fix(NODE-5530): make topology descriptions JSON stringifiable

src/sdam/topology_description.ts

@@ -1,4 +1,4 @@
-import type { ObjectId } from '../bson';
+import { EJSON, type ObjectId } from '../bson';
 import * as WIRE_CONSTANTS from '../cmap/wire_protocol/constants';
 import { type MongoError, MongoRuntimeError } from '../error';
 import { compareObjectId, shuffle } from '../utils';
@@ -342,6 +342,16 @@ export class TopologyDescription {
   hasServer(address: string): boolean {
     return this.servers.has(address);
   }
+
+  /**
+   * Returns a JSON-serializable representation of the TopologyDescription.  This is primarily
+   * intended for use with JSON.stringify().
+   *
+   * This method will not throw.
+   */
+  toJSON() {
+    return EJSON.serialize(this);
+  }
 }
 
 function topologyTypeForServerType(serverType: ServerType): TopologyType {

test/integration/node-specific/errors.test.ts

@@ -1,6 +1,6 @@
 import { expect } from 'chai';
 
-import { MongoClient, MongoServerSelectionError } from '../../mongodb';
+import { MongoClient, MongoServerSelectionError, ReadPreference } from '../../mongodb';
 
 describe('Error (Integration)', function () {
   it('NODE-5296: handles aggregate errors from dns lookup', async function () {
@@ -10,4 +10,37 @@ describe('Error (Integration)', function () {
     expect(error).to.be.instanceOf(MongoServerSelectionError);
     expect(error.message).not.to.be.empty;
   });
+
+  context('when a server selection error is stringified', function () {
+    it(
+      'the error"s topology description correctly displays the `servers`',
+      { requires: { topology: 'replicaset' } },
+      async function () {
+        const client: MongoClient = this.configuration.newClient({
+          serverSelectionTimeoutMS: 1000
+        });
+        try {
+          await client.connect();
+
+          const error = await client
+            .db('foo')
+            .collection('bar')
+            .find(
+              {},
+              {
+                // Use meaningless read preference tags to ensure that the server selection fails
+                readPreference: new ReadPreference('secondary', [{ ny: 'ny' }])
+              }
+            )
+            .toArray()
+            .catch(e => JSON.parse(JSON.stringify(e)));
+
+          const servers = error.reason.servers;
+          expect(Object.keys(servers).length > 0).to.be.true;
+        } finally {
+          await client.close();
+        }
+      }
+    );
+  });
 });

test/integration/server-discovery-and-monitoring/topology_description.test.ts

@@ -14,7 +14,22 @@ describe('TopologyDescription (integration tests)', function () {
     await client.close();
   });
 
+  beforeEach(async function () {
+    client = this.configuration.newClient();
+    await client.connect();
+  });
+
   context('options', function () {
+    let client: MongoClient;
+
+    afterEach(async function () {
+      await client.close();
+    });
+
+    beforeEach(async function () {
+      client = this.configuration.newClient();
+    });
+
     context('localThresholdMS', function () {
       it('should default to 15ms', async function () {
         const options: MongoClientOptions = {};
@@ -35,15 +50,6 @@ describe('TopologyDescription (integration tests)', function () {
   });
 
   context('topology types', function () {
-    let client: MongoClient;
-    beforeEach(async function () {
-      client = this.configuration.newClient();
-    });
-
-    afterEach(async function () {
-      await client.close();
-    });
-
     const topologyTypesMap = new Map<TopologyTypeRequirement, TopologyType>([
       ['single', TopologyType.Single],
       ['replicaset', TopologyType.ReplicaSetWithPrimary],
@@ -65,4 +71,23 @@ describe('TopologyDescription (integration tests)', function () {
       );
     }
   });
+
+  describe('json stringification', function () {
+    it('can be stringified without error', function () {
+      // eslint-disable-next-line @typescript-eslint/no-non-null-assertion, @typescript-eslint/no-non-null-asserted-optional-chain
+      const description = client.topology?.description!;
+      expect(description).to.exist;
+
+      expect(() => JSON.stringify(description)).not.to.throw;
+    });
+
+    it('properly stringifies the server description map', function () {
+      // eslint-disable-next-line @typescript-eslint/no-non-null-assertion, @typescript-eslint/no-non-null-asserted-optional-chain
+      const description = client.topology?.description!;
+      expect(description).to.exist;
+
+      const { servers } = JSON.parse(JSON.stringify(description));
+      expect(Object.keys(servers).length > 0, '`servers` stringified with no servers.').to.be.true;
+    });
+  });
 });