feat(NODE-3750): make maxConnecting configurable

src/cmap/connection_pool.ts

@@ -67,6 +67,8 @@ export interface ConnectionPoolOptions extends Omit<ConnectionOptions, 'id' | 'g
   maxPoolSize: number;
   /** The minimum number of connections that MUST exist at any moment in a single connection pool. */
   minPoolSize: number;
+  /** The maximum number of connections that may be in the process of being established concurrently by the connection pool. */
+  maxConnecting: number;
   /** The maximum amount of time a connection should remain idle in the connection pool before being marked idle. */
   maxIdleTimeMS: number;
   /** The maximum amount of time operation execution should wait for a connection to become available. The default is 0 which means there is no limit. */
@@ -107,7 +109,7 @@ export type ConnectionPoolEvents = {
  */
 export class ConnectionPool extends TypedEventEmitter<ConnectionPoolEvents> {
   closed: boolean;
-  options: Readonly<ConnectionPoolOptions & { maxConnecting: number }>;
+  options: Readonly<ConnectionPoolOptions>;
   /** @internal */
   [kLogger]: Logger;
   /** @internal */
@@ -199,7 +201,7 @@ export class ConnectionPool extends TypedEventEmitter<ConnectionPoolEvents> {
       connectionType: Connection,
       maxPoolSize: options.maxPoolSize ?? 100,
       minPoolSize: options.minPoolSize ?? 0,
-      maxConnecting: 2,
+      maxConnecting: options.maxConnecting ?? 2,
       maxIdleTimeMS: options.maxIdleTimeMS ?? 0,
       waitQueueTimeoutMS: options.waitQueueTimeoutMS ?? 0,
       autoEncrypter: options.autoEncrypter,

src/connection_string.ts

@@ -856,6 +856,16 @@ export const OPTIONS = {
       return new Logger('MongoClient', { loggerLevel: value as LoggerLevel });
     }
   },
+  maxConnecting: {
+    default: 2,
+    transform({ name, values: [value] }): number {
+      const maxConnecting = getUint(name, value);
+      if (maxConnecting === 0) {
+        throw new MongoInvalidArgumentError('maxConnecting must be > 0 if specified');
+      }
+      return maxConnecting;
+    }
+  },
   maxIdleTimeMS: {
     default: 0,
     type: 'uint'

src/mongo_client.ts

@@ -141,6 +141,8 @@ export interface MongoClientOptions extends BSONSerializeOptions, SupportedNodeC
   maxPoolSize?: number;
   /** The minimum number of connections in the connection pool. */
   minPoolSize?: number;
+  /** The maximum number of connections that may be in the process of being established concurrently by the connection pool. */
+  maxConnecting?: number;
   /** The maximum number of milliseconds that a connection can remain idle in the pool before being removed and closed. */
   maxIdleTimeMS?: number;
   /** The maximum time in milliseconds that a thread can wait for a connection to become available. */
@@ -652,6 +654,7 @@ export interface MongoOptions
         | 'keepAliveInitialDelay'
         | 'localThresholdMS'
         | 'logger'
+        | 'maxConnecting'
         | 'maxIdleTimeMS'
         | 'maxPoolSize'
         | 'minPoolSize'

test/spec/connection-monitoring-and-pooling/pool-checkout-custom-maxConnecting-is-enforced.json

@@ -0,0 +1,81 @@
+{
+  "version": 1,
+  "style": "integration",
+  "description": "custom maxConnecting is enforced",
+  "runOn": [
+    {
+      "minServerVersion": "4.4.0"
+    }
+  ],
+  "failPoint": {
+    "configureFailPoint": "failCommand",
+    "mode": "alwaysOn",
+    "data": {
+      "failCommands": [
+        "isMaster",
+        "hello"
+      ],
+      "closeConnection": false,
+      "blockConnection": true,
+      "blockTimeMS": 500
+    }
+  },
+  "poolOptions": {
+    "maxConnecting": 1,
+    "maxPoolSize": 2,
+    "waitQueueTimeoutMS": 5000
+  },
+  "operations": [
+    {
+      "name": "ready"
+    },
+    {
+      "name": "start",
+      "target": "thread1"
+    },
+    {
+      "name": "start",
+      "target": "thread2"
+    },
+    {
+      "name": "checkOut",
+      "thread": "thread1"
+    },
+    {
+      "name": "waitForEvent",
+      "event": "ConnectionCreated",
+      "count": 1
+    },
+    {
+      "name": "checkOut",
+      "thread": "thread2"
+    },
+    {
+      "name": "waitForEvent",
+      "event": "ConnectionReady",
+      "count": 2
+    }
+  ],
+  "events": [
+    {
+      "type": "ConnectionCreated"
+    },
+    {
+      "type": "ConnectionReady"
+    },
+    {
+      "type": "ConnectionCreated"
+    },
+    {
+      "type": "ConnectionReady"
+    }
+  ],
+  "ignore": [
+    "ConnectionCheckOutStarted",
+    "ConnectionCheckedIn",
+    "ConnectionCheckedOut",
+    "ConnectionClosed",
+    "ConnectionPoolCreated",
+    "ConnectionPoolReady"
+  ]
+}

test/spec/connection-monitoring-and-pooling/pool-checkout-custom-maxConnecting-is-enforced.yml

@@ -0,0 +1,50 @@
+version: 1
+style: integration
+description: custom maxConnecting is enforced
+runOn:
+  -
+    minServerVersion: "4.4.0"
+failPoint:
+  configureFailPoint: failCommand
+  mode: "alwaysOn"
+  data:
+    failCommands: ["isMaster","hello"]
+    closeConnection: false
+    blockConnection: true
+    blockTimeMS: 500
+poolOptions:
+  maxConnecting: 1
+  # gives opportunity for the checkout in thread2 to establish a new connection, which it must not do until thread1 establishes one
+  maxPoolSize: 2
+  waitQueueTimeoutMS: 5000
+operations:
+  - name: ready
+  # thread1 exists to consume the single permit to open a connection,
+  # so that thread2 would be blocked acquiring a permit, which results in ordering its ConnectionCreated event after
+  # the ConnectionReady event from thread1.
+  - name: start
+    target: thread1
+  - name: start
+    target: thread2
+  - name: checkOut
+    thread: thread1
+  - name: waitForEvent
+    event: ConnectionCreated
+    count: 1
+  - name: checkOut
+    thread: thread2
+  - name: waitForEvent
+    event: ConnectionReady
+    count: 2
+events:
+  - type: ConnectionCreated
+  - type: ConnectionReady
+  - type: ConnectionCreated
+  - type: ConnectionReady
+ignore:
+  - ConnectionCheckOutStarted
+  - ConnectionCheckedIn
+  - ConnectionCheckedOut
+  - ConnectionClosed
+  - ConnectionPoolCreated
+  - ConnectionPoolReady

test/spec/uri-options/connection-pool-options.json

@@ -2,15 +2,16 @@
   "tests": [
     {
       "description": "Valid connection pool options are parsed correctly",
-      "uri": "mongodb://example.com/?maxIdleTimeMS=50000&maxPoolSize=5&minPoolSize=3",
+      "uri": "mongodb://example.com/?maxIdleTimeMS=50000&maxPoolSize=5&minPoolSize=3&maxConnecting=1",
       "valid": true,
       "warning": false,
       "hosts": null,
       "auth": null,
       "options": {
         "maxIdleTimeMS": 50000,
         "maxPoolSize": 5,
-        "minPoolSize": 3
+        "minPoolSize": 3,
+        "maxConnecting": 1
       }
     },
     {
@@ -52,6 +53,24 @@
       "options": {
         "minPoolSize": 0
       }
+    },
+    {
+      "description": "maxConnecting=0 causes a warning",
+      "uri": "mongodb://example.com/?maxConnecting=0",
+      "valid": true,
+      "warning": true,
+      "hosts": null,
+      "auth": null,
+      "options": {}
+    },
+    {
+      "description": "maxConnecting<0 causes a warning",
+      "uri": "mongodb://example.com/?maxConnecting=-1",
+      "valid": true,
+      "warning": true,
+      "hosts": null,
+      "auth": null,
+      "options": {}
     }
   ]
 }

test/spec/uri-options/connection-pool-options.yml

@@ -1,7 +1,7 @@
 tests:
     -
         description: "Valid connection pool options are parsed correctly"
-        uri: "mongodb://example.com/?maxIdleTimeMS=50000&maxPoolSize=5&minPoolSize=3"
+        uri: "mongodb://example.com/?maxIdleTimeMS=50000&maxPoolSize=5&minPoolSize=3&maxConnecting=1"
         valid: true
         warning: false
         hosts: ~
@@ -10,6 +10,7 @@ tests:
             maxIdleTimeMS: 50000
             maxPoolSize: 5
             minPoolSize: 3
+            maxConnecting: 1
     -
         description: "Non-numeric maxIdleTimeMS causes a warning"
         uri: "mongodb://example.com/?maxIdleTimeMS=invalid"
@@ -45,4 +46,23 @@ tests:
       hosts: ~
       auth: ~
       options:
-          minPoolSize: 0
+          minPoolSize: 0 
+
+    -
+      description: "maxConnecting=0 causes a warning"
+      uri: "mongodb://example.com/?maxConnecting=0"
+      valid: true
+      warning: true
+      hosts: ~
+      auth: ~
+      options: {}
+
+    -
+      description: "maxConnecting<0 causes a warning"
+      uri: "mongodb://example.com/?maxConnecting=-1"
+      valid: true
+      warning: true
+      hosts: ~
+      auth: ~
+      options: {}
+

test/tools/cmap_spec_runner.ts

@@ -19,6 +19,7 @@ type CmapOperation =
 type CmapPoolOptions = {
   maxPoolSize?: number;
   minPoolSize?: number;
+  maxConnecting?: number;
   maxIdleTimeMS?: number;
   waitQueueTimeoutMS?: number;
 };

test/tools/uri_spec_runner.ts

@@ -309,6 +309,7 @@ export function executeUriValidationTest(
 
       //** DIRECTLY MAPPED OPTIONS **/
       case 'zlibCompressionLevel':
+      case 'maxConnecting':
       case 'maxPoolSize':
       case 'minPoolSize':
       case 'connectTimeoutMS':

test/unit/mongo_client.test.js

@@ -97,6 +97,7 @@ describe('MongoOptions', function () {
     localThresholdMS: 3,
     logger: new Logger('Testing!'),
     loggerLevel: 'info',
+    maxConnecting: 5,
     maxIdleTimeMS: 3,
     maxPoolSize: 2,
     maxStalenessSeconds: 3,
@@ -162,6 +163,7 @@ describe('MongoOptions', function () {
       'heartbeatFrequencyMS=2',
       'journal=true',
       'localThresholdMS=2',
+      'maxConnecting=5',
       'maxIdleTimeMS=2',
       'maxPoolSize=4',
       'maxStalenessSeconds=2',