
   Copy the listed files to the target file or existing directory.