io: fix Copyn EOF handling

Fixes #1383.

R=r
CC=golang-dev
https://golang.org/cl/3821044

src/pkg/io/io.go

@@ -206,7 +206,12 @@ func Copyn(dst Writer, src Reader, n int64) (written int64, err os.Error) {
 	// If the writer has a ReadFrom method, use it to do the copy.
 	// Avoids a buffer allocation and a copy.
 	if rt, ok := dst.(ReaderFrom); ok {
-		return rt.ReadFrom(LimitReader(src, n))
+		written, err = rt.ReadFrom(LimitReader(src, n))
+		if written < n && err == nil {
+			// rt stopped early; must have been EOF.
+			err = os.EOF
+		}
+		return
 	}
 	buf := make([]byte, 32*1024)
 	for written < n {

src/pkg/io/io_test.go

@@ -8,6 +8,7 @@ import (
 	"bytes"
 	. "io"
 	"os"
+	"strings"
 	"testing"
 )
 
@@ -80,6 +81,41 @@ func TestCopynWriteTo(t *testing.T) {
 	}
 }
 
+type noReadFrom struct {
+	w Writer
+}
+
+func (w *noReadFrom) Write(p []byte) (n int, err os.Error) {
+	return w.w.Write(p)
+}
+
+func TestCopynEOF(t *testing.T) {
+	// Test that EOF behavior is the same regardless of whether
+	// argument to Copyn has ReadFrom.
+
+	b := new(bytes.Buffer)
+
+	n, err := Copyn(&noReadFrom{b}, strings.NewReader("foo"), 3)
+	if n != 3 || err != nil {
+		t.Errorf("Copyn(noReadFrom, foo, 3) = %d, %v; want 3, nil", n, err)
+	}
+
+	n, err = Copyn(&noReadFrom{b}, strings.NewReader("foo"), 4)
+	if n != 3 || err != os.EOF {
+		t.Errorf("Copyn(noReadFrom, foo, 4) = %d, %v; want 3, EOF", n, err)
+	}
+
+	n, err = Copyn(b, strings.NewReader("foo"), 3) // b has read from
+	if n != 3 || err != nil {
+		t.Errorf("Copyn(bytes.Buffer, foo, 3) = %d, %v; want 3, nil", n, err)
+	}
+
+	n, err = Copyn(b, strings.NewReader("foo"), 4) // b has read from
+	if n != 3 || err != os.EOF {
+		t.Errorf("Copyn(bytes.Buffer, foo, 4) = %d, %v; want 3, EOF", n, err)
+	}
+}
+
 func TestReadAtLeast(t *testing.T) {
 	var rb bytes.Buffer
 	rb.Write([]byte("0123"))