Introduce xferMap, avoid passing Go pointer into the C code.

Weirdness with memory barriers.

usb/libusb.go

@@ -18,6 +18,7 @@ import (
 	"fmt"
 	"log"
 	"reflect"
+	"sync"
 	"time"
 	"unsafe"
 )
@@ -138,9 +139,9 @@ type libusbIntf interface {
 	setAlt(*libusbDevHandle, uint8, uint8) error
 
 	// transfer
-	alloc(*libusbDevHandle, *EndpointInfo, time.Duration, int, []byte) (*libusbTransfer, error)
+	alloc(*libusbDevHandle, *EndpointInfo, time.Duration, int, []byte, chan struct{}) (*libusbTransfer, error)
 	cancel(*libusbTransfer) error
-	submit(*libusbTransfer, chan struct{}) error
+	submit(*libusbTransfer) error
 	data(*libusbTransfer) (int, TransferStatus)
 	free(*libusbTransfer)
 	setIsoPacketLengths(*libusbTransfer, uint32)
@@ -371,7 +372,7 @@ func (libusbImpl) setAlt(d *libusbDevHandle, iface, setup uint8) error {
 	return fromErrNo(C.libusb_set_interface_alt_setting((*C.libusb_device_handle)(d), C.int(iface), C.int(setup)))
 }
 
-func (libusbImpl) alloc(d *libusbDevHandle, ep *EndpointInfo, timeout time.Duration, isoPackets int, buf []byte) (*libusbTransfer, error) {
+func (libusbImpl) alloc(d *libusbDevHandle, ep *EndpointInfo, timeout time.Duration, isoPackets int, buf []byte, done chan struct{}) (*libusbTransfer, error) {
 	xfer := C.libusb_alloc_transfer(C.int(isoPackets))
 	if xfer == nil {
 		return nil, fmt.Errorf("libusb_alloc_transfer(%d) failed", isoPackets)
@@ -381,17 +382,20 @@ func (libusbImpl) alloc(d *libusbDevHandle, ep *EndpointInfo, timeout time.Durat
 	xfer.timeout = C.uint(timeout / time.Millisecond)
 	xfer._type = C.uchar(ep.TransferType)
 	xfer.num_iso_packets = C.int(isoPackets)
-	xfer.buffer = (*C.uchar)((unsafe.Pointer)(&buf[0]))
+	xfer.buffer = (*C.uchar)(unsafe.Pointer(&buf[0]))
 	xfer.length = C.int(len(buf))
-	return (*libusbTransfer)(xfer), nil
+	ret := (*libusbTransfer)(xfer)
+	xferDoneMap.Lock()
+	xferDoneMap.m[ret] = done
+	xferDoneMap.Unlock()
+	return ret, nil
 }
 
 func (libusbImpl) cancel(t *libusbTransfer) error {
 	return fromErrNo(C.libusb_cancel_transfer((*C.struct_libusb_transfer)(t)))
 }
 
-func (libusbImpl) submit(t *libusbTransfer, done chan struct{}) error {
+func (libusbImpl) submit(t *libusbTransfer) error {
-	t.user_data = (unsafe.Pointer)(&done)
 	return fromErrNo(C.submit((*C.struct_libusb_transfer)(t)))
 }
 
@@ -406,6 +410,9 @@ func (libusbImpl) data(t *libusbTransfer) (int, TransferStatus) {
 
 func (libusbImpl) free(t *libusbTransfer) {
 	C.libusb_free_transfer((*C.struct_libusb_transfer)(t))
+	xferDoneMap.Lock()
+	delete(xferDoneMap.m, t)
+	xferDoneMap.Unlock()
 }
 
 func (libusbImpl) setIsoPacketLengths(t *libusbTransfer, length uint32) {
@@ -420,10 +427,20 @@ var (
 	libusbIsoOffset = unsafe.Offsetof(C.struct_libusb_transfer{}.iso_packet_desc)
 )
 
+// xferDoneMap keeps a map of done callback channels for all allocated transfers.
+var xferDoneMap = struct {
+	m map[*libusbTransfer]chan struct{}
+	sync.RWMutex
+}{
+	m: make(map[*libusbTransfer]chan struct{}),
+}
+
 //export xferCallback
-func xferCallback(cptr unsafe.Pointer) {
+func xferCallback(xfer *C.struct_libusb_transfer) {
-	ch := *(*chan struct{})(cptr)
+	xferDoneMap.RLock()
-	close(ch)
+	ch := xferDoneMap.m[(*libusbTransfer)(xfer)]
+	xferDoneMap.RUnlock()
+	ch <- struct{}{}
 }
 
 // for benchmarking and testing