Automatically retry DB connection on startup

This adds retry logic to the initial DB connection on startup.
HedgeDoc now tries connecting to the database up to 30 times, waiting
one second after each try.
This gives a database that was simultaneously started (e.g. via
docker-compose) enough time to get ready to accept connections.

Signed-off-by: David Mehren <git@herrmehren.de>

app.js

@@ -272,21 +272,38 @@ function startListen () {
   }
 }
 
-// sync db then start listen
-models.sequelize.authenticate().then(function () {
-  models.runMigrations().then(() => {
-    sessionStore.sync()
-    // check if realtime is ready
-    if (realtime.isReady()) {
-      models.Revision.checkAllNotesRevision(function (err, notes) {
-        if (err) throw new Error(err)
-        if (!notes || notes.length <= 0) return startListen()
-      })
+const maxDBTries = 30
+let currentDBTry = 1
+function syncAndListen () {
+  // sync db then start listen
+  models.sequelize.authenticate().then(function () {
+    models.runMigrations().then(() => {
+      sessionStore.sync()
+      // check if realtime is ready
+      if (realtime.isReady()) {
+        models.Revision.checkAllNotesRevision(function (err, notes) {
+          if (err) throw new Error(err)
+          if (!notes || notes.length <= 0) return startListen()
+        })
+      } else {
+        logger.error('server still not ready after db synced')
+        process.exit(1)
+      }
+    })
+  }).catch(() => {
+    if (currentDBTry < maxDBTries) {
+      logger.warn(`Database cannot be reached. Try ${currentDBTry} of ${maxDBTries}.`)
+      currentDBTry++
+      setTimeout(function () {
+        syncAndListen()
+      }, 1000)
     } else {
-      throw new Error('server still not ready after db synced')
+      logger.error('Cannot reach database! Exiting.')
+      process.exit(1)
     }
   })
-})
+}
+syncAndListen()
 
 // log uncaught exception
 process.on('uncaughtException', function (err) {