mime/multipart: moved some code to mime/internal/quotedprintable

The code concerning quoted-printable encoding (RFC 2045) and its
variant for MIME headers (RFC 2047) is currently spread in
mime/multipart and net/mail. It is also not exported.

This commit is the first step to fix that issue. It moves the
quoted-printable decoding code from mime/multipart to
mime/internal/quotedprintable. The exposed API is unchanged.

Concerns #4943.

Change-Id: I11352afbb2edb4d6ef62870b9bc5c87c639eff12
Reviewed-on: https://go-review.googlesource.com/1810
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/go/build/deps_test.go

@@ -311,7 +311,7 @@ var pkgDeps = map[string][]string{
 	"crypto/x509/pkix": {"L4", "CRYPTO-MATH"},
 
 	// Simple net+crypto-aware packages.
-	"mime/multipart": {"L4", "OS", "mime", "crypto/rand", "net/textproto"},
+	"mime/multipart": {"L4", "OS", "mime", "crypto/rand", "net/textproto", "mime/internal/quotedprintable"},
 	"net/smtp":       {"L4", "CRYPTO", "NET", "crypto/tls"},
 
 	// HTTP, kingpin of dependencies.

src/mime/internal/quotedprintable/quotedprintable.go

@@ -8,7 +8,7 @@
 // 2. it will pass through a '\r' or '\n' not preceded by '=', consistent
 //    with other broken QP encoders & decoders.
 
-package multipart
+package quotedprintable
 
 import (
 	"bufio"
@@ -23,7 +23,7 @@ type qpReader struct {
 	line []byte // to be consumed before more of br
 }
 
-func newQuotedPrintableReader(r io.Reader) io.Reader {
+func NewReader(r io.Reader) io.Reader {
 	return &qpReader{
 		br: bufio.NewReader(r),
 	}

src/mime/internal/quotedprintable/quotedprintable_test.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-package multipart
+package quotedprintable
 
 import (
 	"bufio"
@@ -65,7 +65,7 @@ func TestQuotedPrintable(t *testing.T) {
 	}
 	for _, tt := range tests {
 		var buf bytes.Buffer
-		_, err := io.Copy(&buf, newQuotedPrintableReader(strings.NewReader(tt.in)))
+		_, err := io.Copy(&buf, NewReader(strings.NewReader(tt.in)))
 		if got := buf.String(); got != tt.want {
 			t.Errorf("for %q, got %q; want %q", tt.in, got, tt.want)
 		}
@@ -116,7 +116,7 @@ func TestQPExhaustive(t *testing.T) {
 			return
 		}
 		buf.Reset()
-		_, err := io.Copy(&buf, newQuotedPrintableReader(strings.NewReader(s)))
+		_, err := io.Copy(&buf, NewReader(strings.NewReader(s)))
 		if err != nil {
 			errStr := err.Error()
 			if strings.Contains(errStr, "invalid bytes after =:") {

src/mime/multipart/multipart.go

@@ -19,6 +19,7 @@ import (
 	"io"
 	"io/ioutil"
 	"mime"
+	"mime/internal/quotedprintable"
 	"net/textproto"
 )
 
@@ -111,7 +112,7 @@ func newPart(mr *Reader) (*Part, error) {
 	const cte = "Content-Transfer-Encoding"
 	if bp.Header.Get(cte) == "quoted-printable" {
 		bp.Header.Del(cte)
-		bp.r = newQuotedPrintableReader(bp.r)
+		bp.r = quotedprintable.NewReader(bp.r)
 	}
 	return bp, nil
 }