Add keepalive comment messages, per spec

keepalive.cc

@@ -0,0 +1,42 @@
+#include <poll.h>
+#include <sys/eventfd.h>
+
+#include "keepalive.h"
+
+namespace firesse {
+
+KeepAlive::KeepAlive(const std::chrono::nanoseconds& max_stale, Index* index)
+		: max_stale_(max_stale),
+		  index_(index),
+		  shutdown_(eventfd(0, 0)) {
+	PCHECK(shutdown_ >= 0) << "eventfd()";
+}
+
+void KeepAlive::Start() {
+	thread_ = std::thread([this]() {
+		int timeout = 0;	
+		constexpr auto num_fds = 1;
+		pollfd fds[num_fds] = {
+			{
+				.fd = shutdown_,
+				.events = POLLIN,
+			},
+		};
+		while (poll(fds, num_fds, timeout) <= 0) {
+			auto sleep = index_->WithStalest([](Stream* stream) {
+				stream->WriteRaw(":\n");
+			}, max_stale_);
+			timeout = std::chrono::duration_cast<std::chrono::milliseconds>(sleep).count();
+		}
+	});
+}
+
+void KeepAlive::Stop() {
+	CHECK(thread_.joinable());
+
+	uint64_t shutdown = 1;
+	PCHECK(write(shutdown_, &shutdown, sizeof(shutdown)) == sizeof(shutdown));
+	thread_.join();
+}
+
+} // namespace firesse