SERVER-66960 Cache highest child txnNumber on checked out sessions

(cherry picked from commit b2f773225a555d581e547527a8cb67bdeed7fadc)

3 files changed, 19 insertions, 12 deletions

diff --git a/src/mongo/db/session.h b/src/mongo/db/session.h
index 256f7ff28b1..14b6a9839ef 100644
--- a/src/mongo/db/session.h
+++ b/src/mongo/db/session.h
@@ -62,6 +62,10 @@ public:
         return _parentSession;
     }
 
+    int getCachedHighestTxnNumberWithChildSessions() {
+        return _cachedHighestTxnNumberWithChildSessions;
+    }
+
 private:
     // The session id of the transaction session that this object represents.
     const LogicalSessionId _sessionId;
@@ -74,6 +78,11 @@ private:
     // Counts how many threads are blocked waiting for this Session to become available. Used to
     // block reaping of this Session from the SessionCatalog.
     int _numWaitingToCheckOut{0};
+
+    // Cache of the highest txnNumber on seen on a child session for this session's logical session.
+    // Cached to avoid taking the session catalog mutex to read and because this is only best
+    // effort to minimize sessions read when refreshing a transaction participant.
+    int _cachedHighestTxnNumberWithChildSessions{kUninitializedTxnNumber};
 };
 
 }  // namespace mongo
diff --git a/src/mongo/db/session_catalog.cpp b/src/mongo/db/session_catalog.cpp
index bc5b97b8566..032e9d8b90e 100644
--- a/src/mongo/db/session_catalog.cpp
+++ b/src/mongo/db/session_catalog.cpp
@@ -105,6 +105,7 @@ SessionCatalog::ScopedCheckedOutSession SessionCatalog::_checkOutSessionInner(
 
     sri->checkoutOpCtx = opCtx;
     sri->lastCheckout = Date_t::now();
+    session->_cachedHighestTxnNumberWithChildSessions = sri->highestTxnNumberWithChildSessions;
 
     return ScopedCheckedOutSession(*this, std::move(sri), session, std::move(killToken));
 }
@@ -321,6 +322,7 @@ void SessionCatalog::_releaseSession(SessionRuntimeInfo* sri,
     }
 
     sri->checkoutOpCtx = nullptr;
+    session->_cachedHighestTxnNumberWithChildSessions = kUninitializedTxnNumber;
     sri->availableCondVar.notify_all();
 
     if (killToken) {
diff --git a/src/mongo/db/transaction_participant.cpp b/src/mongo/db/transaction_participant.cpp
index d7f95a93fa9..6aba931a08b 100644
--- a/src/mongo/db/transaction_participant.cpp
+++ b/src/mongo/db/transaction_participant.cpp
@@ -334,14 +334,8 @@ ActiveTransactionHistory fetchActiveTransactionHistory(OperationContext* opCtx,
  * found in the session catalog and the config.transactions collection.
  */
 TxnNumber fetchHighestTxnNumberWithInternalSessions(OperationContext* opCtx,
-                                                    const LogicalSessionId& parentLsid) {
-    TxnNumber highestTxnNumber{kUninitializedTxnNumber};
-
-    const auto sessionCatalog = SessionCatalog::get(opCtx);
-    sessionCatalog->scanSession(parentLsid, [&](const ObservableSession& osession) {
-        highestTxnNumber = osession.getHighestTxnNumberWithChildSessions();
-    });
-
+                                                    const LogicalSessionId& parentLsid,
+                                                    TxnNumber cachedHighestTxnNumber) {
     try {
         performReadWithNoTimestampDBDirectClient(opCtx, [&](DBDirectClient* client) {
             FindCommandRequest findRequest{NamespaceString::kSessionTransactionsTableNamespace};
@@ -349,7 +343,7 @@ TxnNumber fetchHighestTxnNumberWithInternalSessions(OperationContext* opCtx,
                                        << parentLsid.toBSON()
                                        << (SessionTxnRecord::kSessionIdFieldName + "." +
                                            LogicalSessionId::kTxnNumberFieldName)
-                                       << BSON("$gte" << highestTxnNumber)));
+                                       << BSON("$gte" << cachedHighestTxnNumber)));
             findRequest.setSort(BSON((SessionTxnRecord::kSessionIdFieldName + "." +
                                       LogicalSessionId::kTxnNumberFieldName)
                                      << -1));
@@ -363,7 +357,8 @@ TxnNumber fetchHighestTxnNumberWithInternalSessions(OperationContext* opCtx,
                 const auto doc = cursor->next();
                 const auto childLsid = LogicalSessionId::parse(
                     IDLParserErrorContext("LogicalSessionId"), doc.getObjectField("_id"));
-                highestTxnNumber = std::max(highestTxnNumber, *childLsid.getTxnNumber());
+                cachedHighestTxnNumber =
+                    std::max(cachedHighestTxnNumber, *childLsid.getTxnNumber());
                 invariant(!cursor->more());
             }
         });
@@ -372,7 +367,7 @@ TxnNumber fetchHighestTxnNumberWithInternalSessions(OperationContext* opCtx,
         throw;
     }
 
-    return highestTxnNumber;
+    return cachedHighestTxnNumber;
 }
 
 void updateSessionEntry(OperationContext* opCtx,
@@ -2857,7 +2852,8 @@ void TransactionParticipant::Participant::_refreshSelfFromStorageIfNeeded(Operat
     if (feature_flags::gFeatureFlagInternalTransactions.isEnabled(
             serverGlobalParams.featureCompatibility) &&
         !_isInternalSession()) {
-        const auto txnNumber = fetchHighestTxnNumberWithInternalSessions(opCtx, _sessionId());
+        const auto txnNumber = fetchHighestTxnNumberWithInternalSessions(
+            opCtx, _sessionId(), _session()->getCachedHighestTxnNumberWithChildSessions());
         if (txnNumber > o().activeTxnNumberAndRetryCounter.getTxnNumber()) {
             _setNewTxnNumberAndRetryCounter(opCtx, {txnNumber, kUninitializedTxnRetryCounter});
         }