Linux: try harder not to lose any data

We would previously lose any data that was present on a cancelled URB.
Work harder to make sure this doesn't happen.

1 files changed, 35 insertions, 11 deletions

diff --git a/libusb/os/linux_usbfs.c b/libusb/os/linux_usbfs.c
index f9899cf..1280188 100644
--- a/libusb/os/linux_usbfs.c
+++ b/libusb/os/linux_usbfs.c
@@ -1710,22 +1710,41 @@ static int handle_bulk_completion(struct usbi_transfer *itransfer,
 
 	tpriv->num_retired++;
 
-	if (urb->status == 0 ||
-			(urb->status == -EOVERFLOW && urb->actual_length > 0))
-		itransfer->transferred += urb->actual_length;
-
 	if (tpriv->reap_action != NORMAL) {
 		/* cancelled, submit_fail, or completed early */
-		if (urb->status == 0 && tpriv->reap_action == COMPLETED_EARLY) {
-			/* FIXME we could solve this extreme corner case with a memmove
-			 * or something */
-			usbi_warn(ITRANSFER_CTX(itransfer), "SOME DATA LOST! "
-				"(completed early but remaining urb completed)");
+		usbi_dbg("abnormal reap: urb status %d", urb->status);
+
+		/* even though we're in the process of cancelling, it's possible that
+		 * we may receive some data in these URBs that we don't want to lose.
+		 * examples:
+		 * 1. while the kernel is cancelling all the packets that make up an
+		 *    URB, a few of them might complete. so we get back a successful
+		 *    cancellation *and* some data.
+		 * 2. we receive a short URB which marks the early completion condition,
+		 *    so we start cancelling the remaining URBs. however, we're too
+		 *    slow and another URB completes (or at least completes partially).
+		 *
+		 * When this happens, our objectives are not to lose any "surplus" data,
+		 * and also to stick it at the end of the previously-received data
+		 * (closing any holes), so that libusb reports the total amount of
+		 * transferred data and presents it in a contiguous chunk.
+		 */
+		if (urb->actual_length > 0) {
+			struct libusb_transfer *transfer =
+				__USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
+			unsigned char *target = transfer->buffer + itransfer->transferred;
+			usbi_dbg("received %d bytes of surplus data", urb->actual_length);
+			if (urb->buffer != target) {
+				usbi_dbg("moving surplus data from offset %d to offset %d",
+					(unsigned char *) urb->buffer - transfer->buffer,
+					target - transfer->buffer);
+				memmove(target, urb->buffer, urb->actual_length);
+			}
+			itransfer->transferred += urb->actual_length;
 		}
-		usbi_dbg("CANCEL: urb status %d", urb->status);
 
 		if (tpriv->num_retired == num_urbs) {
-			usbi_dbg("CANCEL: last URB handled, reporting");
+			usbi_dbg("abnormal reap: last URB handled, reporting");
 			if (tpriv->reap_action == CANCELLED) {
 				free(tpriv->urbs);
 				tpriv->urbs = NULL;
@@ -1739,6 +1758,11 @@ static int handle_bulk_completion(struct usbi_transfer *itransfer,
 		return 0;
 	}
 
+	if (urb->status == 0 ||
+			(urb->status == -EOVERFLOW && urb->actual_length > 0))
+		itransfer->transferred += urb->actual_length;
+
+
 	switch (urb->status) {
 	case 0:
 		break;