io: copy slice argument in MultiReader and MultiWriter

Replaces CL 91240045.
Fixes #7809.

LGTM=bradfitz
R=golang-codereviews, minux.ma
CC=adg, bradfitz, golang-codereviews, iant, r
https://golang.org/cl/94380043

src/pkg/io/multi.go

@@ -29,7 +29,9 @@ func (mr *multiReader) Read(p []byte) (n int, err error) {
 // inputs have returned EOF, Read will return EOF.  If any of the readers
 // return a non-nil, non-EOF error, Read will return that error.
 func MultiReader(readers ...Reader) Reader {
-	return &multiReader{readers}
+	r := make([]Reader, len(readers))
+	copy(r, readers)
+	return &multiReader{r}
 }
 
 type multiWriter struct {
@@ -53,5 +55,7 @@ func (t *multiWriter) Write(p []byte) (n int, err error) {
 // MultiWriter creates a writer that duplicates its writes to all the
 // provided writers, similar to the Unix tee(1) command.
 func MultiWriter(writers ...Writer) Writer {
-	return &multiWriter{writers}
+	w := make([]Writer, len(writers))
+	copy(w, writers)
+	return &multiWriter{w}
 }

src/pkg/io/multi_test.go

@@ -9,6 +9,7 @@ import (
 	"crypto/sha1"
 	"fmt"
 	. "io"
+	"io/ioutil"
 	"strings"
 	"testing"
 )
@@ -86,3 +87,29 @@ func TestMultiWriter(t *testing.T) {
 		t.Errorf("expected %q; got %q", sourceString, sink.String())
 	}
 }
+
+// Test that MultiReader copies the input slice and is insulated from future modification.
+func TestMultiReaderCopy(t *testing.T) {
+	slice := []Reader{strings.NewReader("hello world")}
+	r := MultiReader(slice...)
+	slice[0] = nil
+	data, err := ioutil.ReadAll(r)
+	if err != nil || string(data) != "hello world" {
+		t.Errorf("ReadAll() = %q, %v, want %q, nil", data, err, "hello world")
+	}
+}
+
+// Test that MultiWriter copies the input slice and is insulated from future modification.
+func TestMultiWriterCopy(t *testing.T) {
+	var buf bytes.Buffer
+	slice := []Writer{&buf}
+	w := MultiWriter(slice...)
+	slice[0] = nil
+	n, err := w.Write([]byte("hello world"))
+	if err != nil || n != 11 {
+		t.Errorf("Write(`hello world`) = %d, %v, want 11, nil", n, err)
+	}
+	if buf.String() != "hello world" {
+		t.Errorf("buf.String() = %q, want %q", buf.String(), "hello world")
+	}
+}