SERVER-51918 create feature flag for lock free reads

4 files changed, 12 insertions, 6 deletions

diff --git a/src/mongo/db/SConscript b/src/mongo/db/SConscript
index a3265eacdeb..e7c7382705a 100644
--- a/src/mongo/db/SConscript
+++ b/src/mongo/db/SConscript
@@ -385,6 +385,7 @@ env.Library(
         '$BUILD_DIR/mongo/db/repl/repl_settings',
         '$BUILD_DIR/mongo/db/repl/replica_set_messages',
         '$BUILD_DIR/mongo/db/storage/storage_options',
+        '$BUILD_DIR/mongo/idl/feature_flag',
         '$BUILD_DIR/mongo/util/options_parser/options_parser',
         'global_settings',
         'server_options_base',
diff --git a/src/mongo/db/mongod_options.cpp b/src/mongo/db/mongod_options.cpp
index 5b60719af64..bc436427d07 100644
--- a/src/mongo/db/mongod_options.cpp
+++ b/src/mongo/db/mongod_options.cpp
@@ -54,6 +54,7 @@
 #include "mongo/db/server_options_base.h"
 #include "mongo/db/server_options_nongeneral_gen.h"
 #include "mongo/db/server_options_server_helpers.h"
+#include "mongo/db/storage/storage_parameters_gen.h"
 #include "mongo/logv2/log.h"
 #include "mongo/logv2/log_domain_global.h"
 #include "mongo/logv2/log_manager.h"
@@ -481,6 +482,11 @@ Status storeMongodOptions(const moe::Environment& params) {
         storageGlobalParams.noTableScan.store(params["notablescan"].as<bool>());
     }
 
+    // Initialize lock-free reads support from feature flag. This may be adjusted later based on
+    // replica set config.
+    storageGlobalParams.disableLockFreeReads =
+        !feature_flags::gLockFreeReads.isEnabledAndIgnoreFCV();
+
     repl::ReplSettings replSettings;
     if (params.count("replication.replSet")) {
         /* seed list of hosts for the repl set */
diff --git a/src/mongo/db/storage/storage_options.h b/src/mongo/db/storage/storage_options.h
index e7fe5331f96..8de0425455b 100644
--- a/src/mongo/db/storage/storage_options.h
+++ b/src/mongo/db/storage/storage_options.h
@@ -122,7 +122,7 @@ struct StorageGlobalParams {
     // Disables lock-free reads, adjustable via setParameter. Can be disabled by certain user
     // settings with which lock-free reads are incompatible: standalone mode; and
     // enableMajorityReadConcern=false.
-    bool disableLockFreeReads;
+    bool disableLockFreeReads = true;
 
     // Delay in seconds between triggering the next checkpoint after the completion of the previous
     // one. A value of 0 indicates that checkpointing will be skipped.
diff --git a/src/mongo/db/storage/storage_parameters.idl b/src/mongo/db/storage/storage_parameters.idl
index 68db905d4ae..9a35d61be07 100644
--- a/src/mongo/db/storage/storage_parameters.idl
+++ b/src/mongo/db/storage/storage_parameters.idl
@@ -78,13 +78,12 @@ server_parameters:
         default: 2048
         validator:
             gte: 1
-    disableLockFreeReads:
-        description: "Disables the lock-free reads feature."
-        set_at: [ startup ]
-        cpp_varname: 'storageGlobalParams.disableLockFreeReads'
-        default: true
 
 feature_flags:
+    featureFlagLockFreeReads:
+        description: "Support for lock-free reads."
+        cpp_varname: feature_flags::gLockFreeReads
+        default: false
     featureFlagTimeseriesCollection:
         description: "When enabled, support for time-series collections"
         cpp_varname: feature_flags::gTimeseriesCollection