fix(sessions): ensure an error is thrown when attempting sharded tran…

…sactions

Fixes NODE-1931

lib/core/sessions.js

@@ -15,6 +15,10 @@ const isPromiseLike = require('./utils').isPromiseLike;
 const ReadPreference = require('./topologies/read_preference');
 const isTransactionCommand = require('./transactions').isTransactionCommand;
 const resolveClusterTime = require('./topologies/shared').resolveClusterTime;
+const isSharded = require('./wireprotocol/shared').isSharded;
+const maxWireVersion = require('./utils').maxWireVersion;
+
+const MAX_FOR_TRANSACTIONS = 7;
 
 function assertAlive(session, callback) {
   if (session.serverSession == null) {
@@ -185,6 +189,14 @@ class ClientSession extends EventEmitter {
       throw new MongoError('Transaction already in progress');
     }
 
+    const topologyMaxWireVersion = maxWireVersion(this.topology);
+    if (
+      isSharded(this.topology) ||
+      (topologyMaxWireVersion != null && topologyMaxWireVersion < MAX_FOR_TRANSACTIONS)
+    ) {
+      throw new MongoError('Transactions are not supported on sharded clusters in MongoDB < 4.2.');
+    }
+
     // increment txnNumber
     this.incrementTransactionNumber();
 

test/functional/transactions_tests.js

@@ -205,6 +205,32 @@ describe('Transactions', function() {
       }
     });
   });
+
+  describe('startTransaction', function() {
+    it('should error if transactions are not supported', {
+      metadata: { requires: { topology: ['sharded'], mongodb: '>4.0.0' } },
+      test: function(done) {
+        const configuration = this.configuration;
+        const client = configuration.newClient(configuration.writeConcernMax(), { poolSize: 1 });
+
+        client.connect((err, client) => {
+          const session = client.startSession();
+          const db = client.db(configuration.db);
+          const coll = db.collection('transaction_error_test');
+          coll.insertOne({ a: 1 }, err => {
+            expect(err).to.not.exist;
+            expect(() => session.startTransaction()).to.throw(
+              'Transactions are not supported on sharded clusters in MongoDB < 4.2.'
+            );
+
+            session.endSession(() => {
+              client.close(done);
+            });
+          });
+        });
+      }
+    });
+  });
 });
 
 function parseTopologies(topologies) {