fix(NODE-5925): driver throws error when non-read operation in a transaction has a ReadPreferenceMode other than 'primary'

src/sessions.ts

@@ -41,6 +41,7 @@ import {
   ByteUtils,
   calculateDurationInMs,
   type Callback,
+  commandIsReadOperation,
   commandSupportsReadConcern,
   isPromiseLike,
   List,
@@ -194,7 +195,7 @@ export class ClientSession extends TypedEventEmitter<ClientSessionEvents> {
 
     this.operationTime = undefined;
     this.owner = options.owner;
-    this.defaultTransactionOptions = Object.assign({}, options.defaultTransactionOptions);
+    this.defaultTransactionOptions = { ...options.defaultTransactionOptions };
     this.transaction = new Transaction();
   }
 
@@ -1039,6 +1040,16 @@ export function applySession(
       command.readConcern = readConcern;
     }
 
+    if (
+      commandIsReadOperation(command) &&
+      ((typeof session.transaction.options.readPreference === 'string' &&
+        session.transaction.options.readPreference !== 'primary') ||
+        (typeof session.transaction.options.readPreference === 'object' &&
+          session.transaction.options.readPreference.mode !== 'primary'))
+    ) {
+      throw new MongoTransactionError('read preference in a transaction must be primary');
+    }
+
     if (session.supports.causalConsistency && session.operationTime) {
       command.readConcern = command.readConcern || {};
       Object.assign(command.readConcern, { afterClusterTime: session.operationTime });

src/utils.ts

@@ -1077,6 +1077,20 @@ export function commandSupportsReadConcern(command: Document): boolean {
   return false;
 }
 
+export function commandIsReadOperation(command: Document): boolean {
+  if (
+    command.find ||
+    command.findOne ||
+    command.ensureIndex ||
+    command.count ||
+    command.aggregate ||
+    command.distinct
+  ) {
+    return true;
+  }
+  return false;
+}
+
 /**
  * Compare objectIds. `null` is always less
  * - `+1 = oid1 is greater than oid2`

test/integration/transactions/transactions.spec.test.ts

@@ -4,8 +4,6 @@ import { loadSpecTests } from '../../spec';
 import { runUnifiedSuite } from '../../tools/unified-spec-runner/runner';
 
 const SKIPPED_TESTS = [
-  // TODO(NODE-5925) - secondary read preference not allowed in transactions.
-  'readPreference inherited from defaultTransactionOptions',
   // TODO(NODE-5924) - Fix modification of readConcern object post message send.
   'readConcern local in defaultTransactionOptions',
   'defaultTransactionOptions override client options',
@@ -17,7 +15,7 @@ const SKIPPED_TESTS = [
 describe('Transactions Spec Unified Tests', function () {
   runUnifiedSuite(loadSpecTests(path.join('transactions', 'unified')), test => {
     return SKIPPED_TESTS.includes(test.description)
-      ? 'TODO(NODE-5924/NODE-5925): Skipping failing transaction tests'
+      ? 'TODO(NODE-5924): Skipping failing transaction tests'
       : false;
   });
 });

test/tools/unified-spec-runner/entities.ts

@@ -620,9 +620,8 @@ export class EntitiesMap<E = Entity> extends Map<string, E> {
               WriteConcern.fromOptions(defaultOptions);
           }
           if (defaultOptions.readPreference) {
-            options.defaultTransactionOptions.readPreference = ReadPreference.fromOptions(
-              defaultOptions.readPreference
-            );
+            options.defaultTransactionOptions.readPreference =
+              ReadPreference.fromOptions(defaultOptions);
           }
           if (typeof defaultOptions.maxCommitTimeMS === 'number') {
             options.defaultTransactionOptions.maxCommitTimeMS = defaultOptions.maxCommitTimeMS;