SERVER-39811 Add a threshold of the total size of buffered oplog entries in a transaction and make it tunable, defaulting to unlimited

3 files changed, 43 insertions, 0 deletions

diff --git a/src/mongo/db/transaction_participant.cpp b/src/mongo/db/transaction_participant.cpp
index 34f83ba6888..3b963c9d7b8 100644
--- a/src/mongo/db/transaction_participant.cpp
+++ b/src/mongo/db/transaction_participant.cpp
@@ -1128,6 +1128,13 @@ void TransactionParticipant::Participant::addTransactionOperation(
     p().transactionOperations.push_back(operation);
     p().transactionOperationBytes += repl::OplogEntry::getDurableReplOperationSize(operation);
 
+    auto transactionSizeLimitBytes = gTransactionSizeLimitBytes.load();
+    uassert(ErrorCodes::TransactionTooLarge,
+            str::stream() << "Total size of all transaction operations must be less than "
+                          << "server parameter 'transactionSizeLimitBytes' = "
+                          << transactionSizeLimitBytes,
+            p().transactionOperationBytes <= static_cast<size_t>(transactionSizeLimitBytes));
+
     // Creating transactions larger than 16MB requires a new oplog format only available in FCV 4.2.
     const auto isFCV42 = serverGlobalParams.featureCompatibility.getVersion() ==
         ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo42;
diff --git a/src/mongo/db/transaction_participant.idl b/src/mongo/db/transaction_participant.idl
index 55a408ac30e..7e0a0194613 100644
--- a/src/mongo/db/transaction_participant.idl
+++ b/src/mongo/db/transaction_participant.idl
@@ -74,3 +74,12 @@ server_parameters:
         cpp_varname: gMaxNumberOfTransactionOperationsInSingleOplogEntry
         default: 2147483647  # INT_MAX
         validator: { gte: 1 }
+
+    transactionSizeLimitBytes:
+        description: >-
+            Maximum total size of operations in a multi-document transaction.
+        set_at: [ startup, runtime ]
+        cpp_vartype: AtomicWord<long long>
+        cpp_varname: gTransactionSizeLimitBytes
+        default:
+          expr: std::numeric_limits<long long>::max()
diff --git a/src/mongo/db/transaction_participant_test.cpp b/src/mongo/db/transaction_participant_test.cpp
index ea04f5b7829..8dbdd4aa6f5 100644
--- a/src/mongo/db/transaction_participant_test.cpp
+++ b/src/mongo/db/transaction_participant_test.cpp
@@ -1097,6 +1097,33 @@ TEST_F(TxnParticipantTest, TransactionTooLargeWhileBuilding) {
                        ErrorCodes::TransactionTooLarge);
 }
 
+// Tests that a transaction aborts if it becomes too large based on the server parameter
+// 'transactionLimitBytes'.
+TEST_F(TxnParticipantTest, TransactionExceedsSizeParameter) {
+    auto sessionCheckout = checkOutSession();
+    auto txnParticipant = TransactionParticipant::get(opCtx());
+
+    txnParticipant.unstashTransactionResources(opCtx(), "insert");
+    auto oldLimit = gTransactionSizeLimitBytes.load();
+    ON_BLOCK_EXIT([oldLimit] { gTransactionSizeLimitBytes.store(oldLimit); });
+
+    // Set a limit of 2.5 MB
+    gTransactionSizeLimitBytes.store(2 * 1024 * 1024 + 512 * 1024);
+
+    // Two 1MB operations should succeed; three 1MB operations should fail.
+    constexpr size_t kBigDataSize = 1 * 1024 * 1024;
+    std::unique_ptr<uint8_t[]> bigData(new uint8_t[kBigDataSize]());
+    auto operation = repl::OplogEntry::makeInsertOperation(
+        kNss,
+        _uuid,
+        BSON("_id" << 0 << "data" << BSONBinData(bigData.get(), kBigDataSize, BinDataGeneral)));
+    txnParticipant.addTransactionOperation(opCtx(), operation);
+    txnParticipant.addTransactionOperation(opCtx(), operation);
+    ASSERT_THROWS_CODE(txnParticipant.addTransactionOperation(opCtx(), operation),
+                       AssertionException,
+                       ErrorCodes::TransactionTooLarge);
+}
+
 TEST_F(TxnParticipantTest, StashInNestedSessionIsANoop) {
     auto outerScopedSession = checkOutSession();
     Locker* originalLocker = opCtx()->lockState();