gob: when possible, allow sequential decoders on the same input stream.

This can work only if there is no type info required to initialize the decoder,
but it's easy and gains a few percent in the basic benchmarks by avoiding
bufio when it's a bytes.Buffer - a testing-only scenario, I admit.
Add a comment about what Decode expects from the input.

R=rsc
CC=golang-dev
https://golang.org/cl/5165048

src/pkg/gob/decoder.go

@@ -29,9 +29,15 @@ type Decoder struct {
 }
 
 // NewDecoder returns a new decoder that reads from the io.Reader.
+// If r does not also implement io.ByteReader, it will be wrapped in a
+// bufio.Reader.
 func NewDecoder(r io.Reader) *Decoder {
 	dec := new(Decoder)
-	dec.r = bufio.NewReader(r)
+	// We use the ability to read bytes as a plausible surrogate for buffering.
+	if _, ok := r.(io.ByteReader); !ok {
+		r = bufio.NewReader(r)
+	}
+	dec.r = r
 	dec.wireType = make(map[typeId]*wireType)
 	dec.decoderCache = make(map[reflect.Type]map[typeId]**decEngine)
 	dec.ignorerCache = make(map[typeId]**decEngine)

src/pkg/gob/encoder_test.go

@@ -638,3 +638,28 @@ func TestBadCount(t *testing.T) {
 		t.Error("expected bad count error; got", err)
 	}
 }
+
+// Verify that sequential Decoders built on a single input will
+// succeed if the input implements ReadByte and there is no
+// type information in the stream.
+func TestSequentialDecoder(t *testing.T) {
+	b := new(bytes.Buffer)
+	enc := NewEncoder(b)
+	const count = 10
+	for i := 0; i < count; i++ {
+		s := fmt.Sprintf("%d", i)
+		if err := enc.Encode(s); err != nil {
+			t.Error("encoder fail:", err)
+		}
+	}
+	for i := 0; i < count; i++ {
+		dec := NewDecoder(b)
+		var s string
+		if err := dec.Decode(&s); err != nil {
+			t.Fatal("decoder fail:", err)
+		}
+		if s != fmt.Sprintf("%d", i) {
+			t.Fatalf("decode expected %d got %s", i, s)
+		}
+	}
+}