Fix mixed Write and ReadFrom calls (#282)

Flush any pending writes when ReadFrom is used.

Author: klauspost

zstd/encoder.go

@@ -311,7 +311,13 @@ func (e *Encoder) ReadFrom(r io.Reader) (n int64, err error) {
 	if debug {
 		println("Using ReadFrom")
 	}
-	// Maybe handle stuff queued?
+
+	// Flush any current writes.
+	if len(e.state.filling) > 0 {
+		if err := e.nextBlock(false); err != nil {
+			return 0, err
+		}
+	}
 	e.state.filling = e.state.filling[:e.o.blockSize]
 	src := e.state.filling
 	for {
@@ -328,7 +334,7 @@ func (e *Encoder) ReadFrom(r io.Reader) (n int64, err error) {
 			if debug {
 				println("ReadFrom: got EOF final block:", len(e.state.filling))
 			}
-			return n, e.nextBlock(true)
+			return n, nil
 		default:
 			if debug {
 				println("ReadFrom: got error:", err)

zstd/encoder_test.go

@@ -695,6 +695,48 @@ func TestEncoderReadFrom(t *testing.T) {
 	dec.Close()
 }
 
+func TestInterleavedWriteReadFrom(t *testing.T) {
+	var encoded bytes.Buffer
+
+	enc, err := NewWriter(&encoded)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if _, err := enc.Write([]byte("write1")); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := enc.Write([]byte("write2")); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := enc.ReadFrom(strings.NewReader("readfrom1")); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := enc.Write([]byte("write3")); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := enc.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	dec, err := NewReader(&encoded)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer dec.Close()
+
+	gotb, err := ioutil.ReadAll(dec)
+	if err != nil {
+		t.Fatal(err)
+	}
+	got := string(gotb)
+
+	if want := "write1write2readfrom1write3"; got != want {
+		t.Errorf("got decoded %q, want %q", got, want)
+	}
+}
+
 func TestEncoder_EncodeAllEmpty(t *testing.T) {
 	if testing.Short() {
 		t.SkipNow()