Re: [Libguestfs] nbdcpy: from scratch nbdcopy using io_uring

I ran into a problem while working on receiving data from nbd source (Reply to NBD_CMD_READ) As you know we need to parse the error in Reply Header before we can proceed reading the data. Let's say an error occurred so instead of HEADER1_DATA1..._HEADER2_DATA2... we will get HEADER1_HEADER2_ DATA2... (as far as I know) so submitting a recv request to io_uring with length = sizeof(HEADER1+DATA1) would cause problem as it won't detect NBD packet boundaries and will give us as many bytes we ask it (I may be wrong here that's what I read till now). A remedy to this would be just submit 'header reads' to io_uring when we get a read and if header says there were no errors we can be sure there is length bytes ready to be read in the buffer(rest of the NBD packet) and read won't block. Now, as far as I can tell this would work as I expect but our main concern is avoiding copy_user_enhanced_fast_string so this won't be nice. Also attaching metadata (Operation) to read SQE doesn't make sense because As far as I know io_uring won't be able to tell the difference the read is for which io_uring request, Reply Header's handle will tell us which operation in operations vector does this NBD packet belong to. Another solution would be opening multiple sockets one for each slot in operations vector, only one NBD operation runs on a socket i.e. only one inflight request per socket, that too sounds like a bad idea. Thanks and regards, Abhay