fix(NODE-3144): pool clear event ordering and retryability tests (#3407)

src/cmap/connection_pool.ts

@@ -423,11 +423,10 @@ export class ConnectionPool extends TypedEventEmitter<ConnectionPoolEvents> {
     this[kPoolState] = PoolState.paused;
 
     this.clearMinPoolSizeTimer();
-    this.processWaitQueue();
-
     if (!alreadyPaused) {
       this.emit(ConnectionPool.CONNECTION_POOL_CLEARED, new ConnectionPoolClearedEvent(this));
     }
+    this.processWaitQueue();
   }
 
   /** Close the pool */

src/cmap/errors.ts

@@ -24,7 +24,6 @@ export class PoolClosedError extends MongoDriverError {
  * @category Error
  */
 export class PoolClearedError extends MongoNetworkError {
-  // TODO(NODE-3144): needs to extend RetryableError or be marked retryable in some other way per spec
   /** The address of the connection pool */
   address: string;
 

test/integration/retryable-reads/retryable_reads.spec.prose.test.ts

@@ -0,0 +1,140 @@
+/* eslint-disable @typescript-eslint/no-non-null-assertion */
+import { expect } from 'chai';
+
+import { Collection, MongoClient } from '../../../src';
+
+describe('Retryable Reads Spec Prose', () => {
+  let client: MongoClient, failPointName;
+
+  afterEach(async () => {
+    try {
+      if (failPointName) {
+        await client.db('admin').command({ configureFailPoint: failPointName, mode: 'off' });
+      }
+    } finally {
+      failPointName = undefined;
+      await client?.close();
+    }
+  });
+
+  describe('PoolClearedError Retryability Test', () => {
+    // This test will be used to ensure drivers properly retry after encountering PoolClearedErrors.
+    // It MUST be implemented by any driver that implements the CMAP specification.
+    // This test requires MongoDB 4.2.9+ for blockConnection support in the failpoint.
+
+    let cmapEvents: Array<{ name: string; event: Record<string, any> }>;
+    let commandStartedEvents: Array<Record<string, any>>;
+    let testCollection: Collection;
+    beforeEach(async function () {
+      // 1. Create a client with maxPoolSize=1 and retryReads=true.
+      client = this.configuration.newClient(
+        this.configuration.url({
+          useMultipleMongoses: false // If testing against a sharded deployment, be sure to connect to only a single mongos.
+        }),
+        { maxPoolSize: 1, retryReads: true, monitorCommands: true }
+      );
+
+      console.log(client.options);
+
+      testCollection = client.db('retryable-reads-prose').collection('pool-clear-retry');
+      await testCollection.drop().catch(() => null);
+      await testCollection.insertMany([{ test: 1 }, { test: 2 }]);
+
+      // 2. Enable the following failpoint:
+      // NOTE: "9. Disable the failpoint" is done in afterEach
+      failPointName = 'failCommand';
+      const failPoint = await client.db('admin').command({
+        configureFailPoint: failPointName,
+        mode: { times: 1 },
+        data: {
+          failCommands: ['find'],
+          errorCode: 91,
+          blockConnection: true,
+          blockTimeMS: 1000
+        }
+      });
+
+      expect(failPoint).to.have.property('ok', 1);
+
+      cmapEvents = [];
+      commandStartedEvents = [];
+      for (const observedEvent of [
+        'connectionCheckOutStarted',
+        'connectionCheckedOut',
+        'connectionCheckOutFailed',
+        'connectionPoolCleared'
+      ]) {
+        client.on(observedEvent, ev => {
+          cmapEvents.push({ name: observedEvent, event: ev });
+        });
+      }
+
+      client.on('commandStarted', ev => {
+        commandStartedEvents.push(ev);
+      });
+    });
+
+    it('should emit events in the expected sequence', {
+      metadata: { requires: { mongodb: '>=4.2.9', topology: '!load-balanced' } },
+      test: async function () {
+        // 3. Start two threads and attempt to perform a findOne simultaneously on both.
+        const results = await Promise.all([
+          testCollection.findOne({ test: 1 }),
+          testCollection.findOne({ test: 2 })
+        ]);
+
+        client.removeAllListeners();
+        // 4. Verify that both findOne attempts succeed.
+        expect(results[0]).to.have.property('test', 1);
+        expect(results[1]).to.have.property('test', 2);
+
+        // NOTE: For the subsequent checks, we rely on the exact sequence of ALL events
+        // for ease of readability; however, only the relative order matters for
+        // the purposes of this test, so if this ever becomes an issue, the test
+        // can be refactored to assert on relative index values instead
+
+        // 5. Via CMAP monitoring, assert that the first check out succeeds.
+        expect(cmapEvents.shift()).to.have.property(
+          'name',
+          'connectionCheckOutStarted',
+          'expected 1) checkout 1 to start'
+        );
+        expect(cmapEvents.shift()).to.have.property(
+          'name',
+          'connectionCheckOutStarted',
+          'expected 2) checkout 2 to start'
+        );
+        expect(cmapEvents.shift()).to.have.property(
+          'name',
+          'connectionCheckedOut',
+          'expected 3) first checkout to succeed'
+        );
+
+        // 6. Via CMAP monitoring, assert that a PoolClearedEvent is then emitted.
+        expect(cmapEvents.shift()).to.have.property(
+          'name',
+          'connectionPoolCleared',
+          'expected 4) pool to clear'
+        );
+
+        // 7. Via CMAP monitoring, assert that the second check out then fails due to a connection error.
+        const nextEvent = cmapEvents.shift();
+        expect(nextEvent).to.have.property(
+          'name',
+          'connectionCheckOutFailed',
+          'expected 5) checkout 2 to fail'
+        );
+        expect(nextEvent!.event).to.have.property('reason', 'connectionError');
+
+        // 8. Via Command Monitoring, assert that exactly three find CommandStartedEvents were observed in total.
+        const observedFindCommandStartedEvents = commandStartedEvents.filter(
+          ({ commandName }) => commandName === 'find'
+        );
+        expect(observedFindCommandStartedEvents).to.have.lengthOf(
+          3,
+          'expected 3 find command started events'
+        );
+      }
+    });
+  });
+});