SERVER-41560 default the snapshot window size to 0 when running with the WT inMemory storage engine

Author: Dianna Hohensee

src/mongo/db/SConscript

@@ -1402,6 +1402,7 @@ env.Library(
     ],
     LIBDEPS_PRIVATE=[
         'snapshot_window_options',
+        '$BUILD_DIR/mongo/db/commands/test_commands_enabled',
         '$BUILD_DIR/mongo/db/service_context',
     ],
 )

src/mongo/db/db.cpp

@@ -611,7 +611,11 @@ ExitCode _initAndListen(int listenPort) {
     // release periodically in order to avoid storage cache pressure build up.
     if (storageEngine->supportsReadConcernSnapshot()) {
         PeriodicThreadToAbortExpiredTransactions::get(serviceContext)->start();
-        PeriodicThreadToDecreaseSnapshotHistoryCachePressure::get(serviceContext)->start();
+        // The inMemory engine is not yet used for replica or sharded transactions in production so
+        // it does not currently maintain snapshot history. It is live in testing, however.
+        if (!storageEngine->isEphemeral() || getTestCommandsEnabled()) {
+            PeriodicThreadToDecreaseSnapshotHistoryCachePressure::get(serviceContext)->start();
+        }
     }
 
     // Set up the logical session cache

src/mongo/db/snapshot_window_util.cpp

@@ -33,6 +33,7 @@
 
 #include "mongo/db/snapshot_window_util.h"
 
+#include "mongo/db/commands/test_commands_enabled.h"
 #include "mongo/db/operation_context.h"
 #include "mongo/db/service_context.h"
 #include "mongo/db/snapshot_window_options.h"
@@ -109,6 +110,7 @@ void increaseTargetSnapshotWindowSize(OperationContext* opCtx) {
     // the window size.
     StorageEngine* engine = opCtx->getServiceContext()->getStorageEngine();
     if (engine && engine->isCacheUnderPressure(opCtx)) {
+        invariant(!engine->isEphemeral() || getTestCommandsEnabled());
         warning() << "Attempted to increase the time window of available snapshots for "
                      "point-in-time operations (readConcern level 'snapshot' or transactions), but "
                      "the storage engine cache pressure, per the cachePressureThreshold setting of "
@@ -152,6 +154,7 @@ void decreaseTargetSnapshotWindowSize(OperationContext* opCtx) {
 
     StorageEngine* engine = opCtx->getServiceContext()->getStorageEngine();
     if (engine && engine->isCacheUnderPressure(opCtx)) {
+        invariant(!engine->isEphemeral() || getTestCommandsEnabled());
         _decreaseTargetSnapshotWindowSize(lock, opCtx);
     }
 }

src/mongo/db/snapshot_window_util_test.cpp

@@ -32,6 +32,7 @@
 #include "mongo/db/snapshot_window_util.h"
 
 #include "mongo/db/client.h"
+#include "mongo/db/commands/test_commands_enabled.h"
 #include "mongo/db/service_context.h"
 #include "mongo/db/service_context_devnull_test_fixture.h"
 #include "mongo/db/snapshot_window_options.h"
@@ -51,6 +52,7 @@ class SnapshotWindowTest : public ServiceContextDevnullTestFixture {
     void setUp() override {
         ServiceContextDevnullTestFixture::setUp();
         _opCtx = cc().makeOperationContext();
+        setTestCommandsEnabled(true);
     }
 
     void tearDown() override {

src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.cpp

@@ -684,6 +684,13 @@ WiredTigerKVEngine::WiredTigerKVEngine(const std::string& canonicalName,
         _checkpointThread->go();
     }
 
+    if (_ephemeral && !getTestCommandsEnabled()) {
+        // We do not maintain any snapshot history for the ephemeral storage engine in production
+        // because replication and sharded transactions do not currently run on the inMemory engine.
+        // It is live in testing, however.
+        snapshotWindowParams.targetSnapshotHistoryWindowInSeconds.store(0);
+    }
+
     _sizeStorerUri = _uri("sizeStorer");
     WiredTigerSession session(_conn);
     if (!_readOnly && repair && _hasUri(session.getSession(), _sizeStorerUri)) {
@@ -1675,6 +1682,11 @@ Timestamp WiredTigerKVEngine::_calculateHistoryLagFromStableTimestamp(Timestamp
     // The oldest_timestamp should lag behind the stable_timestamp by
     // 'targetSnapshotHistoryWindowInSeconds' seconds.
 
+    if (_ephemeral && !getTestCommandsEnabled()) {
+        // No history should be maintained for the inMemory engine because it is not used yet.
+        invariant(snapshotWindowParams.targetSnapshotHistoryWindowInSeconds.load() == 0);
+    }
+
     if (stableTimestamp.getSecs() <
         static_cast<unsigned>(snapshotWindowParams.targetSnapshotHistoryWindowInSeconds.load())) {
         // The history window is larger than the timestamp history thus far. We must wait for