[IO] document and adjust tests to changes in io_uring's broken link handling

Until kernel commit cf10960426515 io_uring submitted sqe until it encountered
an invalid one. Now io_uring will catch the error on subimssion and
will cancel all sqe's contained in the link prior to the invalid one.

emper/io/IoContext.cpp

@@ -133,13 +133,18 @@ void IoContext::submit(Future &future) {
 	// Because we submit every Future right away multiple prepared sqes can only
 	// occur for Future chains.
 	// If we could not submit the whole chain cancel all non submitted Futures
-	// because we can not guaranty the soundness of the chain and invalidate their
-	// prepared sqe
+	// because we can not guaranty the soundness of the chain have to invalidate
+	// non submitted but prepared sqes
 	// req1 -> invalid_req -> req3
 	// will submit only 2 instead of all 3 prepared sqes leaving a sqe for req3
 	// in the SQ but unsubmitted
 	// See: https://github.com/axboe/liburing/issues/186
 	// https://github.com/axboe/liburing/issues/92
+	//
+	// Since kernel commit cf10960426515 io_uring does no longer submit parts of a
+	// broken chains. This means that all sqes in a chain except the broken one will
+	// result in cqes with result -ECANCELD and the invalid one will
+	// generate a cqe with the appropriate error code
 	if (unlikely(static_cast<unsigned>(submitted) < prepared)) {
 		unsigned unsubmitted = io_uring_sq_ready(&ring);
 		Future *unsubmittedFuture = &future;

tests/io/LinkFutureTest.cpp

@@ -119,7 +119,10 @@ static void failureChainCorInvCor() {
 	assert(res == -EBADF);
 
 	res = correctFuture1.wait();
-	assert(res == (int32_t)buf.size());
+	// Since the kernel commit cf10960426515 io_uring does not
+	// submit broken links and completes any request before a
+	// invalid one as canceled.
+	assert(res == -ECANCELED || res == (int32_t)buf.size());
 
 	emper::io::closeAndWait(fd);
 }