chore: add test to demonstrate check in

Author: nbbeeken

test/integration/connection-monitoring-and-pooling/connection_pool.test.ts

@@ -2,7 +2,14 @@ import { once } from 'node:events';
 
 import { expect } from 'chai';
 
-import { type ConnectionPoolCreatedEvent, type Db, type MongoClient } from '../../mongodb';
+import {
+  type ConnectionCheckedInEvent,
+  type ConnectionCheckedOutEvent,
+  type ConnectionPoolCreatedEvent,
+  type Db,
+  type MongoClient
+} from '../../mongodb';
+import { clearFailPoint, configureFailPoint, sleep } from '../../tools/utils';
 
 describe('Connection Pool', function () {
   let client: MongoClient;
@@ -64,5 +71,59 @@ describe('Connection Pool', function () {
         });
       });
     });
+
+    describe.only('ConnectionCheckedInEvent', function () {
+      let client;
+
+      beforeEach(async function () {
+        client = this.configuration.newClient();
+        configureFailPoint(this.configuration, {
+          configureFailPoint: 'failCommand',
+          mode: 'alwaysOn',
+          data: {
+            failCommands: ['insert'],
+            blockConnection: true,
+            blockTimeMS: 60_000
+          }
+        });
+      });
+
+      afterEach(async function () {
+        clearFailPoint(this.configuration);
+        await client.close();
+      });
+
+      describe('when a MongoClient is closed', function () {
+        it('a connection pool emits checked in events for closed connections', async () => {
+          const connectionCheckedOutEvents: ConnectionCheckedOutEvent[] = [];
+          client.on('connectionCheckedOut', event => connectionCheckedOutEvents.push(event));
+          const connectionCheckedInEvents: ConnectionCheckedInEvent[] = [];
+          client.on('connectionCheckedIn', event => connectionCheckedInEvents.push(event));
+
+          const pingPromises = Promise.allSettled([
+            client.db('test').collection('test').insertOne({ a: 1 }),
+            client.db('test').collection('test').insertOne({ a: 1 }),
+            client.db('test').collection('test').insertOne({ a: 1 })
+          ]);
+
+          // wait until all pings are pending on the server
+          while (connectionCheckedOutEvents.length < 3) await sleep(1);
+
+          const pingConnectionIds = connectionCheckedOutEvents.map(
+            ({ connectionId }) => connectionId
+          );
+
+          await client.close();
+
+          const pingCheckIns = connectionCheckedInEvents.filter(checkIn =>
+            pingConnectionIds.includes(checkIn.connectionId)
+          );
+
+          expect(pingCheckIns).to.have.lengthOf(3);
+
+          await pingPromises;
+        });
+      });
+    });
   });
 });