cmd/go: allow "stdout" and "stderr" as inputs to script_test "cp" command

Updates #30241

Change-Id: I543d8914faf810835d3327baa3c84b3dff124156
Reviewed-on: https://go-review.googlesource.com/c/163519Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/script_test.go

@@ -509,16 +509,33 @@ func (ts *testScript) cmdCp(neg bool, args []string) {
 	}
 
 	for _, arg := range args[:len(args)-1] {
-		src := ts.mkabs(arg)
-		info, err := os.Stat(src)
-		ts.check(err)
-		data, err := ioutil.ReadFile(src)
-		ts.check(err)
+		var (
+			src  string
+			data []byte
+			mode os.FileMode
+		)
+		switch arg {
+		case "stdout":
+			src = arg
+			data = []byte(ts.stdout)
+			mode = 0666
+		case "stderr":
+			src = arg
+			data = []byte(ts.stderr)
+			mode = 0666
+		default:
+			src = ts.mkabs(arg)
+			info, err := os.Stat(src)
+			ts.check(err)
+			mode = info.Mode() & 0777
+			data, err = ioutil.ReadFile(src)
+			ts.check(err)
+		}
 		targ := dst
 		if dstDir {
 			targ = filepath.Join(dst, filepath.Base(src))
 		}
-		ts.check(ioutil.WriteFile(targ, data, info.Mode()&0777))
+		ts.check(ioutil.WriteFile(targ, data, mode))
 	}
 }
 

src/cmd/go/testdata/script/README

@@ -108,6 +108,8 @@ The commands are:
 
 - cp src... dst
   Copy the listed files to the target file or existing directory.
+  src can include "stdout" or "stderr" to use the standard output or standard error
+  from the most recent exec or go command.
 
 - env [key=value...]
   With no arguments, print the environment (useful for debugging).