SERVER-66852 Eagerly erase retryable child sessions from SessionCatalog

8 files changed, 667 insertions, 36 deletions

diff --git a/src/mongo/db/logical_session_id_helpers.cpp b/src/mongo/db/logical_session_id_helpers.cpp
index 035b54b061d..c780813e68a 100644
--- a/src/mongo/db/logical_session_id_helpers.cpp
+++ b/src/mongo/db/logical_session_id_helpers.cpp
@@ -84,6 +84,11 @@ SHA256Block getLogicalSessionUserDigestFor(StringData user, StringData db) {
     return SHA256Block::computeHash({ConstDataRange(fn.c_str(), fn.size())});
 }
 
+bool isParentSessionId(const LogicalSessionId& sessionId) {
+    // All child sessions must have a txnUUID.
+    return !sessionId.getTxnUUID();
+}
+
 boost::optional<LogicalSessionId> getParentSessionId(const LogicalSessionId& sessionId) {
     if (sessionId.getTxnUUID()) {
         return LogicalSessionId{sessionId.getId(), sessionId.getUid()};
diff --git a/src/mongo/db/logical_session_id_helpers.h b/src/mongo/db/logical_session_id_helpers.h
index 631ca0d979a..3d692c701e5 100644
--- a/src/mongo/db/logical_session_id_helpers.h
+++ b/src/mongo/db/logical_session_id_helpers.h
@@ -50,6 +50,12 @@ SHA256Block getLogicalSessionUserDigestForLoggedInUser(const OperationContext* o
 SHA256Block getLogicalSessionUserDigestFor(StringData user, StringData db);
 
 /**
+ * Returns if the given session is a parent session, ie only has fields that could have come from an
+ * external client.
+ */
+bool isParentSessionId(const LogicalSessionId& sessionId);
+
+/**
  * Returns the parent session id for the given session id if there is one.
  */
 boost::optional<LogicalSessionId> getParentSessionId(const LogicalSessionId& sessionId);
diff --git a/src/mongo/db/session_catalog.cpp b/src/mongo/db/session_catalog.cpp
index a0c97676370..7760427aa9a 100644
--- a/src/mongo/db/session_catalog.cpp
+++ b/src/mongo/db/session_catalog.cpp
@@ -89,7 +89,7 @@ SessionCatalog::ScopedCheckedOutSession SessionCatalog::_checkOutSessionInner(
     stdx::unique_lock<Latch> ul(_mutex);
 
     auto sri = _getOrCreateSessionRuntimeInfo(ul, lsid);
-    auto session = sri->getSession(lsid);
+    auto session = sri->getSession(ul, lsid);
     invariant(session);
 
     if (killToken) {
@@ -148,7 +148,7 @@ void SessionCatalog::scanSession(const LogicalSessionId& lsid,
     stdx::lock_guard<Latch> lg(_mutex);
 
     if (auto sri = _getSessionRuntimeInfo(lg, lsid)) {
-        auto session = sri->getSession(lsid);
+        auto session = sri->getSession(lg, lsid);
         invariant(session);
 
         ObservableSession osession(lg, sri, session);
@@ -259,7 +259,7 @@ SessionCatalog::KillToken SessionCatalog::killSession(const LogicalSessionId& ls
 
     auto sri = _getSessionRuntimeInfo(lg, lsid);
     uassert(ErrorCodes::NoSuchSession, "Session not found", sri);
-    auto session = sri->getSession(lsid);
+    auto session = sri->getSession(lg, lsid);
     uassert(ErrorCodes::NoSuchSession, "Session not found", session);
     return ObservableSession(lg, sri, session).kill();
 }
@@ -275,7 +275,7 @@ void SessionCatalog::createSessionIfDoesNotExist(const LogicalSessionId& lsid) {
 }
 
 SessionCatalog::SessionRuntimeInfo* SessionCatalog::_getSessionRuntimeInfo(
-    WithLock, const LogicalSessionId& lsid) {
+    WithLock wl, const LogicalSessionId& lsid) {
     auto parentLsid = castToParentSessionId(lsid);
     auto sriIt = _sessions.find(parentLsid);
 
@@ -284,7 +284,7 @@ SessionCatalog::SessionRuntimeInfo* SessionCatalog::_getSessionRuntimeInfo(
     }
 
     auto sri = sriIt->second.get();
-    auto session = sri->getSession(lsid);
+    auto session = sri->getSession(wl, lsid);
 
     if (session) {
         return sri;
@@ -322,7 +322,8 @@ SessionCatalog::SessionRuntimeInfo* SessionCatalog::_getOrCreateSessionRuntimeIn
 
 void SessionCatalog::_releaseSession(SessionRuntimeInfo* sri,
                                      Session* session,
-                                     boost::optional<KillToken> killToken) {
+                                     boost::optional<KillToken> killToken,
+                                     boost::optional<TxnNumber> clientTxnNumberStarted) {
     stdx::lock_guard<Latch> lg(_mutex);
 
     // Make sure we have exactly the same session on the map and that it is still associated with an
@@ -341,9 +342,30 @@ void SessionCatalog::_releaseSession(SessionRuntimeInfo* sri,
         invariant(sri->killsRequested > 0);
         --sri->killsRequested;
     }
+
+    if (clientTxnNumberStarted.has_value()) {
+        // Since the given txnNumber successfully started, we know any child sessions with older
+        // txnNumbers can be discarded. This needed to wait until a transaction started because that
+        // can fail, e.g. if the active transaction is prepared.
+        auto numReaped = stdx::erase_if(sri->childSessions, [&](auto&& it) {
+            ObservableSession osession(lg, sri, &it.second);
+            if (it.first.getTxnNumber() && *it.first.getTxnNumber() < *clientTxnNumberStarted) {
+                osession.markForReap(ObservableSession::ReapMode::kExclusive);
+            }
+            return osession._shouldBeReaped();
+        });
+
+        LOGV2_DEBUG(6685200,
+                    4,
+                    "Erased child sessions",
+                    "releasedLsid"_attr = session->getSessionId(),
+                    "clientTxnNumber"_attr = *clientTxnNumberStarted,
+                    "childSessionsRemaining"_attr = sri->childSessions.size(),
+                    "numReaped"_attr = numReaped);
+    }
 }
 
-Session* SessionCatalog::SessionRuntimeInfo::getSession(const LogicalSessionId& lsid) {
+Session* SessionCatalog::SessionRuntimeInfo::getSession(WithLock, const LogicalSessionId& lsid) {
     if (lsid == parentSession._sessionId) {
         return &parentSession;
     }
@@ -488,4 +510,36 @@ void OperationContextSession::checkOut(OperationContext* opCtx) {
     checkedOutSession.emplace(std::move(scopedCheckedOutSession));
 }
 
+void OperationContextSession::observeNewTxnNumberStarted(OperationContext* opCtx,
+                                                         const LogicalSessionId& lsid,
+                                                         TxnNumber txnNumber) {
+    auto& checkedOutSession = operationSessionDecoration(opCtx);
+    invariant(checkedOutSession);
+
+    LOGV2_DEBUG(6685201,
+                4,
+                "Observing new retryable write number started on session",
+                "lsid"_attr = lsid,
+                "txnNumber"_attr = txnNumber);
+
+    const auto& checkedOutLsid = (*checkedOutSession)->getSessionId();
+    if (isParentSessionId(lsid)) {
+        // Observing a new transaction/retryable write on a parent session.
+
+        // The operation must have checked out the parent session itself or a child session of the
+        // parent. This is safe because both share the same SessionRuntimeInfo.
+        dassert(lsid == checkedOutLsid || lsid == *getParentSessionId(checkedOutLsid));
+
+        checkedOutSession->observeNewClientTxnNumberStarted(txnNumber);
+    } else if (isInternalSessionForRetryableWrite(lsid)) {
+        // Observing a new internal transaction on a retryable session.
+
+        // A transaction on a child session is always begun on an operation that checked it out
+        // directly.
+        dassert(lsid == checkedOutLsid);
+
+        checkedOutSession->observeNewClientTxnNumberStarted(*lsid.getTxnNumber());
+    }
+}
+
 }  // namespace mongo
diff --git a/src/mongo/db/session_catalog.h b/src/mongo/db/session_catalog.h
index 937889c82cc..8186e2fcc60 100644
--- a/src/mongo/db/session_catalog.h
+++ b/src/mongo/db/session_catalog.h
@@ -152,7 +152,7 @@ private:
             invariant(!getParentSessionId(lsid));
         }
 
-        Session* getSession(const LogicalSessionId& lsid);
+        Session* getSession(WithLock, const LogicalSessionId& lsid);
 
         // Must only be accessed by the OperationContext which currently has this logical session
         // checked out.
@@ -207,11 +207,13 @@ private:
     SessionRuntimeInfo* _getOrCreateSessionRuntimeInfo(WithLock lk, const LogicalSessionId& lsid);
 
     /**
-     * Makes a session, previously checked out through 'checkoutSession', available again.
+     * Makes a session, previously checked out through 'checkoutSession', available again. Will free
+     * any retryable sessions with txnNumbers before clientTxnNumberStarted if it is set.
      */
     void _releaseSession(SessionRuntimeInfo* sri,
                          Session* session,
-                         boost::optional<KillToken> killToken);
+                         boost::optional<KillToken> killToken,
+                         boost::optional<TxnNumber> clientTxnNumberStarted);
 
     // Protects the state below
     mutable Mutex _mutex =
@@ -239,6 +241,7 @@ public:
 
     ScopedCheckedOutSession(ScopedCheckedOutSession&& other)
         : _catalog(other._catalog),
+          _clientTxnNumberStarted(other._clientTxnNumberStarted),
           _sri(other._sri),
           _session(other._session),
           _killToken(std::move(other._killToken)) {
@@ -251,7 +254,8 @@ public:
 
     ~ScopedCheckedOutSession() {
         if (_sri) {
-            _catalog._releaseSession(_sri, _session, std::move(_killToken));
+            _catalog._releaseSession(
+                _sri, _session, std::move(_killToken), _clientTxnNumberStarted);
         }
     }
 
@@ -275,10 +279,20 @@ public:
         return bool(_killToken);
     }
 
+    void observeNewClientTxnNumberStarted(TxnNumber txnNumber) {
+        _clientTxnNumberStarted = txnNumber;
+    }
+
 private:
     // The owning session catalog into which the session should be checked back
     SessionCatalog& _catalog;
 
+    // If this session began a retryable write or transaction while checked out, this is set to the
+    // "client txnNumber" of that transaction, which is the top-level txnNumber for a retryable
+    // write or transaction sent by a client or the txnNumber in the sessionId for a retryable
+    // child transaction.
+    boost::optional<TxnNumber> _clientTxnNumberStarted;
+
     SessionCatalog::SessionRuntimeInfo* _sri;
     Session* _session;
     boost::optional<SessionCatalog::KillToken> _killToken;
@@ -489,6 +503,14 @@ public:
     static void checkIn(OperationContext* opCtx, CheckInReason reason);
     static void checkOut(OperationContext* opCtx);
 
+    /**
+     * Notifies the session catalog when a new transaction/retryable write is begun on the operation
+     * context's checked out session.
+     */
+    static void observeNewTxnNumberStarted(OperationContext* opCtx,
+                                           const LogicalSessionId& lsid,
+                                           TxnNumber txnNumber);
+
 private:
     OperationContext* const _opCtx;
 };
diff --git a/src/mongo/db/transaction_participant.cpp b/src/mongo/db/transaction_participant.cpp
index 6944763d31b..0a8495b859c 100644
--- a/src/mongo/db/transaction_participant.cpp
+++ b/src/mongo/db/transaction_participant.cpp
@@ -2733,6 +2733,14 @@ void TransactionParticipant::Participant::_setNewTxnNumberAndRetryCounter(
 
     // Reset the transactional state
     _resetTransactionStateAndUnlock(&lk, TransactionState::kNone);
+
+    invariant(!lk);
+    if (isParentSessionId(_sessionId())) {
+        // Only observe parent sessions because retryable transactions begin the same txnNumber on
+        // their parent session.
+        OperationContextSession::observeNewTxnNumberStarted(
+            opCtx, _sessionId(), txnNumberAndRetryCounter.getTxnNumber());
+    }
 }
 
 void RetryableWriteTransactionParticipantCatalog::addParticipant(
diff --git a/src/mongo/db/transaction_participant_test.cpp b/src/mongo/db/transaction_participant_test.cpp
index 37cca9a0099..736020b0bf1 100644
--- a/src/mongo/db/transaction_participant_test.cpp
+++ b/src/mongo/db/transaction_participant_test.cpp
@@ -47,6 +47,7 @@
 #include "mongo/db/repl/storage_interface_mock.h"
 #include "mongo/db/server_transactions_metrics.h"
 #include "mongo/db/service_context.h"
+#include "mongo/db/session_catalog.h"
 #include "mongo/db/session_catalog_mongod.h"
 #include "mongo/db/stats/fill_locker_info.h"
 #include "mongo/db/transaction_participant.h"
@@ -350,6 +351,33 @@ protected:
         return opCtxSession;
     }
 
+    void checkOutSessionFromDiferentOpCtx(const LogicalSessionId& lsid,
+                                          bool beginOrContinueTxn,
+                                          boost::optional<TxnNumber> txnNumber = boost::none,
+                                          boost::optional<bool> autocommit = boost::none,
+                                          boost::optional<bool> startTransaction = boost::none,
+                                          bool commitTxn = false) {
+        runFunctionFromDifferentOpCtx([&](OperationContext* opCtx) {
+            opCtx->setLogicalSessionId(lsid);
+            if (txnNumber) {
+                opCtx->setTxnNumber(*txnNumber);
+                opCtx->setInMultiDocumentTransaction();
+            }
+
+            auto opCtxSession = std::make_unique<MongoDOperationContextSession>(opCtx);
+
+            if (beginOrContinueTxn) {
+                auto txnParticipant = TransactionParticipant::get(opCtx);
+                txnParticipant.beginOrContinue(
+                    opCtx, {*opCtx->getTxnNumber()}, autocommit, startTransaction);
+
+                if (commitTxn) {
+                    txnParticipant.commitUnpreparedTransaction(opCtx);
+                }
+            }
+        });
+    }
+
     const LogicalSessionId _sessionId{makeLogicalSessionIdForTest()};
     const TxnNumber _txnNumber{20};
     const UUID _uuid = UUID::gen();
@@ -1662,6 +1690,30 @@ protected:
         TxnParticipantTest::tearDown();
     }
 
+    void runRetryableWrite(LogicalSessionId lsid, TxnNumber txnNumber) {
+        runFunctionFromDifferentOpCtx([&](OperationContext* opCtx) {
+            opCtx->setLogicalSessionId(lsid);
+            opCtx->setTxnNumber(txnNumber);
+            auto opCtxSession = std::make_unique<MongoDOperationContextSession>(opCtx);
+            TransactionParticipant::get(opCtx).beginOrContinue(
+                opCtx, {*opCtx->getTxnNumber()}, boost::none, boost::none);
+        });
+    }
+
+    void runAndCommitTransaction(LogicalSessionId lsid, TxnNumber txnNumber) {
+        runFunctionFromDifferentOpCtx([&](OperationContext* opCtx) {
+            opCtx->setLogicalSessionId(lsid);
+            opCtx->setTxnNumber(txnNumber);
+            opCtx->setInMultiDocumentTransaction();
+            auto opCtxSession = std::make_unique<MongoDOperationContextSession>(opCtx);
+
+            auto txnParticipant = TransactionParticipant::get(opCtx);
+            txnParticipant.beginOrContinue(opCtx, {*opCtx->getTxnNumber()}, false, true);
+            txnParticipant.unstashTransactionResources(opCtx, "find");
+            txnParticipant.commitUnpreparedTransaction(opCtx);
+        });
+    }
+
     RAIIServerParameterControllerForTest _controller{"featureFlagInternalTransactions", true};
 };
 
@@ -5432,5 +5484,397 @@ TEST_F(TxnParticipantTest,
     }
 }
 
+bool doesExistInCatalog(const LogicalSessionId& lsid, SessionCatalog* sessionCatalog) {
+    bool existsInCatalog{false};
+    sessionCatalog->scanSession(lsid,
+                                [&](const ObservableSession& session) { existsInCatalog = true; });
+    return existsInCatalog;
+}
+
+TEST_F(ShardTxnParticipantTest, EagerlyReapRetryableSessionsUponNewClientTransaction) {
+    auto sessionCatalog = SessionCatalog::get(getServiceContext());
+    ASSERT_EQ(sessionCatalog->size(), 0);
+
+    // Add a parent session with one retryable child.
+
+    auto parentLsid = makeLogicalSessionIdForTest();
+    auto parentTxnNumber = 0;
+    runRetryableWrite(parentLsid, parentTxnNumber);
+
+    auto retryableChildLsid =
+        makeLogicalSessionIdWithTxnNumberAndUUIDForTest(parentLsid, parentTxnNumber);
+    runAndCommitTransaction(retryableChildLsid, 0);
+
+    ASSERT_EQ(sessionCatalog->size(), 1);
+    ASSERT(doesExistInCatalog(parentLsid, sessionCatalog));
+    ASSERT(doesExistInCatalog(retryableChildLsid, sessionCatalog));
+
+    // Start a higher txnNumber client transaction and verify the child was erased.
+
+    parentTxnNumber++;
+    runAndCommitTransaction(parentLsid, parentTxnNumber);
+
+    ASSERT_EQ(sessionCatalog->size(), 1);
+    ASSERT(doesExistInCatalog(parentLsid, sessionCatalog));
+    ASSERT_FALSE(doesExistInCatalog(retryableChildLsid, sessionCatalog));
+}
+
+TEST_F(ShardTxnParticipantTest, EagerlyReapRetryableSessionsUponNewClientRetryableWrite) {
+    auto sessionCatalog = SessionCatalog::get(getServiceContext());
+    ASSERT_EQ(sessionCatalog->size(), 0);
+
+    // Add a parent session with one retryable child.
+
+    auto parentLsid = makeLogicalSessionIdForTest();
+    auto parentTxnNumber = 0;
+    runRetryableWrite(parentLsid, parentTxnNumber);
+
+    auto retryableChildLsid =
+        makeLogicalSessionIdWithTxnNumberAndUUIDForTest(parentLsid, parentTxnNumber);
+    runAndCommitTransaction(retryableChildLsid, 0);
+
+    ASSERT_EQ(sessionCatalog->size(), 1);
+    ASSERT(doesExistInCatalog(parentLsid, sessionCatalog));
+    ASSERT(doesExistInCatalog(retryableChildLsid, sessionCatalog));
+
+    // Start a higher txnNumber retryable write and verify the child was erased.
+
+    parentTxnNumber++;
+    runRetryableWrite(parentLsid, parentTxnNumber);
+
+    ASSERT_EQ(sessionCatalog->size(), 1);
+    ASSERT(doesExistInCatalog(parentLsid, sessionCatalog));
+    ASSERT_FALSE(doesExistInCatalog(retryableChildLsid, sessionCatalog));
+}
+
+TEST_F(ShardTxnParticipantTest, EagerlyReapRetryableSessionsUponNewRetryableTransaction) {
+    auto sessionCatalog = SessionCatalog::get(getServiceContext());
+    ASSERT_EQ(sessionCatalog->size(), 0);
+
+    // Add a parent session with one retryable child.
+
+    auto parentLsid = makeLogicalSessionIdForTest();
+    auto parentTxnNumber = 0;
+    runRetryableWrite(parentLsid, parentTxnNumber);
+
+    auto retryableChildLsid =
+        makeLogicalSessionIdWithTxnNumberAndUUIDForTest(parentLsid, parentTxnNumber);
+    runAndCommitTransaction(retryableChildLsid, 0);
+
+    ASSERT_EQ(sessionCatalog->size(), 1);
+    ASSERT(doesExistInCatalog(parentLsid, sessionCatalog));
+    ASSERT(doesExistInCatalog(retryableChildLsid, sessionCatalog));
+
+    // Start a higher txnNumber retryable transaction and verify the child was erased.
+
+    parentTxnNumber++;
+    auto higherRetryableChildLsid =
+        makeLogicalSessionIdWithTxnNumberAndUUIDForTest(parentLsid, parentTxnNumber);
+    runAndCommitTransaction(higherRetryableChildLsid, 0);
+
+    ASSERT_EQ(sessionCatalog->size(), 1);
+    ASSERT(doesExistInCatalog(parentLsid, sessionCatalog));
+    ASSERT_FALSE(doesExistInCatalog(retryableChildLsid, sessionCatalog));
+    ASSERT(doesExistInCatalog(higherRetryableChildLsid, sessionCatalog));
+}
+
+TEST_F(
+    ShardTxnParticipantTest,
+    EagerlyReapRetryableSessionsOnlyUponNewTransactionBegunAndIgnoresNonRetryableAndUnrelatedSessions) {
+    auto sessionCatalog = SessionCatalog::get(getServiceContext());
+    ASSERT_EQ(sessionCatalog->size(), 0);
+
+    // Add a parent session with two retryable children and one non-retryable child.
+
+    auto parentLsid = makeLogicalSessionIdForTest();
+    auto parentTxnNumber = 0;
+    runRetryableWrite(parentLsid, parentTxnNumber);
+
+    auto retryableChildLsid1 =
+        makeLogicalSessionIdWithTxnNumberAndUUIDForTest(parentLsid, parentTxnNumber);
+    runAndCommitTransaction(retryableChildLsid1, 0);
+
+    auto retryableChildLsid2 =
+        makeLogicalSessionIdWithTxnNumberAndUUIDForTest(parentLsid, parentTxnNumber);
+    runAndCommitTransaction(retryableChildLsid2, 0);
+
+    auto nonRetryableChildLsid = makeLogicalSessionIdWithTxnUUIDForTest(parentLsid);
+    runAndCommitTransaction(nonRetryableChildLsid, 0);
+
+    // Add entries for unrelated sessions to verify they aren't affected.
+
+    auto parentLsidOther = makeLogicalSessionIdForTest();
+    runRetryableWrite(parentLsidOther, 0);
+
+    auto retryableChildLsidOther =
+        makeLogicalSessionIdWithTxnNumberAndUUIDForTest(parentLsidOther, parentTxnNumber);
+    runAndCommitTransaction(retryableChildLsidOther, 0);
+
+    auto nonRetryableChildLsidOther =
+        makeLogicalSessionIdWithTxnNumberAndUUIDForTest(parentLsidOther, parentTxnNumber);
+    runAndCommitTransaction(nonRetryableChildLsidOther, 0);
+
+    ASSERT_EQ(sessionCatalog->size(), 2);
+    ASSERT(doesExistInCatalog(parentLsid, sessionCatalog));
+    ASSERT(doesExistInCatalog(nonRetryableChildLsid, sessionCatalog));
+    ASSERT(doesExistInCatalog(retryableChildLsid1, sessionCatalog));
+    ASSERT(doesExistInCatalog(retryableChildLsid2, sessionCatalog));
+    ASSERT(doesExistInCatalog(parentLsidOther, sessionCatalog));
+    ASSERT(doesExistInCatalog(retryableChildLsidOther, sessionCatalog));
+    ASSERT(doesExistInCatalog(nonRetryableChildLsidOther, sessionCatalog));
+
+    // Check out with a higher txnNumber and verify we don't reap until a transaction has begun on
+    // it.
+
+    parentTxnNumber++;
+
+    // Does not call beginOrContinue.
+    runFunctionFromDifferentOpCtx([&](OperationContext* opCtx) {
+        opCtx->setLogicalSessionId(parentLsid);
+        opCtx->setTxnNumber(parentTxnNumber);
+        auto opCtxSession = std::make_unique<MongoDOperationContextSession>(opCtx);
+    });
+    // Does not call beginOrContinue.
+    auto higherRetryableChildLsid =
+        makeLogicalSessionIdWithTxnNumberAndUUIDForTest(parentLsid, parentTxnNumber);
+    runFunctionFromDifferentOpCtx([&](OperationContext* opCtx) {
+        opCtx->setLogicalSessionId(higherRetryableChildLsid);
+        opCtx->setTxnNumber(0);
+        opCtx->setInMultiDocumentTransaction();
+        auto opCtxSession = std::make_unique<MongoDOperationContextSession>(opCtx);
+    });
+    // beginOrContinue fails because no startTransaction=true.
+    runFunctionFromDifferentOpCtx([&](OperationContext* opCtx) {
+        opCtx->setLogicalSessionId(parentLsid);
+        opCtx->setTxnNumber(parentTxnNumber);
+        opCtx->setInMultiDocumentTransaction();
+        auto opCtxSession = std::make_unique<MongoDOperationContextSession>(opCtx);
+
+        auto txnParticipant = TransactionParticipant::get(opCtx);
+        ASSERT_THROWS_CODE(
+            txnParticipant.beginOrContinue(opCtx, {*opCtx->getTxnNumber()}, false, boost::none),
+            AssertionException,
+            ErrorCodes::NoSuchTransaction);
+    });
+    // Non-retryable child sessions shouldn't affect retryable sessions.
+    auto newNonRetryableChildLsid = makeLogicalSessionIdWithTxnUUIDForTest(parentLsid);
+    runAndCommitTransaction(newNonRetryableChildLsid, 0);
+
+    // No sessions should have been reaped.
+    ASSERT_EQ(sessionCatalog->size(), 2);
+    ASSERT(doesExistInCatalog(parentLsid, sessionCatalog));
+    ASSERT(doesExistInCatalog(nonRetryableChildLsid, sessionCatalog));
+    ASSERT(doesExistInCatalog(retryableChildLsid1, sessionCatalog));
+    ASSERT(doesExistInCatalog(retryableChildLsid2, sessionCatalog));
+    ASSERT(doesExistInCatalog(parentLsidOther, sessionCatalog));
+    ASSERT(doesExistInCatalog(retryableChildLsidOther, sessionCatalog));
+    ASSERT(doesExistInCatalog(nonRetryableChildLsidOther, sessionCatalog));
+
+    // Call beginOrContinue for a higher txnNumber and verify we do erase old sessions for the
+    // active session.
+
+    runRetryableWrite(parentLsid, parentTxnNumber);
+
+    // The two retryable children for parentLsid should have been reaped.
+    ASSERT_EQ(sessionCatalog->size(), 2);
+    ASSERT(doesExistInCatalog(parentLsid, sessionCatalog));
+    ASSERT(doesExistInCatalog(nonRetryableChildLsid, sessionCatalog));
+    ASSERT_FALSE(doesExistInCatalog(retryableChildLsid1, sessionCatalog));
+    ASSERT_FALSE(doesExistInCatalog(retryableChildLsid2, sessionCatalog));
+    ASSERT(doesExistInCatalog(parentLsidOther, sessionCatalog));
+    ASSERT(doesExistInCatalog(retryableChildLsidOther, sessionCatalog));
+    ASSERT(doesExistInCatalog(nonRetryableChildLsidOther, sessionCatalog));
+}
+
+TEST_F(ShardTxnParticipantTest, EagerlyReapLowerTxnNumbers) {
+    auto sessionCatalog = SessionCatalog::get(getServiceContext());
+    ASSERT_EQ(sessionCatalog->size(), 0);
+
+    // Add a parent session with two retryable children.
+
+    auto parentLsid = makeLogicalSessionIdForTest();
+    auto parentTxnNumber = 1;
+    runRetryableWrite(parentLsid, parentTxnNumber);
+
+    auto retryableChildLsid1 =
+        makeLogicalSessionIdWithTxnNumberAndUUIDForTest(parentLsid, parentTxnNumber);
+    runAndCommitTransaction(retryableChildLsid1, 0);
+
+    auto retryableChildLsid2 =
+        makeLogicalSessionIdWithTxnNumberAndUUIDForTest(parentLsid, parentTxnNumber);
+    runAndCommitTransaction(retryableChildLsid2, 0);
+
+    auto lowerRetryableChildLsid =
+        makeLogicalSessionIdWithTxnNumberAndUUIDForTest(parentLsid, parentTxnNumber - 1);
+    runFunctionFromDifferentOpCtx([&](OperationContext* opCtx) {
+        opCtx->setLogicalSessionId(lowerRetryableChildLsid);
+        opCtx->setTxnNumber(0);
+        opCtx->setInMultiDocumentTransaction();
+        auto opCtxSession = std::make_unique<MongoDOperationContextSession>(opCtx);
+    });
+
+    ASSERT_EQ(sessionCatalog->size(), 1);
+    ASSERT(doesExistInCatalog(parentLsid, sessionCatalog));
+    ASSERT(doesExistInCatalog(retryableChildLsid1, sessionCatalog));
+    ASSERT(doesExistInCatalog(retryableChildLsid2, sessionCatalog));
+    ASSERT(doesExistInCatalog(lowerRetryableChildLsid, sessionCatalog));
+
+    // Start a higher txnNumber retryable transaction and verify the child was erased.
+
+    parentTxnNumber++;
+    runRetryableWrite(parentLsid, parentTxnNumber);
+
+    ASSERT_EQ(sessionCatalog->size(), 1);
+    ASSERT(doesExistInCatalog(parentLsid, sessionCatalog));
+    ASSERT_FALSE(doesExistInCatalog(retryableChildLsid1, sessionCatalog));
+    ASSERT_FALSE(doesExistInCatalog(retryableChildLsid2, sessionCatalog));
+    ASSERT_FALSE(doesExistInCatalog(lowerRetryableChildLsid, sessionCatalog));
+}
+
+TEST_F(ShardTxnParticipantTest, EagerlyReapSkipsHigherUnusedTxnNumbers) {
+    auto sessionCatalog = SessionCatalog::get(getServiceContext());
+    ASSERT_EQ(sessionCatalog->size(), 0);
+
+    // Add a parent session with one retryable child.
+
+    auto parentLsid = makeLogicalSessionIdForTest();
+    auto parentTxnNumber = 1;
+    runRetryableWrite(parentLsid, parentTxnNumber);
+
+    auto retryableChildLsid =
+        makeLogicalSessionIdWithTxnNumberAndUUIDForTest(parentLsid, parentTxnNumber);
+    runAndCommitTransaction(retryableChildLsid, 0);
+
+    ASSERT_EQ(sessionCatalog->size(), 1);
+    ASSERT(doesExistInCatalog(parentLsid, sessionCatalog));
+    ASSERT(doesExistInCatalog(retryableChildLsid, sessionCatalog));
+
+    // Check out a higher txnNumber retryable transaction but do not start it and verify it does not
+    // reap and is not reaped.
+
+    auto higherUnusedRetryableChildLsid =
+        makeLogicalSessionIdWithTxnNumberAndUUIDForTest(parentLsid, parentTxnNumber + 10);
+    runFunctionFromDifferentOpCtx([&](OperationContext* opCtx) {
+        opCtx->setLogicalSessionId(higherUnusedRetryableChildLsid);
+        opCtx->setTxnNumber(0);
+        opCtx->setInMultiDocumentTransaction();
+        auto opCtxSession = std::make_unique<MongoDOperationContextSession>(opCtx);
+    });
+
+    ASSERT_EQ(sessionCatalog->size(), 1);
+    ASSERT(doesExistInCatalog(parentLsid, sessionCatalog));
+    ASSERT(doesExistInCatalog(retryableChildLsid, sessionCatalog));
+    ASSERT(doesExistInCatalog(higherUnusedRetryableChildLsid, sessionCatalog));
+
+    parentTxnNumber++;  // Still less than in higherUnusedRetryableChildLsid.
+    runRetryableWrite(parentLsid, parentTxnNumber);
+
+    ASSERT_EQ(sessionCatalog->size(), 1);
+    ASSERT(doesExistInCatalog(parentLsid, sessionCatalog));
+    ASSERT_FALSE(doesExistInCatalog(retryableChildLsid, sessionCatalog));
+    ASSERT(doesExistInCatalog(higherUnusedRetryableChildLsid, sessionCatalog));
+}
+
+TEST_F(ShardTxnParticipantTest, EagerlyReapSkipsKilledSessions) {
+    auto sessionCatalog = SessionCatalog::get(getServiceContext());
+    ASSERT_EQ(sessionCatalog->size(), 0);
+
+    // Add a parent session with two retryable children.
+
+    auto parentLsid = makeLogicalSessionIdForTest();
+    auto parentTxnNumber = 1;
+    runRetryableWrite(parentLsid, parentTxnNumber);
+
+    auto retryableChildLsid1 =
+        makeLogicalSessionIdWithTxnNumberAndUUIDForTest(parentLsid, parentTxnNumber);
+    runAndCommitTransaction(retryableChildLsid1, 0);
+
+    auto retryableChildLsid2 =
+        makeLogicalSessionIdWithTxnNumberAndUUIDForTest(parentLsid, parentTxnNumber);
+    runAndCommitTransaction(retryableChildLsid2, 0);
+
+    ASSERT_EQ(sessionCatalog->size(), 1);
+    ASSERT(doesExistInCatalog(parentLsid, sessionCatalog));
+    ASSERT(doesExistInCatalog(retryableChildLsid1, sessionCatalog));
+    ASSERT(doesExistInCatalog(retryableChildLsid2, sessionCatalog));
+
+    // Kill one retryable child and verify no sessions in its SRI can be reaped until it has been
+    // checked out by its killer.
+
+    parentTxnNumber++;
+    boost::optional<SessionCatalog::KillToken> killToken;
+    runFunctionFromDifferentOpCtx([&](OperationContext* opCtx) {
+        opCtx->setLogicalSessionId(parentLsid);
+        opCtx->setTxnNumber(parentTxnNumber);
+        auto opCtxSession = std::make_unique<MongoDOperationContextSession>(opCtx);
+
+        TransactionParticipant::get(opCtx).beginOrContinue(
+            opCtx, {*opCtx->getTxnNumber()}, boost::none, boost::none);
+
+        // Kill after checking out the session because we can't check out the session again
+        // after a kill without checking out with the killToken first.
+        killToken = sessionCatalog->killSession(retryableChildLsid1);
+    });
+
+    ASSERT_EQ(sessionCatalog->size(), 1);
+    ASSERT(doesExistInCatalog(parentLsid, sessionCatalog));
+    ASSERT(doesExistInCatalog(retryableChildLsid1, sessionCatalog));
+    ASSERT(doesExistInCatalog(retryableChildLsid2, sessionCatalog));
+
+    // Check out for kill the killed retryable session and now both retryable sessions can be
+    // reaped.
+    runFunctionFromDifferentOpCtx([&](OperationContext* opCtx) {
+        sessionCatalog->checkOutSessionForKill(opCtx, std::move(*killToken));
+    });
+
+    // A new client txnNumber must be seen to trigger the reaping, so the sessions shouldn't have
+    // been reaped upon releasing the killed session.
+    ASSERT_EQ(sessionCatalog->size(), 1);
+    ASSERT(doesExistInCatalog(parentLsid, sessionCatalog));
+    ASSERT(doesExistInCatalog(retryableChildLsid1, sessionCatalog));
+    ASSERT(doesExistInCatalog(retryableChildLsid2, sessionCatalog));
+
+    parentTxnNumber++;
+    runRetryableWrite(parentLsid, parentTxnNumber);
+
+    ASSERT_EQ(sessionCatalog->size(), 1);
+    ASSERT(doesExistInCatalog(parentLsid, sessionCatalog));
+    ASSERT_FALSE(doesExistInCatalog(retryableChildLsid1, sessionCatalog));
+    ASSERT_FALSE(doesExistInCatalog(retryableChildLsid2, sessionCatalog));
+}
+
+TEST_F(ShardTxnParticipantTest, CheckingOutEagerlyReapedSessionDoesNotCrash) {
+    auto sessionCatalog = SessionCatalog::get(getServiceContext());
+    ASSERT_EQ(sessionCatalog->size(), 0);
+
+    // Add a parent session with one retryable child.
+
+    auto parentLsid = makeLogicalSessionIdForTest();
+    auto parentTxnNumber = 0;
+    runRetryableWrite(parentLsid, parentTxnNumber);
+
+    auto retryableChildLsid =
+        makeLogicalSessionIdWithTxnNumberAndUUIDForTest(parentLsid, parentTxnNumber);
+    runAndCommitTransaction(retryableChildLsid, 0);
+
+    ASSERT_EQ(sessionCatalog->size(), 1);
+    ASSERT(doesExistInCatalog(parentLsid, sessionCatalog));
+    ASSERT(doesExistInCatalog(retryableChildLsid, sessionCatalog));
+
+    // Start a higher txnNumber client transaction and verify the child was erased.
+
+    parentTxnNumber++;
+    runAndCommitTransaction(parentLsid, parentTxnNumber);
+
+    ASSERT_EQ(sessionCatalog->size(), 1);
+    ASSERT(doesExistInCatalog(parentLsid, sessionCatalog));
+    ASSERT_FALSE(doesExistInCatalog(retryableChildLsid, sessionCatalog));
+
+    // Check out the child again to verify this doesn't crash.
+    ASSERT_THROWS_CODE(runAndCommitTransaction(retryableChildLsid, 1),
+                       AssertionException,
+                       ErrorCodes::TransactionTooOld);
+}
+
 }  // namespace
 }  // namespace mongo
diff --git a/src/mongo/s/transaction_router.cpp b/src/mongo/s/transaction_router.cpp
index 955fb56861e..1368e3dc342 100644
--- a/src/mongo/s/transaction_router.cpp
+++ b/src/mongo/s/transaction_router.cpp
@@ -1436,34 +1436,39 @@ void TransactionRouter::Router::appendRecoveryToken(BSONObjBuilder* builder) con
 
 void TransactionRouter::Router::_resetRouterState(
     OperationContext* opCtx, const TxnNumberAndRetryCounter& txnNumberAndRetryCounter) {
-    stdx::lock_guard<Client> lk(*opCtx->getClient());
+    {
+        stdx::lock_guard<Client> lk(*opCtx->getClient());
 
-    uassert(ErrorCodes::ConflictingOperationInProgress,
-            "Cannot start a new transaction while the previous is yielded",
-            o(lk).activeYields == 0);
-
-    o(lk).txnNumberAndRetryCounter.setTxnNumber(txnNumberAndRetryCounter.getTxnNumber());
-    o(lk).txnNumberAndRetryCounter.setTxnRetryCounter(
-        *txnNumberAndRetryCounter.getTxnRetryCounter());
-    o(lk).commitType = CommitType::kNotInitiated;
-    p().isRecoveringCommit = false;
-    o(lk).participants.clear();
-    o(lk).coordinatorId.reset();
-    p().recoveryShardId.reset();
-    o(lk).apiParameters = {};
-    o(lk).readConcernArgs = {};
-    o(lk).atClusterTime.reset();
-    o(lk).abortCause = std::string();
-    o(lk).metricsTracker.emplace(opCtx->getServiceContext());
-    p().terminationInitiated = false;
+        uassert(ErrorCodes::ConflictingOperationInProgress,
+                "Cannot start a new transaction while the previous is yielded",
+                o(lk).activeYields == 0);
 
-    auto tickSource = opCtx->getServiceContext()->getTickSource();
-    o(lk).metricsTracker->trySetActive(tickSource, tickSource->getTicks());
+        o(lk).txnNumberAndRetryCounter.setTxnNumber(txnNumberAndRetryCounter.getTxnNumber());
+        o(lk).txnNumberAndRetryCounter.setTxnRetryCounter(
+            *txnNumberAndRetryCounter.getTxnRetryCounter());
+        o(lk).commitType = CommitType::kNotInitiated;
+        p().isRecoveringCommit = false;
+        o(lk).participants.clear();
+        o(lk).coordinatorId.reset();
+        p().recoveryShardId.reset();
+        o(lk).apiParameters = {};
+        o(lk).readConcernArgs = {};
+        o(lk).atClusterTime.reset();
+        o(lk).abortCause = std::string();
+        o(lk).metricsTracker.emplace(opCtx->getServiceContext());
+        p().terminationInitiated = false;
+
+        auto tickSource = opCtx->getServiceContext()->getTickSource();
+        o(lk).metricsTracker->trySetActive(tickSource, tickSource->getTicks());
+
+        // TODO SERVER-37115: Parse statement ids from the client and remember the statement id
+        // of the command that started the transaction, if one was included.
+        p().latestStmtId = kDefaultFirstStmtId;
+        p().firstStmtId = kDefaultFirstStmtId;
+    }
 
-    // TODO SERVER-37115: Parse statement ids from the client and remember the statement id
-    // of the command that started the transaction, if one was included.
-    p().latestStmtId = kDefaultFirstStmtId;
-    p().firstStmtId = kDefaultFirstStmtId;
+    OperationContextSession::observeNewTxnNumberStarted(
+        opCtx, _sessionId(), txnNumberAndRetryCounter.getTxnNumber());
 };
 
 void TransactionRouter::Router::_resetRouterStateForStartTransaction(
diff --git a/src/mongo/s/transaction_router_test.cpp b/src/mongo/s/transaction_router_test.cpp
index addefae213c..ef933e911c1 100644
--- a/src/mongo/s/transaction_router_test.cpp
+++ b/src/mongo/s/transaction_router_test.cpp
@@ -200,6 +200,26 @@ protected:
         });
     }
 
+    void runFunctionFromDifferentOpCtx(std::function<void(OperationContext*)> func) {
+        auto newClientOwned = getServiceContext()->makeClient("newClient");
+        AlternativeClientRegion acr(newClientOwned);
+        auto newOpCtx = cc().makeOperationContext();
+        func(newOpCtx.get());
+    }
+
+    void runTransactionLeaveOpen(LogicalSessionId lsid, TxnNumber txnNumber) {
+        runFunctionFromDifferentOpCtx([&](OperationContext* opCtx) {
+            opCtx->setLogicalSessionId(lsid);
+            opCtx->setTxnNumber(txnNumber);
+            opCtx->setInMultiDocumentTransaction();
+            auto opCtxSession = std::make_unique<RouterOperationContextSession>(opCtx);
+
+            auto txnRouter = TransactionRouter::get(opCtx);
+            txnRouter.beginOrContinueTxn(
+                opCtx, *opCtx->getTxnNumber(), TransactionRouter::TransactionActions::kStart);
+        });
+    }
+
 private:
     // Enables the transaction router to retry within a transaction on stale version and snapshot
     // errors for the duration of each test.
@@ -5302,5 +5322,72 @@ TEST_F(TransactionRouterMetricsTest, IsTrackingOverIfTxnImplicitlyAborted) {
     implicitAbortInProgress();
     ASSERT(txnRouter().isTrackingOver());
 }
+
+bool doesExistInCatalog(const LogicalSessionId& lsid, SessionCatalog* sessionCatalog) {
+    bool existsInCatalog{false};
+    sessionCatalog->scanSession(lsid,
+                                [&](const ObservableSession& session) { existsInCatalog = true; });
+    return existsInCatalog;
+}
+
+TEST_F(TransactionRouterTest, EagerlyReapRetryableSessionsUponNewClientTransaction) {
+    auto sessionCatalog = SessionCatalog::get(getServiceContext());
+    ASSERT_EQ(sessionCatalog->size(), 0);
+
+    // Add a parent session with one retryable child.
+
+    auto parentLsid = makeLogicalSessionIdForTest();
+    auto parentTxnNumber = 0;
+    runTransactionLeaveOpen(parentLsid, parentTxnNumber);
+
+    auto retryableChildLsid =
+        makeLogicalSessionIdWithTxnNumberAndUUIDForTest(parentLsid, parentTxnNumber);
+    runTransactionLeaveOpen(retryableChildLsid, 0);
+
+    ASSERT_EQ(sessionCatalog->size(), 1);
+    ASSERT(doesExistInCatalog(parentLsid, sessionCatalog));
+    ASSERT(doesExistInCatalog(retryableChildLsid, sessionCatalog));
+
+    // Start a higher txnNumber client transaction and verify the child was erased.
+
+    parentTxnNumber++;
+    runTransactionLeaveOpen(parentLsid, parentTxnNumber);
+
+    ASSERT_EQ(sessionCatalog->size(), 1);
+    ASSERT(doesExistInCatalog(parentLsid, sessionCatalog));
+    ASSERT_FALSE(doesExistInCatalog(retryableChildLsid, sessionCatalog));
+}
+
+TEST_F(TransactionRouterTest, EagerlyReapRetryableSessionsUponNewRetryableTransaction) {
+    auto sessionCatalog = SessionCatalog::get(getServiceContext());
+    ASSERT_EQ(sessionCatalog->size(), 0);
+
+    // Add a parent session with one retryable child.
+
+    auto parentLsid = makeLogicalSessionIdForTest();
+    auto parentTxnNumber = 0;
+    runTransactionLeaveOpen(parentLsid, parentTxnNumber);
+
+    auto retryableChildLsid =
+        makeLogicalSessionIdWithTxnNumberAndUUIDForTest(parentLsid, parentTxnNumber);
+    runTransactionLeaveOpen(retryableChildLsid, 0);
+
+    ASSERT_EQ(sessionCatalog->size(), 1);
+    ASSERT(doesExistInCatalog(parentLsid, sessionCatalog));
+    ASSERT(doesExistInCatalog(retryableChildLsid, sessionCatalog));
+
+    // Start a higher txnNumber retryable transaction and verify the child was erased.
+
+    parentTxnNumber++;
+    auto higherRetryableChildLsid =
+        makeLogicalSessionIdWithTxnNumberAndUUIDForTest(parentLsid, parentTxnNumber);
+    runTransactionLeaveOpen(higherRetryableChildLsid, 0);
+
+    ASSERT_EQ(sessionCatalog->size(), 1);
+    ASSERT(doesExistInCatalog(parentLsid, sessionCatalog));
+    ASSERT_FALSE(doesExistInCatalog(retryableChildLsid, sessionCatalog));
+    ASSERT(doesExistInCatalog(higherRetryableChildLsid, sessionCatalog));
+}
+
 }  // unnamed namespace
 }  // namespace mongo