encoding/ascii85: handle non-data bytes correctly

Previously Read wouldn't return once its internal input buffer
is filled with non-data bytes.
Fixes #7875.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/90820043

src/pkg/encoding/ascii85/ascii85.go

@@ -281,6 +281,18 @@ func (d *decoder) Read(p []byte) (n int, err error) {
 				d.nbuf = copy(d.buf[0:], d.buf[nsrc:d.nbuf])
 				continue // copy out and return
 			}
+			if ndst == 0 && d.err == nil {
+				// Special case: input buffer is mostly filled with non-data bytes.
+				// Filter out such bytes to make room for more input.
+				off := 0
+				for i := 0; i < d.nbuf; i++ {
+					if d.buf[i] > ' ' {
+						d.buf[off] = d.buf[i]
+						off++
+					}
+				}
+				d.nbuf = off
+			}
 		}
 
 		// Out of input, out of decoded output.  Check errors.

src/pkg/encoding/ascii85/ascii85_test.go

@@ -197,3 +197,14 @@ func TestBig(t *testing.T) {
 		t.Errorf("Decode(Encode(%d-byte string)) failed at offset %d", n, i)
 	}
 }
+
+func TestDecoderInternalWhitespace(t *testing.T) {
+	s := strings.Repeat(" ", 2048) + "z"
+	decoded, err := ioutil.ReadAll(NewDecoder(strings.NewReader(s)))
+	if err != nil {
+		t.Errorf("Decode gave error %v", err)
+	}
+	if want := []byte("\000\000\000\000"); !bytes.Equal(want, decoded) {
+		t.Errorf("Decode failed: got %v, want %v", decoded, want)
+	}
+}