closingd: Ensure proper closing of TCP socket.

Fixes: #1457

common/socket_close.h

@@ -0,0 +1,23 @@
+/* common/socket_close - Properly close a socket,
+ * ensuring that any data we write just before
+ * the close has been transmitted to the other
+ * side, and ignoring any data the other side
+ * has sent at the time the close was started.
+ *
+ * Reference:
+ *
+ * http://ia800504.us.archive.org/3/items/TheUltimateSo_lingerPageOrWhyIsMyTcpNotReliable/the-ultimate-so_linger-page-or-why-is-my-tcp-not-reliable.html
+ */
+#ifndef LIGHTNING_COMMON_SOCKET_CLOSE_H
+#define LIGHTNING_COMMON_SOCKET_CLOSE_H
+#include "config.h"
+#include <stdbool.h>
+
+/* Return false if something failed, true if
+ * nothing failed.
+ * If something failed, error is stored in
+ * `errno.
+ */
+bool socket_close(int fd);
+
+#endif /* LIGHTNING_COMMON_SOCKET_CLOSE_H */