add config.performance.unfurlSrvCacheTtl option for caching resolveSrv calls

lib/servers.js

@@ -1,4 +1,5 @@
 const crypto = require('crypto')
+const dns = require('dns')
 
 const TimeTracker = require('./time')
 const Server = require('./server')
@@ -313,6 +314,62 @@ class ServerRegistration {
       color: this.data.color
     }
   }
+
+  unfurlSrv (callback) {
+    // Skip unfurling SRV, instantly return pre-configured data
+    if (config.performance && config.performance.skipUnfurlSrv) {
+      callback(this.data.ip, this.data.port)
+      return
+    }
+
+    const timestamp = new Date().getTime()
+
+    // If a cached copy exists and is within its TTL, instantly return
+    if (this._lastUnfurlSrv && timestamp - this._lastUnfurlSrv.timestamp <= config.performance.unfurlSrvCacheTtl) {
+      callback(this._lastUnfurlSrv.ip, this._lastUnfurlSrv.port)
+      return
+    }
+
+    // Group callbacks into an array
+    // Once resolved, fire callbacks sequentially
+    // This avoids callbacks possibly executing out of order
+    if (!this._unfurlSrvCallbackQueue) {
+      this._unfurlSrvCallbackQueue = []
+    }
+
+    this._unfurlSrvCallbackQueue.push(callback)
+
+    // Prevent multiple #resolveSrv calls per ServerRegistration
+    if (!this._isUnfurlingSrv) {
+      this._isUnfurlingSrv = true
+
+      dns.resolveSrv('_minecraft._tcp' + this.data.ip, (_, records) => {
+        this._lastUnfurlSrv = {
+          timestamp
+        }
+
+        if (records && records.length > 0) {
+          this._lastUnfurlSrv.ip = records[0].name
+          this._lastUnfurlSrv.port = records[0].port
+        } else {
+          // Provide fallback to pre-configured data
+          this._lastUnfurlSrv.ip = this.data.ip
+          this._lastUnfurlSrv.port = this.data.port
+        }
+
+        // Fire the waiting callbacks in queue
+        // Release blocking flag to allow new #resolveSrv calls
+        this._isUnfurlingSrv = false
+
+        for (const callback of this._unfurlSrvCallbackQueue) {
+          callback(this._lastUnfurlSrv.ip, this._lastUnfurlSrv.port)
+        }
+
+        // Reset the callback queue
+        this._unfurlSrvCallbackQueue = []
+      })
+    }
+  }
 }
 
 module.exports = ServerRegistration