How to flush IN bulk endpoint buffer on device - usb

Question

I'd like to make sure that on the device connected to Chrome (via WebUSB) IN endpoint doesn't contain messages from previous bulk transmission. I checked the API for the WebUSB:
https://wicg.github.io/webusb/
and I don't see any kind of flush function that would allow emptying buffer. I was thinking about reading data until device returns NAK - something like this:
/* #1 Make sure that IN endpoint contains no more data */
while (true) {
let result = await data.transferIn(1, 6);
if (result.length === 0) {
break;
}
}
/* #2 Send request */
await data.transferOut(0x81, message);
/* #3 Receive valid request */
let result = await data.transferIn(1, 6);
but unfortunately it looks that there is no good solution:
when there is no more data to read the transferIn() becomes blocking
function - so we cannot relay on async calling transferIn()
when transferIn() is called in the promise with timeout we can end
with more than one promise waiting for incoming data (which is bad
since we don't know which promise would receive data)
What would be the best approach for making sure the device IN endpoint contains no data?

Answer 1

The concept of an "IN bulk endpoint buffer" doesn't exist in the USB specification. Whether a device responds to an IN PID with DATA or NACK is entirely up the device. The answer may be generated on the fly based on the device state, or be fed from an internal buffer. There is no way for the host to know that the buffer is "empty". This is something that has to be defined at a higher protocol layer between the host and device.
For example, the host or device may indicate the amount of data that is expected to be transfered. The host then knows how much data it can expect to read from the IN endpoint before the current operation is complete. This is how the USB Mass Storage protocol works.
If the protocol between the host and device doesn't define these kinds of message boundaries the best way to flush to buffer is not to try. Instead, always be reading from the IN endpoint and interpret data as it is received. This may include using setTimeout() to checking whether a response has been received to a particular request within a given deadline. Data received not in response to a request could be discarded if it is uninteresting.