diff --git a/usb/device_test.go b/usb/device_test.go
index 26fd8e4..d1613a2 100644
--- a/usb/device_test.go
+++ b/usb/device_test.go
@@ -20,9 +20,8 @@ import (
 )
 
 func TestOpenEndpoint(t *testing.T) {
-	origLib := libusb
-	defer func() { libusb = origLib }()
-	libusb = newFakeLibusb()
+	_, done := newFakeLibusb()
+	defer done()
 
 	c := NewContext()
 	defer c.Close()
diff --git a/usb/endpoint_test.go b/usb/endpoint_test.go
index 4d45c3f..eddea21 100644
--- a/usb/endpoint_test.go
+++ b/usb/endpoint_test.go
@@ -20,7 +20,9 @@ import (
 )
 
 func TestEndpoint(t *testing.T) {
-	defer func(i libusbIntf) { libusb = i }(libusb)
+	lib, done := newFakeLibusb()
+	defer done()
+
 	for _, epData := range []struct {
 		ei   EndpointInfo
 		intf InterfaceSetting
@@ -83,9 +85,6 @@ func TestEndpoint(t *testing.T) {
 				wantErr: true,
 			},
 		} {
-			lib := newFakeLibusb()
-			libusb = lib
-
 			ep := newEndpoint(nil, epData.intf, epData.ei)
 			go func() {
 				fakeT := lib.waitForSubmitted()
@@ -150,8 +149,8 @@ func TestEndpointInfo(t *testing.T) {
 func TestEndpointIn(t *testing.T) {
 	defer func(i libusbIntf) { libusb = i }(libusb)
 
-	lib := newFakeLibusb()
-	libusb = lib
+	lib, done := newFakeLibusb()
+	defer done()
 
 	ctx := NewContext()
 	defer ctx.Close()
@@ -187,8 +186,8 @@ func TestEndpointIn(t *testing.T) {
 func TestEndpointOut(t *testing.T) {
 	defer func(i libusbIntf) { libusb = i }(libusb)
 
-	lib := newFakeLibusb()
-	libusb = lib
+	lib, done := newFakeLibusb()
+	defer done()
 
 	ctx := NewContext()
 	defer ctx.Close()
diff --git a/usb/transfer_test.go b/usb/transfer_test.go
index d07a51b..dd653f1 100644
--- a/usb/transfer_test.go
+++ b/usb/transfer_test.go
@@ -20,8 +20,8 @@ import (
 )
 
 func TestNewTransfer(t *testing.T) {
-	defer func(i libusbIntf) { libusb = i }(libusb)
-	libusb = newFakeLibusb()
+	_, done := newFakeLibusb()
+	defer done()
 
 	for _, tc := range []struct {
 		desc        string
@@ -62,6 +62,7 @@ func TestNewTransfer(t *testing.T) {
 		if err != nil {
 			t.Fatalf("newUSBTransfer(): %v", err)
 		}
+		defer xfer.free()
 		if got, want := len(xfer.buf), tc.wantLength; got != want {
 			t.Errorf("xfer.buf: got %d bytes, want %d", got, want)
 		}
@@ -69,10 +70,8 @@ func TestNewTransfer(t *testing.T) {
 }
 
 func TestTransferProtocol(t *testing.T) {
-	defer func(i libusbIntf) { libusb = i }(libusb)
-
-	f := newFakeLibusb()
-	libusb = f
+	f, done := newFakeLibusb()
+	defer done()
 
 	xfers := make([]*usbTransfer, 2)
 	var err error
@@ -140,3 +139,20 @@ func TestTransferProtocol(t *testing.T) {
 		x.free()
 	}
 }
+
+func BenchmarkSubSlice(b *testing.B) {
+	x := make([]byte, 512)
+	start, len := 50, 50
+	b.Run("start:start+len", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			y := x
+			y = y[start : start+len]
+		}
+	})
+	b.Run("[start:][:len]", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			y := x
+			y = y[start:][:len]
+		}
+	})
+}
diff --git a/usb/usb_test.go b/usb/usb_test.go
index 60a66bd..fb9943f 100644
--- a/usb/usb_test.go
+++ b/usb/usb_test.go
@@ -18,9 +18,8 @@ package usb
 import "testing"
 
 func TestListDevices(t *testing.T) {
-	orig := libusb
-	defer func() { libusb = orig }()
-	libusb = newFakeLibusb()
+	_, done := newFakeLibusb()
+	defer done()
 
 	c := NewContext()
 	defer c.Close()
@@ -55,9 +54,8 @@ func TestListDevices(t *testing.T) {
 }
 
 func TestOpenDeviceWithVidPid(t *testing.T) {
-	orig := libusb
-	defer func() { libusb = orig }()
-	libusb = newFakeLibusb()
+	_, done := newFakeLibusb()
+	defer done()
 
 	for _, d := range []struct {
 		vid, pid ID