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Don't print an error when libusb_cancel_transfer() fails with NOT_FOUND

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As stated in the documentation for libusb_cancel_transfer(),
LIBUSB_ERROR_NOT_FOUND is an expected return value for
libusb_cancel_transfer() under certain circumstances, so
printing an error every time this happens is undesirable.

Even more so because under Linux IOCTL_USBFS_DISCARDURB sets errno
to EINVAL when the kernel can't not find the urb in the kernel's
urbs-in-flight list, which means that the urb has already completed
at the host controller level but it has not necessarily been reaped
yet. IOW under Linux libusb_cancel_transfer() may yield a result of
LIBUSB_ERROR_NOT_FOUND *before* the transfer's callback has been
called! In conclusion there is no way for applications to avoid
calling libusb_cancel_transfer() on already completed transfers,
and these errors can and do happen frequently for some USB traffic.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
This commit is contained in:
Hans de Goede
2012-03-09 11:31:07 +01:00
committed by Peter Stuge
parent 4db8275da5
commit 98bc7b8d12
1 changed files with 5 additions and 2 deletions
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libusb/io.c
+5 -2
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@@ -1353,8 +1353,11 @@ int API_EXPORTED libusb_cancel_transfer(struct libusb_transfer *transfer)
usbi_mutex_lock(&itransfer->lock);
r = usbi_backend->cancel_transfer(itransfer);
if (r < 0) {
usbi_err(TRANSFER_CTX(transfer),
"cancel transfer failed error %d", r);
if (r != LIBUSB_ERROR_NOT_FOUND)
usbi_err(TRANSFER_CTX(transfer),
"cancel transfer failed error %d", r);
else
usbi_dbg("cancel transfer failed error %d", r);
if (r == LIBUSB_ERROR_NO_DEVICE)
itransfer->flags |= USBI_TRANSFER_DEVICE_DISAPPEARED;