fix(replset): introduce a fixed-time server selection loop

This is required because we are moving server selection to live
outside of topologies. The legacy topologies would use the
disconnectHandler to implement a form of server selection.

Author: mbroadst

lib/core/topologies/replset.js

@@ -19,6 +19,7 @@ const isRetryableWritesSupported = require('./shared').isRetryableWritesSupporte
 const relayEvents = require('../utils').relayEvents;
 const isRetryableError = require('../error').isRetryableError;
 const BSON = retrieveBSON();
+const calculateDurationInMs = require('../utils').calculateDurationInMs;
 
 //
 // States
@@ -1110,19 +1111,36 @@ ReplSet.prototype.selectServer = function(selector, options, callback) {
     readPreference = options.readPreference || ReadPreference.primary;
   }
 
-  const server = this.s.replicaSetState.pickServer(readPreference);
-  if (server == null) {
-    callback(new MongoError('server selection failed'));
-    return;
-  }
+  let lastError;
+  const start = process.hrtime();
+  const _selectServer = () => {
+    if (calculateDurationInMs(start) >= 10000) {
+      if (lastError != null) {
+        callback(lastError, null);
+      } else {
+        callback(new MongoError('Server selection timed out'));
+      }
 
-  if (!(server instanceof Server)) {
-    callback(server, null);
-    return;
-  }
+      return;
+    }
+
+    const server = this.s.replicaSetState.pickServer(readPreference);
+    if (server == null) {
+      setTimeout(_selectServer, 1000);
+      return;
+    }
+
+    if (!(server instanceof Server)) {
+      lastError = server;
+      setTimeout(_selectServer, 1000);
+      return;
+    }
+
+    if (this.s.debug) this.emit('pickedServer', options.readPreference, server);
+    callback(null, server);
+  };
 
-  if (this.s.debug) this.emit('pickedServer', options.readPreference, server);
-  callback(null, server);
+  _selectServer();
 };
 
 /**