fix(cmap): error wait queue members on failed connection creation

When a pool is empty and an operation enters the wait queue, the
pool will attempt to immediately create a new connection for the
requested operation. If that immediate creation fails, the error
should be returned to that member, rather than letting the member
sit in the queue indefinitely

NODE-2424
NODE-2372

Author: mbroadst

lib/cmap/connection_pool.js

@@ -424,7 +424,6 @@ function createConnection(pool, callback) {
 
     // if a callback has been provided, check out the connection immediately
     if (typeof callback === 'function') {
-      pool.emit('connectionCheckedOut', new ConnectionCheckedOutEvent(pool, connection));
       callback(undefined, connection);
       return;
     }
@@ -474,7 +473,30 @@ function processWaitQueue(pool) {
 
   const maxPoolSize = pool.options.maxPoolSize;
   if (pool[kWaitQueue].length && (maxPoolSize <= 0 || pool.totalConnectionCount < maxPoolSize)) {
-    createConnection(pool);
+    createConnection(pool, (err, connection) => {
+      const waitQueueMember = pool[kWaitQueue].shift();
+      if (waitQueueMember == null) {
+        if (err == null) {
+          pool[kConnections].push(connection);
+        }
+
+        return;
+      }
+
+      if (waitQueueMember[kCancelled]) {
+        return;
+      }
+
+      if (err) {
+        pool.emit('connectionCheckOutFailed', new ConnectionCheckOutFailedEvent(pool, err));
+      } else {
+        pool.emit('connectionCheckedOut', new ConnectionCheckedOutEvent(pool, connection));
+      }
+
+      clearTimeout(waitQueueMember.timer);
+      waitQueueMember.callback(err, connection);
+    });
+
     return;
   }
 }

test/unit/cmap/connection_pool.test.js

@@ -158,13 +158,13 @@ describe('Connection Pool', function() {
 
       const callback = err => {
         expect(err).to.exist;
-        expect(err).to.match(/Timed out/);
+        expect(err).to.match(/closed/);
         pool.close(done);
       };
 
       pool.withConnection((err, conn, cb) => {
         expect(err).to.exist;
-        expect(err).to.match(/Timed out/);
+        expect(err).to.match(/closed/);
         cb(err);
       }, callback);
     });