auto restart prometheus metrics server (#669)

src/main/kotlin/io/emeraldpay/dshackle/monitoring/MonitoringSetup.kt

@@ -32,7 +32,9 @@ import org.slf4j.LoggerFactory
 import org.springframework.beans.factory.annotation.Autowired
 import org.springframework.stereotype.Service
 import java.io.IOException
+import java.net.InetAddress
 import java.net.InetSocketAddress
+import java.net.ServerSocket
 import java.util.zip.GZIPOutputStream
 import javax.annotation.PostConstruct
 
@@ -45,6 +47,20 @@ class MonitoringSetup(
         private val log = LoggerFactory.getLogger(MonitoringSetup::class.java)
     }
 
+    private fun isTcpPortAvailable(host: String, port: Int): Boolean {
+        try {
+            ServerSocket().use { serverSocket ->
+                // setReuseAddress(false) is required only on macOS,
+                // otherwise the code will not work correctly on that platform
+                serverSocket.reuseAddress = false
+                serverSocket.bind(InetSocketAddress(InetAddress.getByName(host), port), 1)
+                return true
+            }
+        } catch (ex: java.lang.Exception) {
+            return false
+        }
+    }
+
     @PostConstruct
     fun setup() {
         val prometheusRegistry = PrometheusMeterRegistry(PrometheusConfig.DEFAULT)
@@ -75,6 +91,10 @@ class MonitoringSetup(
         if (monitoringConfig.prometheus.enabled) {
             // use standard JVM server with a single thread blocking processing
             // prometheus is a single thread periodic call, no reason to setup anything complex
+            var started = false
+            while (true) {
+                if (isTcpPortAvailable(monitoringConfig.prometheus.host, monitoringConfig.prometheus.port)) {
+                    started = true
                     try {
                         log.info("Run Prometheus metrics on ${monitoringConfig.prometheus.host}:${monitoringConfig.prometheus.port}${monitoringConfig.prometheus.path}")
                         val server = HttpServer.create(
@@ -99,6 +119,13 @@ class MonitoringSetup(
                     } catch (e: IOException) {
                         log.error("Failed to start Prometheus Server", e)
                     }
+                } else {
+                    if (!started) {
+                        log.error("Can't start prometheus metrics on ${monitoringConfig.prometheus.host}:${monitoringConfig.prometheus.port}${monitoringConfig.prometheus.path}")
+                    }
+                    Thread.sleep(1000)
+                }
+            }
         }
     }
 }