common/msg_queue: send backtrace on oversize queues.

Scary looking, but great for debugging!

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

common/msg_queue.c

@@ -2,10 +2,13 @@
 #include <assert.h>
 #include <ccan/cast/cast.h>
 #include <ccan/membuf/membuf.h>
+#include <common/daemon.h>
 #include <common/msg_queue.h>
 #include <common/utils.h>
 #include <wire/wire.h>
 
+static bool warned_once;
+
 struct msg_queue {
 	bool fd_passing;
 	MEMBUF(const u8 *) mb;
@@ -60,6 +63,12 @@ static void do_enqueue(struct msg_queue *q, const u8 *add TAKES)
 
 	*msg = tal_dup_talarr(q, u8, add);
 
+	if (!warned_once && msg_queue_length(q) > 100000) {
+		/* Can cause re-entry, so set flag first! */
+		warned_once = true;
+		send_backtrace("excessive queue length");
+	}
+
 	/* In case someone is waiting */
 	io_wake(q);
 }