From 5f25043b9442b7b92488f6dbe98dd02c7f8b46ae Mon Sep 17 00:00:00 2001
From: Michael Stapelberg <stapelberg@google.com>
Date: Thu, 31 Dec 2020 16:42:12 +0100
Subject: [PATCH] dhcp4d: only publish to MQTT when channel is ready to prevent
 deadlocks

---
 cmd/dhcp4d/dhcp4d.go | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/cmd/dhcp4d/dhcp4d.go b/cmd/dhcp4d/dhcp4d.go
index 5c48e6f..5a9b9e1 100644
--- a/cmd/dhcp4d/dhcp4d.go
+++ b/cmd/dhcp4d/dhcp4d.go
@@ -420,10 +420,16 @@ func newSrv(permDir string) (*srv, error) {
 		if identifier == "" {
 			identifier = latest.HardwareAddr
 		}
-		mayqtt <- PublishRequest{
+		select {
+		case mayqtt <- PublishRequest{
 			Topic:    "router7/dhcp4d/lease/" + identifier,
 			Retained: true,
 			Payload:  leaseJSON,
+		}:
+		default:
+			// Channel not ready? skip publishing this lease (best-effort).
+			// This is an easy way of breaking circular dependencies between
+			// MQTT broker and DHCP server, and avoiding deadlocks.
 		}
 	}
 	conn, err := conn.NewUDP4BoundListener(*iface, ":67")