feature: mdns retry with indexed names

Add mdns advertisement retry with instance names that
have the retry count appended to the name. Retries
9 times and gives up.

src/mdns.js

@@ -98,27 +98,7 @@ module.exports = function mdnsResponder(app) {
 
   const ads = []
   types.forEach((type, i) => {
-    debug(
-      'Starting mDNS ad: ' +
-        type.type +
-        ' ' +
-        app.config.getExternalHostname() +
-        ':' +
-        type.port
-    )
-    let name
-    if (instanceName) {
-      name = toUtfMaxLength(i === 0 ? `SK ${instanceName}` : instanceName)
-    }
-    const optionsForType = { name, ...options }
-    debug(optionsForType)
-    const ad = new mdns.Advertisement(type.type, type.port, optionsForType)
-    ad.on('error', err => {
-      console.log(type.type.name)
-      console.error(err)
-    })
-    ad.start()
-    ads.push(ad)
+    startAdWithNamedRetry(mdns, type, i, options, instanceName, ads, 0)
   })
 
   return {
@@ -131,19 +111,71 @@ module.exports = function mdnsResponder(app) {
   }
 }
 
+const MAX_RETRIES = 9
+
+function startAdWithNamedRetry(
+  mdns,
+  type,
+  i,
+  options,
+  instanceName,
+  ads,
+  retryIndex
+) {
+  if (retryIndex > MAX_RETRIES) {
+    return
+  }
+
+  debug(`Starting mDNS ad: ${type.type} ${type.host} ${type.port}`)
+
+  let name
+  if (instanceName) {
+    name = toLengthCappedIndexedName(
+      i === 0 ? `SK ${instanceName}` : instanceName,
+      retryIndex
+    )
+  }
+  const optionsForType = { name, ...options }
+  debug(optionsForType)
+  const ad = new mdns.Advertisement(type.type, type.port, optionsForType)
+  ad.on('error', err => {
+    console.log(type.type.name)
+    console.error(err)
+    try {
+      ad.stop()
+    } catch (e) {
+      console.error(e)
+    }
+    startAdWithNamedRetry(
+      mdns,
+      type,
+      i,
+      options,
+      instanceName,
+      ads,
+      retryIndex + 1
+    )
+  })
+  ad.start()
+  ads[i] = ad
+}
+
 const AD_NAME_MAX_UTF_LENGTH = 63 - 3 //allow prefix 'SK ' for http
 
 function getInstanceName(signalk) {
   const full = signalk.retrieve()
   return _.get(full, `${_.get(full, 'self')}.name`)
 }
 
-function toUtfMaxLength(s) {
+// return the string with utf length capped to 60
+// with retry count appended as -n for n >0
+function toLengthCappedIndexedName(s, retryIndex) {
   let result = s
-  while (utfLength(result) > AD_NAME_MAX_UTF_LENGTH) {
+  const maxLength = AD_NAME_MAX_UTF_LENGTH - (retryIndex > 0 ? '-X'.length : 0)
+  while (utfLength(result) > maxLength) {
     result = result.slice(0, result.length - 1)
   }
-  return result
+  return result + (retryIndex > 0 ? `-${retryIndex}` : '')
 }
 
 function utfLength(s) {