refactor(pool): support creating connections in destroying state

We have some edge cases in our testing where `endSessions` is sent
during `destroy`, but the pool might not have enough open
connections in that case.

Author: mbroadst

lib/core/connection/pool.js

@@ -560,18 +560,19 @@ Pool.prototype.connect = function(callback) {
     }
 
     stateTransition(this, CONNECTED);
-    if (typeof callback === 'function') {
-      callback(null, conn);
-    } else {
-      this.emit('connect', this, conn);
-    }
 
     // create min connections
     if (this.minSize) {
       for (let i = 0; i < this.minSize; i++) {
         createConnection(this);
       }
     }
+
+    if (typeof callback === 'function') {
+      callback(null, conn);
+    } else {
+      this.emit('connect', this, conn);
+    }
   });
 };
 
@@ -687,7 +688,6 @@ Pool.prototype.destroy = function(force, callback) {
       }
 
       destroy(self, connections, { force: false }, callback);
-      // } else if (self.queue.length > 0 && !this.reconnectId) {
     } else {
       // Ensure we empty the queue
       _execute(self)();
@@ -942,7 +942,7 @@ function removeConnection(self, connection) {
 }
 
 function createConnection(pool, callback) {
-  if (pool.state === DESTROYED || pool.state === DESTROYING) {
+  if (pool.state === DESTROYED) {
     if (typeof callback === 'function') {
       callback(new MongoError('Cannot create connection when pool is destroyed'));
     }
@@ -983,7 +983,7 @@ function createConnection(pool, callback) {
     }
 
     // the pool might have been closed since we started creating the connection
-    if (pool.state === DESTROYED || pool.state === DESTROYING) {
+    if (pool.state === DESTROYED) {
       if (typeof callback === 'function') {
         callback(new MongoError('Pool was destroyed after connection creation'));
       }
@@ -1050,6 +1050,12 @@ function _execute(self) {
       if (self.availableConnections.length === 0) {
         // Flush any monitoring operations
         flushMonitoringOperations(self.queue);
+
+        // attempt to grow the pool
+        if (totalConnections < self.options.size) {
+          createConnection(self);
+        }
+
         break;
       }
 
@@ -1114,10 +1120,7 @@ function _execute(self) {
           }
 
           // Re-execute the operation
-          setTimeout(function() {
-            _execute(self)();
-          }, 10);
-
+          setTimeout(() => _execute(self)(), 10);
           break;
         }
       }