lightningd/io_loop_with_timers.c: Move mainloop to its own source file, have chaintopology use it.

Fixes: #2687

lightningd/io_loop_with_timers.c

@@ -0,0 +1,36 @@
+#include "lightningd/io_loop_with_timers.h"
+
+#include <ccan/io/io.h>
+#include <ccan/timer/timer.h>
+#include <common/timeout.h>
+#include <lightningd/lightningd.h>
+#include <wallet/db.h>
+#include <wallet/wallet.h>
+
+void *io_loop_with_timers(struct lightningd *ld)
+{
+	void *retval = NULL;
+	struct timer *expired;
+
+	while (!retval) {
+		/* ~ccan/io's io_loop() continuously calls
+		 * io_poll_lightningd() for all file descriptors registered
+		 * with it, then calls their callbacks or closes them if they
+		 * fail, as appropriate.
+		 *
+		 * It will only exit if there's an expired timer, *or* someone
+		 * calls io_break, or if there are no more file descriptors
+		 * (which never happens in our code). */
+		retval = io_loop(&ld->timers, &expired);
+
+		/*~ Notice that timers are called here in the event loop like
+		 * anything else, so there are no weird concurrency issues. */
+		if (expired) {
+			db_begin_transaction(ld->wallet->db);
+			timer_expired(ld, expired);
+			db_commit_transaction(ld->wallet->db);
+		}
+	}
+
+	return retval;
+}