fix: allow event loop to process during wait queue processing

Running `processWaitQueue` on the next tick allows the event loop
to process while the connection pool is processing large numbers of
wait queue members.

NODE-2803

Author: mbroadst

lib/cmap/connection_pool.js

@@ -218,7 +218,6 @@ class ConnectionPool extends EventEmitter {
       return;
     }
 
-    // add this request to the wait queue
     const waitQueueMember = { callback };
 
     const pool = this;
@@ -233,11 +232,8 @@ class ConnectionPool extends EventEmitter {
       }, waitQueueTimeoutMS);
     }
 
-    // place the member at the end of the wait queue
     this[kWaitQueue].push(waitQueueMember);
-
-    // process the wait queue
-    processWaitQueue(this);
+    process.nextTick(() => processWaitQueue(this));
   }
 
   /**
@@ -250,10 +246,8 @@ class ConnectionPool extends EventEmitter {
     const stale = connectionIsStale(this, connection);
     const willDestroy = !!(poolClosed || stale || connection.closed);
 
-    // Properly adjust state of connection
     if (!willDestroy) {
       connection.markAvailable();
-
       this[kConnections].push(connection);
     }
 
@@ -264,7 +258,7 @@ class ConnectionPool extends EventEmitter {
       destroyConnection(this, connection, reason);
     }
 
-    processWaitQueue(this);
+    process.nextTick(() => processWaitQueue(this));
   }
 
   /**
@@ -434,7 +428,7 @@ function createConnection(pool, callback) {
 
     // otherwise add it to the pool for later acquisition, and try to process the wait queue
     pool[kConnections].push(connection);
-    processWaitQueue(pool);
+    process.nextTick(() => processWaitQueue(pool));
   });
 }
 

test/unit/cmap/connection_pool.test.js

@@ -145,11 +145,13 @@ describe('Connection Pool', function() {
         sinon.stub(pool, 'availableConnectionCount').get(() => 0);
         pool.checkIn(conn);
 
-        expect(pool)
-          .property('waitQueueSize')
-          .to.equal(0);
+        process.nextTick(() => {
+          expect(pool)
+            .property('waitQueueSize')
+            .to.equal(0);
 
-        done();
+          done();
+        });
       });
     });
   });