crypto_sync: sync_crypto_write_no_delay to flush TCP after critical packets.

Avoid that 200ms loss.  We don't want to disable nagle generally,
since it's great for gossip and other traffic; we just want to push at
critical times.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/crypto_sync.c

@@ -7,6 +7,9 @@
 #include <common/utils.h>
 #include <errno.h>
 #include <inttypes.h>
+#include <netinet/in.h>
+#include <netinet/tcp.h>
+#include <sys/socket.h>
 #include <wire/wire.h>
 #include <wire/wire_sync.h>
 
@@ -48,6 +51,51 @@ void sync_crypto_write(struct crypto_state *cs, int fd, const void *msg TAKES)
 #endif
 }
 
+/* We're happy for the kernel to batch update and gossip messages, but a
+ * commitment message, for example, should be instantly sent.  There's no
+ * great way of doing this, unfortunately.
+ *
+ * Setting TCP_NODELAY on Linux flushes the socket, which really means
+ * we'd want to toggle on then off it *after* sending.  But Linux has
+ * TCP_CORK.  On FreeBSD, it seems (looking at source) not to, so
+ * there we'd want to set it before the send, and reenable it
+ * afterwards.  Even if this is wrong on other non-Linux platforms, it
+ * only means one extra packet.
+ */
+void sync_crypto_write_no_delay(struct crypto_state *cs, int fd,
+				const void *msg TAKES)
+{
+	int val;
+	int opt;
+	const char *optname;
+	static bool complained = false;
+
+#ifdef TCP_CORK
+	opt = TCP_CORK;
+	optname = "TCP_CORK";
+#elif defined(TCP_NODELAY)
+	opt = TCP_NODELAY;
+	optname = "TCP_NODELAY";
+#else
+#error "Please report platform with neither TCP_CORK nor TCP_NODELAY?"
+#endif
+
+	val = 1;
+	if (setsockopt(fd, IPPROTO_TCP, opt, &val, sizeof(val)) != 0) {
+		/* This actually happens in testing, where we blackhole the fd */
+		if (!complained) {
+			status_broken("setsockopt %s=1: %s",
+				      optname,
+				      strerror(errno));
+			complained = true;
+		}
+	}
+	sync_crypto_write(cs, fd, msg);
+
+	val = 0;
+	setsockopt(fd, IPPROTO_TCP, opt, &val, sizeof(val));
+}
+
 u8 *sync_crypto_read(const tal_t *ctx, struct crypto_state *cs, int fd)
 {
 	u8 hdr[18], *enc, *dec;

common/crypto_sync.h

@@ -9,6 +9,10 @@ struct crypto_state;
 /* Exits with peer_failed_connection_lost() if write fails. */
 void sync_crypto_write(struct crypto_state *cs, int fd, const void *msg TAKES);
 
+/* Same, but disabled nagle for this message. */
+void sync_crypto_write_no_delay(struct crypto_state *cs, int fd,
+				const void *msg TAKES);
+
 /* Exits with peer_failed_connection_lost() if can't read packet. */
 u8 *sync_crypto_read(const tal_t *ctx, struct crypto_state *cs, int fd);