SERVER-31882 start a transaction when beginning a UnitOfWork

In order to avoid implicitly starting a transaction under a hot mutex for vectored inserts,
this commit explicitly starts a transaction when beginUnitOfWork is called.

2 files changed, 3 insertions, 1 deletions

diff --git a/src/mongo/db/storage/wiredtiger/wiredtiger_record_store.cpp b/src/mongo/db/storage/wiredtiger/wiredtiger_record_store.cpp
index 4000740b2a0..1995e1ffa64 100644
--- a/src/mongo/db/storage/wiredtiger/wiredtiger_record_store.cpp
+++ b/src/mongo/db/storage/wiredtiger/wiredtiger_record_store.cpp
@@ -1671,7 +1671,7 @@ void WiredTigerRecordStore::cappedTruncateAfter(OperationContext* opCtx,
 
 Status WiredTigerRecordStore::oplogDiskLocRegister(OperationContext* opCtx,
                                                    const Timestamp& opTime) {
-    // This starts a new transaction and gives it a timestamp.
+    // This labels the current transaction with a timestamp.
     // This is required for oplog visibility to work correctly, as WiredTiger uses the transaction
     // list to determine where there are holes in the oplog.
     return opCtx->recoveryUnit()->setTimestamp(SnapshotName(opTime));
diff --git a/src/mongo/db/storage/wiredtiger/wiredtiger_recovery_unit.cpp b/src/mongo/db/storage/wiredtiger/wiredtiger_recovery_unit.cpp
index 8be67c449a5..67ddcb9d93d 100644
--- a/src/mongo/db/storage/wiredtiger/wiredtiger_recovery_unit.cpp
+++ b/src/mongo/db/storage/wiredtiger/wiredtiger_recovery_unit.cpp
@@ -119,6 +119,8 @@ void WiredTigerRecoveryUnit::beginUnitOfWork(OperationContext* opCtx) {
     invariant(!_areWriteUnitOfWorksBanned);
     invariant(!_inUnitOfWork);
     _inUnitOfWork = true;
+    // Begin a new transaction, if one is not already started.
+    getSession();
 }
 
 void WiredTigerRecoveryUnit::commitUnitOfWork() {