Add TCP keep-alive on ZMQ socket (#1807)

One of ZMQ's drawbacks is that subscribers on an unreliable network may
silently disconnect from publishers in case of network failures.

In our case, we want to reconnect immediately when that happens, so we set
a tcp keep-alive to ensure this.

Fixes #1789

eclair-core/pom.xml

@@ -196,7 +196,7 @@
         <dependency>
             <groupId>org.zeromq</groupId>
             <artifactId>jeromq</artifactId>
-            <version>0.5.0</version>
+            <version>0.5.2</version>
         </dependency>
         <!-- SERIALIZATION -->
         <dependency>

eclair-core/src/main/scala/fr/acinq/eclair/blockchain/bitcoind/zmq/ZMQActor.scala

@@ -41,6 +41,7 @@ class ZMQActor(address: String, topic: String, connected: Option[Promise[Done]]
   subscriber.monitor("inproc://events", ZMQ.EVENT_CONNECTED | ZMQ.EVENT_DISCONNECTED)
   subscriber.connect(address)
   subscriber.subscribe(topic.getBytes(ZMQ.CHARSET))
+  subscriber.setTCPKeepAlive(1)
 
   val monitor = ctx.createSocket(SocketType.PAIR)
   monitor.connect("inproc://events")
@@ -49,18 +50,18 @@ class ZMQActor(address: String, topic: String, connected: Option[Promise[Done]]
 
   // we check messages in a non-blocking manner with an interval, making sure to retrieve all messages before waiting again
   @tailrec
-  final def checkEvent: Unit = Option(Event.recv(monitor, ZMQ.DONTWAIT)) match {
+  final def checkEvent(): Unit = Option(Event.recv(monitor, ZMQ.DONTWAIT)) match {
     case Some(event) =>
       self ! event
-      checkEvent
+      checkEvent()
     case None => ()
   }
 
   @tailrec
-  final def checkMsg: Unit = Option(ZMsg.recvMsg(subscriber, ZMQ.DONTWAIT)) match {
+  final def checkMsg(): Unit = Option(ZMsg.recvMsg(subscriber, ZMQ.DONTWAIT)) match {
     case Some(msg) =>
       self ! msg
-      checkMsg
+      checkMsg()
     case None => ()
   }
 
@@ -69,11 +70,11 @@ class ZMQActor(address: String, topic: String, connected: Option[Promise[Done]]
 
   override def receive: Receive = {
     case Symbol("checkEvent") =>
-      checkEvent
+      checkEvent()
       context.system.scheduler.scheduleOnce(1 second, self, Symbol("checkEvent"))
 
     case Symbol("checkMsg") =>
-      checkMsg
+      checkMsg()
       context.system.scheduler.scheduleOnce(1 second, self, Symbol("checkMsg"))
 
     case event: Event => event.getEvent match {

eclair-core/src/test/scala/fr/acinq/eclair/blockchain/bitcoind/ZmqWatcherSpec.scala

@@ -84,6 +84,22 @@ class ZmqWatcherSpec extends TestKitBaseClass with AnyFunSuiteLike with Bitcoind
     }
   }
 
+  test("reconnect ZMQ automatically") {
+    withWatcher(f => {
+      import f._
+
+      // When the watcher starts, it broadcasts the current height.
+      val block1 = listener.expectMsgType[CurrentBlockCount]
+      listener.expectNoMessage(100 millis)
+
+      restartBitcoind(probe)
+      generateBlocks(1)
+      val block2 = listener.expectMsgType[CurrentBlockCount]
+      assert(block2.blockCount === block1.blockCount + 1)
+      listener.expectNoMessage(100 millis)
+    })
+  }
+
   test("add/remove watches from/to utxo map") {
     val m0 = Map.empty[OutPoint, Set[Watch[_ <: WatchTriggered]]]
     val txid = randomBytes32