encoding/base64: fix streaming decode of padding-free base64

Fixes #13384.

Change-Id: Id9e827acddc8de139f93c5de0c6486bc4334c7d4
Reviewed-on: https://go-review.googlesource.com/18330Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.6.html

@@ -568,6 +568,17 @@ Also in the <a href="/pkg/encoding/asn1/"><code>encoding/asn1</code></a> package
 <a href="/pkg/encoding/asn1/#Unmarshal"><code>Unmarshal</code></a> now rejects various non-standard integer and length encodings.
 </li>
 
+<li>
+The <a href="/pkg/encoding/base64"><code>encoding/base64</code></a> package's
+<a href="/pkg/encoding/base64/#Decoder"><code>Decoder</code></a> has been fixed
+to process the final bytes of its input. Previously it processed as many four-byte tokens as
+possible but ignore the remainder, up to three bytes.
+The <code>Decoder</code> therefore now handles inputs in unpadded encodings (like
+<a href="/pkg/encoding/base64/#RawURLEncoding">RawURLEncoding</a>) correctly,
+but it also rejects inputs in padded encodings that are truncated or end with invalid bytes,
+such as trailing spaces.
+</li>
+
 <li>
 The <a href="/pkg/encoding/json/"><code>encoding/json</code></a> package
 now checks the syntax of a

src/encoding/base64/base64.go

@@ -347,6 +347,7 @@ func (enc *Encoding) DecodeString(s string) ([]byte, error) {
 
 type decoder struct {
 	err     error
+	readErr error // error from r.Read
 	enc     *Encoding
 	r       io.Reader
 	end     bool       // saw end of message
@@ -357,10 +358,6 @@ type decoder struct {
 }
 
 func (d *decoder) Read(p []byte) (n int, err error) {
-	if d.err != nil {
-		return 0, d.err
-	}
-
 	// Use leftover decoded output from last read.
 	if len(d.out) > 0 {
 		n = copy(p, d.out)
@@ -368,9 +365,14 @@ func (d *decoder) Read(p []byte) (n int, err error) {
 		return n, nil
 	}
 
+	if d.err != nil {
+		return 0, d.err
+	}
+
 	// This code assumes that d.r strips supported whitespace ('\r' and '\n').
 
-	// Read a chunk.
+	// Refill buffer.
+	for d.nbuf < 4 && d.readErr == nil {
 		nn := len(p) / 3 * 4
 		if nn < 4 {
 			nn = 4
@@ -378,9 +380,31 @@ func (d *decoder) Read(p []byte) (n int, err error) {
 		if nn > len(d.buf) {
 			nn = len(d.buf)
 		}
-	nn, d.err = io.ReadAtLeast(d.r, d.buf[d.nbuf:nn], 4-d.nbuf)
+		nn, d.readErr = d.r.Read(d.buf[d.nbuf:nn])
 		d.nbuf += nn
-	if d.err != nil || d.nbuf < 4 {
+	}
+
+	if d.nbuf < 4 {
+		if d.enc.padChar == NoPadding && d.nbuf > 0 {
+			// Decode final fragment, without padding.
+			var nw int
+			nw, _, d.err = d.enc.decode(d.outbuf[:], d.buf[:d.nbuf])
+			d.nbuf = 0
+			d.end = true
+			d.out = d.outbuf[:nw]
+			n = copy(p, d.out)
+			d.out = d.out[n:]
+			if n > 0 || len(p) == 0 && len(d.out) > 0 {
+				return n, nil
+			}
+			if d.err != nil {
+				return 0, d.err
+			}
+		}
+		d.err = d.readErr
+		if d.err == io.EOF && d.nbuf > 0 {
+			d.err = io.ErrUnexpectedEOF
+		}
 		return 0, d.err
 	}
 
@@ -396,13 +420,7 @@ func (d *decoder) Read(p []byte) (n int, err error) {
 		n, d.end, d.err = d.enc.decode(p, d.buf[:nr])
 	}
 	d.nbuf -= nr
-	for i := 0; i < d.nbuf; i++ {
-		d.buf[i] = d.buf[i+nr]
-	}
-
-	if d.err == nil {
-		d.err = err
-	}
+	copy(d.buf[:d.nbuf], d.buf[nr:])
 	return n, d.err
 }
 

src/encoding/base64/base64_test.go

@@ -406,3 +406,28 @@ func BenchmarkDecodeString(b *testing.B) {
 		StdEncoding.DecodeString(data)
 	}
 }
+
+func TestDecoderRaw(t *testing.T) {
+	source := "AAAAAA"
+	want := []byte{0, 0, 0, 0}
+
+	// Direct.
+	dec1, err := RawURLEncoding.DecodeString(source)
+	if err != nil || !bytes.Equal(dec1, want) {
+		t.Errorf("RawURLEncoding.DecodeString(%q) = %x, %v, want %x, nil", source, dec1, err, want)
+	}
+
+	// Through reader. Used to fail.
+	r := NewDecoder(RawURLEncoding, bytes.NewReader([]byte(source)))
+	dec2, err := ioutil.ReadAll(io.LimitReader(r, 100))
+	if err != nil || !bytes.Equal(dec2, want) {
+		t.Errorf("reading NewDecoder(RawURLEncoding, %q) = %x, %v, want %x, nil", source, dec2, err, want)
+	}
+
+	// Should work with padding.
+	r = NewDecoder(URLEncoding, bytes.NewReader([]byte(source+"==")))
+	dec3, err := ioutil.ReadAll(r)
+	if err != nil || !bytes.Equal(dec3, want) {
+		t.Errorf("reading NewDecoder(URLEncoding, %q) = %x, %v, want %x, nil", source+"==", dec3, err, want)
+	}
+}

test/bench/go1/jsondata_test.go

@@ -1816,4 +1816,4 @@ zJE6zEudHD27ZzbOeSgpk/HnkQbT7twqaaJXNvUzMuUt1hyhU7ceZcph42+VTlXU
 cZ9UZZJyYojLjaeJHfJU1UZUEmBfLumu8yW5skuyE9uh2BmVxJZi6KxaXBNwSolw
 BqBcQLj3ucNZIYZLYtirLu3brW6UYgZgZJiDIGiwpsgg7g1AITkgM6FHITxDDnGt
 4SDHzZbL5s8fec5PCq5DOzDRdWS+0h5Y2INZak1D29cpVyb2aVrV3Wlt7rQhLa3e
-m3ZwPNcXywE2Qesk1XN24HvZ2Xa6nlm8Pf/xdyRThQkO1NjuAA== `)
+m3ZwPNcXywE2Qesk1XN24HvZ2Xa6nlm8Pf/xdyRThQkO1NjuAA==`)