io/uring/Ring: add io_uring_for_each_cqe() wrapper

src/io/uring/Ring.hxx

@@ -126,6 +126,35 @@ public:
 	void SeenCompletion(struct io_uring_cqe &cqe) noexcept {
 		io_uring_cqe_seen(&ring, &cqe);
 	}
+
+	/**
+	 * Invoke a function with a reference to each completion (but
+	 * do not mark it "seen" or advance the completion queue
+	 * head).
+	 */
+	unsigned ForEachCompletion(struct io_uring_cqe *cqe,
+				   std::invocable<struct io_uring_cqe &> auto f) noexcept {
+		unsigned dummy, n = 0;
+
+		io_uring_for_each_cqe(&ring, dummy, cqe) {
+			++n;
+			f(*cqe);
+		}
+
+		return n;
+	}
+
+	/**
+	 * Like ForEachCompletion(), but advance the completion queue
+	 * head.
+	 */
+	unsigned VisitCompletions(struct io_uring_cqe *cqe,
+				  std::invocable<const struct io_uring_cqe &> auto f) noexcept {
+		unsigned n = ForEachCompletion(cqe, f);
+		if (n > 0)
+			io_uring_cq_advance(&ring, n);
+		return n;
+	}
 };
 
 } // namespace Uring