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fix(NODE-6589): background task does not prune idle connections when minPoolSize=0 #4569

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fix(NODE-6589): background task does not prune idle connections when minPoolSize=0 #4569

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409f401
Add regression test
baileympearson Jun 26, 2025
1ac3000
fix
baileympearson Jun 26, 2025
8fed111
fix leaks in unit tests
baileympearson Jun 27, 2025
ab28f3d
lint
baileympearson Jun 27, 2025
e054d28
remove accidental test skip
baileympearson Jun 27, 2025
b2c9675
fix flake
baileympearson Jun 27, 2025
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2 changes: 1 addition & 1 deletion src/cmap/connection_pool.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -691,7 +691,7 @@ export class ConnectionPool extends TypedEventEmitter<ConnectionPoolEvents> {

private ensureMinPoolSize() {
const minPoolSize = this.options.minPoolSize;
if (this.poolState !== PoolState.ready || minPoolSize === 0) {
if (this.poolState !== PoolState.ready) {
return;
}

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57 changes: 55 additions & 2 deletions test/integration/connection-monitoring-and-pooling/connection_pool.test.ts
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Original file line number Diff line number Diff line change
@@ -1,8 +1,14 @@
import { once } from 'node:events';

import { expect } from 'chai';

import { type ConnectionPoolCreatedEvent, type Db, type MongoClient } from '../../mongodb';
import * as sinon from 'sinon';

import {
type ConnectionPoolCreatedEvent,
type Db,
type MongoClient,
type Server
} from '../../mongodb';
import { clearFailPoint, configureFailPoint, sleep } from '../../tools/utils';

describe('Connection Pool', function () {
Expand Down Expand Up @@ -150,4 +156,51 @@ describe('Connection Pool', function () {
});
});
});

describe(
'background task cleans up connections when minPoolSize=0',
{ requires: { topology: 'single' } },
function () {
let server: Server;
let ensureMinPoolSizeSpy: sinon.SinonSpy;

beforeEach(async function () {
client = this.configuration.newClient(
{},
{
maxConnecting: 10,
minPoolSize: 0,
maxIdleTimeMS: 100
}
);

await client.connect();

await Promise.all(
Array.from({ length: 10 }).map(() => {
return client.db('foo').collection('bar').insertOne({ a: 1 });
})
);

server = Array.from(client.topology.s.servers.entries())[0][1];
expect(
server.pool.availableConnectionCount,
'pool was not filled with connections'
).to.be.greaterThan(0);

ensureMinPoolSizeSpy = sinon.spy(server.pool, 'ensureMinPoolSize');
});

it(
'prunes idle connections when minPoolSize=0',
{ requires: { topology: 'single' } },
async function () {
await sleep(500);
expect(server.pool.availableConnectionCount).to.equal(0);

expect(ensureMinPoolSizeSpy).to.have.been.called;
}
);
}
);
});
2 changes: 1 addition & 1 deletion test/integration/crud/insert.test.js
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Expand Up @@ -1825,7 +1825,7 @@ describe('crud - insert', function () {
.toArray();
const doc = docs.pop();
expect(doc.a._bsontype).to.equal('Long');
client.close();
await client.close();
}
});

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7 changes: 3 additions & 4 deletions test/unit/assorted/max_staleness.spec.test.js
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Expand Up @@ -45,13 +45,12 @@ describe('Max Staleness (spec)', function () {
});

const specTests = collectStalenessTests(maxStalenessDir);
Object.keys(specTests).forEach(specTestName => {
for (const [specTestName, test] of Object.entries(specTests)) {
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I was debugging these tests for a leak and refactored this as a drive-by improvement.

describe(specTestName, () => {
specTests[specTestName].forEach(testData => {
for (const testData of test)
it(testData.description, async function () {
return executeServerSelectionTest(testData);
});
});
});
});
}
});
2 changes: 2 additions & 0 deletions test/unit/assorted/server_selection_latency_window_utils.ts
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Expand Up @@ -145,4 +145,6 @@ export async function runServerSelectionLatencyWindowTest(test: ServerSelectionL
const observedFrequencies = calculateObservedFrequencies(selectedServers);

compareResultsToExpected(test.outcome, observedFrequencies);

await topology.close();
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This is one of the causes of leaks in unit tests - gotta close the topology.

Before the change in this PR, the pool did not run the background thread by default. With this change, the background thread does run by default, which keeps the event loop open unless the pool's minPoolSize timer is explicitly cleared (which happens in pool.close()).

This isn't an issue for in real scenarios because the server class always closes its pool.

}
4 changes: 3 additions & 1 deletion test/unit/assorted/server_selection_spec_helper.js
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Original file line number Diff line number Diff line change
Expand Up @@ -123,7 +123,7 @@ async function executeServerSelectionTest(testDefinition) {
const readPreference = readPreferenceFromDefinition(testDefinition.read_preference);
selector = ServerSelectors.readPreferenceServerSelector(readPreference);
} catch (e) {
if (testDefinition.error) return;
if (testDefinition.error) return topology.close();
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Same as above.

throw e;
}
} else {
Expand Down Expand Up @@ -179,6 +179,8 @@ async function executeServerSelectionTest(testDefinition) {
// this is another expected error case
if (expectedServers.length === 0 && err instanceof MongoServerSelectionError) return;
throw err;
} finally {
topology.close();
}
}

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15 changes: 10 additions & 5 deletions test/unit/cmap/connection_pool.test.js
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Expand Up @@ -15,6 +15,7 @@ const { TimeoutContext } = require('../../mongodb');
describe('Connection Pool', function () {
let timeoutContext;
let mockMongod;
let pool;
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Same as above.

const stubServer = {
topology: {
client: {
Expand Down Expand Up @@ -50,6 +51,8 @@ describe('Connection Pool', function () {
timeoutContext = TimeoutContext.create({ waitQueueTimeoutMS: 0, serverSelectionTimeoutMS: 0 });
});

afterEach(() => pool?.close());

it('should destroy connections which have been closed', async function () {
mockMongod.setMessageHandler(request => {
const doc = request.document;
Expand All @@ -61,7 +64,7 @@ describe('Connection Pool', function () {
}
});

const pool = new ConnectionPool(stubServer, {
pool = new ConnectionPool(stubServer, {
maxPoolSize: 1,
hostAddress: mockMongod.hostAddress()
});
Expand All @@ -81,6 +84,8 @@ describe('Connection Pool', function () {
expect(events).to.have.length(1);
const closeEvent = events[0];
expect(closeEvent).have.property('reason').equal('error');

conn.destroy();
});

it('should propagate socket timeouts to connections', async function () {
Expand All @@ -93,7 +98,7 @@ describe('Connection Pool', function () {
}
});

const pool = new ConnectionPool(stubServer, {
pool = new ConnectionPool(stubServer, {
maxPoolSize: 1,
socketTimeoutMS: 200,
hostAddress: mockMongod.hostAddress()
Expand All @@ -117,7 +122,7 @@ describe('Connection Pool', function () {
}
});

const pool = new ConnectionPool(stubServer, {
pool = new ConnectionPool(stubServer, {
maxPoolSize: 1,
waitQueueTimeoutMS: 200,
hostAddress: mockMongod.hostAddress()
Expand Down Expand Up @@ -157,7 +162,7 @@ describe('Connection Pool', function () {
});

it('should respect the minPoolSizeCheckFrequencyMS option', function () {
const pool = new ConnectionPool(stubServer, {
pool = new ConnectionPool(stubServer, {
minPoolSize: 2,
minPoolSizeCheckFrequencyMS: 42,
hostAddress: mockMongod.hostAddress()
Expand Down Expand Up @@ -193,7 +198,7 @@ describe('Connection Pool', function () {
});

it('should default minPoolSizeCheckFrequencyMS to 100ms', function () {
const pool = new ConnectionPool(stubServer, {
pool = new ConnectionPool(stubServer, {
minPoolSize: 2,
hostAddress: mockMongod.hostAddress()
});
Expand Down