fix(transactions): fix sharded transaction error logic

Fixed the logic so that startTransaction will not error on
sharded transactions on mongodb > 4.2.0

Author: daprahamian

lib/core/sessions.js

@@ -18,7 +18,7 @@ const resolveClusterTime = require('./topologies/shared').resolveClusterTime;
 const isSharded = require('./wireprotocol/shared').isSharded;
 const maxWireVersion = require('./utils').maxWireVersion;
 
-const MAX_FOR_TRANSACTIONS = 7;
+const minWireVersionForShardedTransactions = 8;
 
 function assertAlive(session, callback) {
   if (session.serverSession == null) {
@@ -191,8 +191,9 @@ class ClientSession extends EventEmitter {
 
     const topologyMaxWireVersion = maxWireVersion(this.topology);
     if (
-      isSharded(this.topology) ||
-      (topologyMaxWireVersion != null && topologyMaxWireVersion < MAX_FOR_TRANSACTIONS)
+      isSharded(this.topology) &&
+      (topologyMaxWireVersion == null ||
+        topologyMaxWireVersion < minWireVersionForShardedTransactions)
     ) {
       throw new MongoError('Transactions are not supported on sharded clusters in MongoDB < 4.2.');
     }

test/functional/transactions_tests.js

@@ -71,7 +71,7 @@ describe('Transactions', function() {
 
   describe('startTransaction', function() {
     it('should error if transactions are not supported', {
-      metadata: { requires: { topology: ['sharded'], mongodb: '>4.0.0' } },
+      metadata: { requires: { topology: ['sharded'], mongodb: '4.0.x' } },
       test: function(done) {
         const configuration = this.configuration;
         const client = configuration.newClient(configuration.writeConcernMax(), { poolSize: 1 });