io/uring/Queue: add method RequireSubmitEntry()

Fixes assertion failure when the submit queue is empty.

src/io/uring/Queue.cxx

@@ -34,6 +34,8 @@
 #include "CancellableOperation.hxx"
 #include "util/DeleteDisposer.hxx"
 
+#include <stdexcept>
+
 namespace Uring {
 
 Queue::Queue(unsigned entries, unsigned flags)
@@ -46,6 +48,23 @@ Queue::~Queue() noexcept
 	operations.clear_and_dispose(DeleteDisposer{});
 }
 
+struct io_uring_sqe &
+Queue::RequireSubmitEntry()
+{
+	auto *sqe = GetSubmitEntry();
+	if (sqe == nullptr) {
+		/* the submit queue is full; submit it to the kernel
+		   and try again */
+		Submit();
+
+		sqe = GetSubmitEntry();
+		if (sqe == nullptr)
+			throw std::runtime_error{"io_uring_get_sqe() failed"};
+	}
+
+	return *sqe;
+}
+
 void
 Queue::AddPending(struct io_uring_sqe &sqe,
 		  Operation &operation) noexcept

src/io/uring/Queue.hxx

@@ -63,6 +63,14 @@ public:
 		return ring.GetSubmitEntry();
 	}
 
+	/**
+	 * Like GetSubmitEntry(), but call Submit() if the submit
+	 * queue is full.
+	 *
+	 * May throw exceptions if Submit() fails.
+	 */
+	struct io_uring_sqe &RequireSubmitEntry();
+
 	bool HasPending() const noexcept {
 		return !operations.empty();
 	}

src/io/uring/ReadOperation.cxx

@@ -45,14 +45,13 @@ ReadOperation::Start(Queue &queue, FileDescriptor fd, off_t offset,
 
 	buffer = std::make_unique<std::byte[]>(size);
 
-	auto *s = queue.GetSubmitEntry();
+	auto &s = queue.RequireSubmitEntry();
-	assert(s != nullptr); // TODO: what if the submit queue is full?
 
 	iov.iov_base = buffer.get();
 	iov.iov_len = size;
 
-	io_uring_prep_readv(s, fd.Get(), &iov, 1, offset);
+	io_uring_prep_readv(&s, fd.Get(), &iov, 1, offset);
-	queue.Push(*s, *this);
+	queue.Push(s, *this);
 }
 
 void