SERVER-42481 Remove FCV checks around prepared transactions

Author: jasonjhchan

jstests/core/txns/abort_unprepared_transactions_on_FCV_downgrade.js

@@ -1,5 +1,9 @@
-// Test that open unprepared transactions are aborted on FCV downgrade.
-// @tags: [uses_transactions]
+/**
+ * Test that open unprepared transactions are aborted on FCV downgrade. This test covers the
+ * behavior between FCV downgrade and unprepared transactions as of v4.2. It is safe to change this
+ * test's behavior or remove this test entirely if the behavior changes post v4.2.
+ * @tags: [uses_transactions]
+ */
 (function() {
 "use strict";
 load("jstests/libs/feature_compatibility_version.js");

jstests/core/txns/await_prepared_transactions_on_FCV_downgrade.js

@@ -1,5 +1,7 @@
 /**
- * Test that we wait for prepared transactions to finish during downgrade to FCV 4.0.
+ * Test that we wait for prepared transactions to finish during FCV downgrade. This test covers the
+ * locking behavior as of v4.2. It is safe to change this test's behavior or remove this test
+ * entirely if the locking behavior changes post v4.2.
  * @tags: [uses_transactions, uses_prepare_transaction]
  */
 (function() {
@@ -50,13 +52,6 @@ try {
     assert.commandWorked(testDB.adminCommand({setFeatureCompatibilityVersion: lastStableFCV}));
     checkFCV(adminDB, lastStableFCV);
 
-    jsTestLog("Verify that we are not allowed to prepare a transaction after downgrading.");
-    session.startTransaction();
-    assert.commandWorked(sessionDB[collName].insert({"b": 2}));
-    assert.commandFailedWithCode(sessionDB.adminCommand({prepareTransaction: 1}),
-                                 ErrorCodes.CommandNotSupported);
-    assert.commandFailedWithCode(session.abortTransaction_forTesting(),
-                                 ErrorCodes.NoSuchTransaction);
 } finally {
     assert.commandWorked(
         testDB.adminCommand({configureFailPoint: "failNonIntentLocksIfWaitNeeded", mode: "off"}));

jstests/core/txns/prepare_requires_fcv42.js

@@ -103,11 +103,6 @@ class PrepareTransactionCmd : public TypedCommand<PrepareTransactionCmd> {
                 << opCtx->getTxnNumber() << " on session "
                 << opCtx->getLogicalSessionId()->toBSON();
 
-            uassert(ErrorCodes::CommandNotSupported,
-                    "'prepareTransaction' is only supported in feature compatibility version 4.2",
-                    (serverGlobalParams.featureCompatibility.getVersion() ==
-                     ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo42));
-
             uassert(ErrorCodes::NoSuchTransaction,
                     "Transaction isn't in progress",
                     txnParticipant.inMultiDocumentTransaction());