mime/multipart: report io.EOF correctly on part ending without newlines

If a part ends with "--boundary--", without a final "\r\n",
that's also a graceful EOF, and we should return io.EOF instead
of the fmt-wrapped io.EOF from bufio.Reader.ReadSlice.

I found this bug parsing an image attachment from gmail.
Minimal test case stripped down from the original
gmail-generated attachment included.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6118043

src/pkg/mime/multipart/multipart.go

@@ -185,6 +185,14 @@ func (r *Reader) NextPart() (*Part, error) {
 	expectNewPart := false
 	for {
 		line, err := r.bufReader.ReadSlice('\n')
+		if err == io.EOF && bytes.Equal(line, r.dashBoundaryDash) {
+			// If the buffer ends in "--boundary--" without the
+			// trailing "\r\n", ReadSlice will return an error
+			// (since it's missing the '\n'), but this is a valid
+			// multipart EOF so we need to return io.EOF instead of
+			// a fmt-wrapped one.
+			return nil, io.EOF
+		}
 		if err != nil {
 			return nil, fmt.Errorf("multipart: NextPart: %v", err)
 		}

src/pkg/mime/multipart/multipart_test.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"io"
 	"io/ioutil"
+	"os"
 	"strings"
 	"testing"
 )
@@ -377,3 +378,52 @@ func TestLineContinuation(t *testing.T) {
 		}
 	}
 }
+
+// Test parsing an image attachment from gmail, which previously failed.
+func TestNested(t *testing.T) {
+	// nested-mime is the body part of a multipart/mixed email
+	// with boundary e89a8ff1c1e83553e304be640612
+	f, err := os.Open("testdata/nested-mime")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+	mr := NewReader(f, "e89a8ff1c1e83553e304be640612")
+	p, err := mr.NextPart()
+	if err != nil {
+		t.Fatalf("error reading first section (alternative): %v", err)
+	}
+
+	// Read the inner text/plain and text/html sections of the multipart/alternative.
+	mr2 := NewReader(p, "e89a8ff1c1e83553e004be640610")
+	p, err = mr2.NextPart()
+	if err != nil {
+		t.Fatalf("reading text/plain part: %v", err)
+	}
+	if b, err := ioutil.ReadAll(p); string(b) != "*body*\r\n" || err != nil {
+		t.Fatalf("reading text/plain part: got %q, %v", b, err)
+	}
+	p, err = mr2.NextPart()
+	if err != nil {
+		t.Fatalf("reading text/html part: %v", err)
+	}
+	if b, err := ioutil.ReadAll(p); string(b) != "<b>body</b>\r\n" || err != nil {
+		t.Fatalf("reading text/html part: got %q, %v", b, err)
+	}
+
+	p, err = mr2.NextPart()
+	if err != io.EOF {
+		t.Fatalf("final inner NextPart = %v; want io.EOF", err)
+	}
+
+	// Back to the outer multipart/mixed, reading the image attachment.
+	_, err = mr.NextPart()
+	if err != nil {
+		t.Fatalf("error reading the image attachment at the end: %v", err)
+	}
+
+	_, err = mr.NextPart()
+	if err != io.EOF {
+		t.Fatalf("final outer NextPart = %v; want io.EOF", err)
+	}
+}

src/pkg/mime/multipart/testdata/nested-mime

@@ -0,0 +1,29 @@
+--e89a8ff1c1e83553e304be640612
+Content-Type: multipart/alternative; boundary=e89a8ff1c1e83553e004be640610
+
+--e89a8ff1c1e83553e004be640610
+Content-Type: text/plain; charset=UTF-8
+
+*body*
+
+--e89a8ff1c1e83553e004be640610
+Content-Type: text/html; charset=UTF-8
+
+<b>body</b>
+
+--e89a8ff1c1e83553e004be640610--
+--e89a8ff1c1e83553e304be640612
+Content-Type: image/png; name="x.png"
+Content-Disposition: attachment; 
+	filename="x.png"
+Content-Transfer-Encoding: base64
+X-Attachment-Id: f_h1edgigu0
+
+iVBORw0KGgoAAAANSUhEUgAAAagAAADrCAIAAACza5XhAAAKMWlDQ1BJQ0MgUHJvZmlsZQAASImd
+lndUU9kWh8+9N71QkhCKlNBraFICSA29SJEuKjEJEErAkAAiNkRUcERRkaYIMijggKNDkbEiioUB
+8b2kqeGaj4aTNftesu5mob4pr07ecMywRwLBvDCJOksqlUyldAZD7g9fxIZRWWPMvXRNJROJRBIG
+Y7Vx0mva1HAwYqibdKONXye3dW4iUonhWFJnqK7OaanU1gGkErFYEgaj0cg8wK+zVPh2ziwnHy07
+U8lYTNapezSzOuevRwLB7CFkqQQCwaJDiBQIBIJFhwh8AoFg0SHUqQUCASRJKkwkhMy/JfODWPEJ
+BIJFhwh8AoFg0TFnQqQ55GtPFopcJsN97e1nYtNuIBYeGBgYCmYrmE3jZ05iaGAoMX0xzxkWz6Hv
+yO7WvrlwzA0uLzrD+VkKqViwl9IfTBVNFMyc/x9alloiPPlqhQAAAABJRU5ErkJggg==
+--e89a8ff1c1e83553e304be640612--