refactor(pool): maintain legacy pool reconnect behavior

lib/core/connection/pool.js

@@ -3,7 +3,7 @@
 const inherits = require('util').inherits;
 const EventEmitter = require('events').EventEmitter;
 const MongoError = require('../error').MongoError;
-const MongoNetworkError = require('../error').MongoNetworkError;
+const MongoTimeoutError = require('../error').MongoTimeoutError;
 const MongoWriteConcernError = require('../error').MongoWriteConcernError;
 const Logger = require('./logger');
 const f = require('util').format;
@@ -113,7 +113,9 @@ var Pool = function(topology, options) {
       reconnectInterval: 1000,
       reconnectTries: 30,
       // Enable domains
-      domainsEnabled: false
+      domainsEnabled: false,
+      // feature flag for determining if we are running with the unified topology or not
+      legacyCompatMode: true
     },
     options
   );
@@ -123,6 +125,7 @@ var Pool = function(topology, options) {
   // Current reconnect retries
   this.retriesLeft = this.options.reconnectTries;
   this.reconnectId = null;
+  this.reconnectError = null;
   // No bson parser passed in
   if (
     !options.bson ||
@@ -289,6 +292,7 @@ function connectionFailureHandler(pool, event, err, conn) {
 
   // Start reconnection attempts
   if (!pool.reconnectId && pool.options.reconnect) {
+    pool.reconnectError = err;
     pool.reconnectId = setTimeout(attemptReconnect(pool), pool.options.reconnectInterval);
   }
 
@@ -315,10 +319,11 @@ function attemptReconnect(pool, callback) {
     if (pool.retriesLeft <= 0) {
       pool.destroy();
 
-      const error = new MongoNetworkError(
+      const error = new MongoTimeoutError(
         `failed to reconnect after ${pool.options.reconnectTries} attempts with interval ${
           pool.options.reconnectInterval
-        } ms`
+        } ms`,
+        pool.reconnectError
       );
 
       pool.emit('reconnectFailed', error);
@@ -333,7 +338,17 @@ function attemptReconnect(pool, callback) {
     pool.reconnectId = null;
 
     // now retry creating a connection
-    createConnection(pool, callback);
+    createConnection(pool, (err, conn) => {
+      if (err == null) {
+        pool.reconnectId = null;
+        pool.retriesLeft = pool.options.reconnectTries;
+        pool.emit('reconnect', pool);
+      }
+
+      if (typeof callback === 'function') {
+        callback(err, conn);
+      }
+    });
   };
 }
 
@@ -943,7 +958,23 @@ function createConnection(pool, callback) {
         pool.logger.debug(`connection attempt failed with error [${JSON.stringify(err)}]`);
       }
 
+      if (pool.options.legacyCompatMode === false) {
+        // The unified topology uses the reported `error` from a pool to track what error
+        // reason is returned to the user during selection timeout. We only want to emit
+        // this if the pool is active because the listeners are removed on destruction.
+        if (pool.state !== DESTROYED && pool.state !== DESTROYING) {
+          pool.emit('error', err);
+        }
+      }
+
+      // check if reconnect is enabled, and attempt retry if so
       if (!pool.reconnectId && pool.options.reconnect) {
+        if (pool.state === CONNECTING && pool.options.legacyCompatMode) {
+          callback(err);
+          return;
+        }
+
+        pool.reconnectError = err;
         pool.reconnectId = setTimeout(
           attemptReconnect(pool, callback),
           pool.options.reconnectInterval
@@ -978,13 +1009,6 @@ function createConnection(pool, callback) {
 
     pool.availableConnections.push(connection);
 
-    // if there is a reconnect in progress, reset state and emit event
-    if (pool.reconnectId) {
-      pool.reconnectId = null;
-      pool.retriesLeft = pool.options.reconnectTries;
-      pool.emit('reconnect', pool);
-    }
-
     // if a callback was provided, return the connection
     if (typeof callback === 'function') {
       callback(null, connection);

lib/core/sdam/server.js

@@ -125,6 +125,7 @@ class Server extends EventEmitter {
 
     // NOTE: this should only be the case if we are connecting to a single server
     poolOptions.reconnect = true;
+    poolOptions.legacyCompatMode = false;
 
     this.s.pool = new Pool(this, poolOptions);