ModuloReader: handle zero reads from inner reader

backend/media/service.go

@@ -522,6 +522,15 @@ func (r *ModuloBufReader) Read(p []byte) (int, error) {
 
 	nr := nr1 + nr2
 	rem := nr % r.modSize
+
+	// if there was an error, return immediately.
+	if err == io.EOF {
+		return nr, err
+	} else if err != nil {
+		return nr - rem, err
+	}
+
+	// write any remainder to the buffer
 	if rem != 0 {
 		// err is always nil
 		_, _ = r.buf.Write(p[nr-rem : nr])

backend/media/service_test.go

@@ -27,9 +27,10 @@ func TestModuloBufReader(t *testing.T) {
 		data: [][]byte{
 			{'a', 'b', 'c', 'd'},
 			{'e', 'f', 'g', 'h', 'i'},
+			{},
 			{'j', 'k', 'l', 'm'},
 			{'n', 'o', 'p'},
-			{'q', 'r', 's', 't'},
+			{'q', 'r', 's', 't', 'u'},
 		},
 	}
 
@@ -47,6 +48,11 @@ func TestModuloBufReader(t *testing.T) {
 	assert.Equal(t, 4, n)
 	assert.Equal(t, []byte{'e', 'f', 'g', 'h'}, out[:n])
 
+	n, err = modReader.Read(out)
+	assert.NoError(t, err)
+	assert.Zero(t, n)
+	assert.Empty(t, out[:n])
+
 	n, err = modReader.Read(out)
 	assert.NoError(t, err)
 	assert.NoError(t, err)
@@ -63,6 +69,11 @@ func TestModuloBufReader(t *testing.T) {
 	assert.Equal(t, 4, n)
 	assert.Equal(t, []byte{'q', 'r', 's', 't'}, out[:n])
 
+	n, err = modReader.Read(out)
+	assert.Equal(t, io.EOF, err)
+	assert.Equal(t, 1, n)
+	assert.Equal(t, []byte{'u'}, out[:n])
+
 	n, err = modReader.Read(out)
 	assert.Zero(t, n)
 	assert.Equal(t, io.EOF, err)