tcp keepalive: user TCP_USER_TIMEOUT on linux if extant

https://github.com/warmcat/libwebsockets/issues/1223

CMakeLists.txt

@@ -1365,6 +1365,11 @@ endif()
 
 CHECK_C_SOURCE_COMPILES("#define _GNU_SOURCE\n#include <unistd.h>\nint main(void) {int fd[2];\n return pipe2(fd, 0);\n}\n" LWS_HAVE_PIPE2)
 
+# tcp keepalive needs this on linux to work practically... but it only exists
+# after kernel 2.6.37
+
+CHECK_C_SOURCE_COMPILES("#include <linux/tcp.h>\nint main(void) { return TCP_USER_TIMEOUT; }\n" LWS_HAVE_TCP_USER_TIMEOUT)
+
 set(CMAKE_REQUIRED_LIBRARIES ${temp})
 # Generate the lws_config.h that includes all the public compilation settings.
 configure_file(

cmake/lws_config_private.h.in

@@ -81,6 +81,8 @@
 /* Define to 1 if you have the <unistd.h> header file. */
 #cmakedefine LWS_HAVE_UNISTD_H
 
+#cmakedefine LWS_HAVE_TCP_USER_TIMEOUT
+
 /* Define to 1 if you have the `vfork' function. */
 #cmakedefine LWS_HAVE_VFORK
 

lib/plat/lws-plat-unix.c

@@ -348,6 +348,14 @@ lws_plat_set_socket_options(struct lws_vhost *vhost, int fd)
 		 */
 #else
 		/* set the keepalive conditions we want on it too */
+
+#if defined(LWS_HAVE_TCP_USER_TIMEOUT)
+		optval = 1000 * (vhost->ka_time +
+				 (vhost->ka_interval * vhost->ka_probes));
+		if (setsockopt(fd, IPPROTO_TCP, TCP_USER_TIMEOUT,
+			       (const void *)&optval, optlen) < 0)
+			return 1;
+#endif
 		optval = vhost->ka_time;
 		if (setsockopt(fd, IPPROTO_TCP, TCP_KEEPIDLE,
 			       (const void *)&optval, optlen) < 0)