SERVER-51918 create feature flag for lock free reads

etc/evergreen.yml

@@ -11053,7 +11053,7 @@ buildvariants:
   - rhel62-small
   expansions: &enterprise-rhel-62-64-bit-lock-free-reads-expansions
     test_flags: >-
-      --mongodSetParameters="{disableLockFreeReads: false}" --excludeWithAnyTags=incompatible_with_lockfreereads
+      --mongodSetParameters="{featureFlagLockFreeReads: true}" --excludeWithAnyTags=incompatible_with_lockfreereads
     compile_flags: >-
       -j$(grep -c ^processor /proc/cpuinfo)
       --ssl

jstests/noPassthrough/disable_lock_free_reads_server_parameter.js

@@ -1,9 +1,9 @@
 /**
- * Tests the 'disableLockFreeReads' startup setParameter.
+ * Tests the 'featureFlagLockFreeReads' startup setParameter.
  *
- * User set disableLockFreeReads will be overridden to true (disabled) if:
+ * User set featureFlagLockFreeReads will be overridden to false (disabled) if:
  *   - with enableMajorityReadConcern=false
- * Otherwise, the default for disableLockFreeReads is true.
+ * Otherwise, the default for featureFlagLockFreeReads is false.
  *
  * This test is not compatible with the special Lock Free Reads build variant because
  * disableLockFreeReads is overridden there.
@@ -22,48 +22,52 @@
 
 const replSetName = 'disable_lock_free_reads_server_parameter';
 
-jsTest.log("Starting server with disableLockFreeReads=false in standalone mode: this should turn " +
-           "on lock-free reads.");
+jsTest.log(
+    "Starting server with featureFlagLockFreeReads=true in standalone mode: this should turn " +
+    "on lock-free reads.");
 
-let conn = MongoRunner.runMongod({setParameter: "disableLockFreeReads=false"});
+let conn = MongoRunner.runMongod({setParameter: "featureFlagLockFreeReads=true"});
 assert(conn);
 checkLog.containsJson(conn, 4788403);  // Logging that lock-free reads is enabled.
 MongoRunner.stopMongod(conn);
 
-jsTest.log("Starting server with disableLockFreeReads=true in standalone mode: this is the " +
+jsTest.log("Starting server with featureFlagLockFreeReads=false in standalone mode: this is the " +
            "default and nothing should happen.");
 
-conn = MongoRunner.runMongod({setParameter: "disableLockFreeReads=true"});
+conn = MongoRunner.runMongod({setParameter: "featureFlagLockFreeReads=false"});
 assert(conn);
 assert(!checkLog.checkContainsOnce(conn, "disabling lock-free reads"));
 checkLog.containsJson(conn, 4788402);  // Logging that lock-free reads is disabled.
 MongoRunner.stopMongod(conn);
 
-jsTest.log("Starting server with disableLockFreeReads=false and enableMajorityReadConcern=false: " +
-           "this should override the setting to true.");
+jsTest.log(
+    "Starting server with featureFlagLockFreeReads=true and enableMajorityReadConcern=false: " +
+    "this should override the setting to true.");
 
 conn = MongoRunner.runMongod({
     replSet: replSetName,
     enableMajorityReadConcern: false,
-    setParameter: "disableLockFreeReads=false"
+    setParameter: "featureFlagLockFreeReads=true"
 });
 assert(conn);
 checkLog.containsJson(conn, 4788401);  // Logging eMRCf disables lock-free reads.
 checkLog.containsJson(conn, 4788402);  // Logging that lock-free reads is disabled.
 MongoRunner.stopMongod(conn);
 
-jsTest.log("Starting server as a replica set member with disableLockFreeReads=false: this should " +
-           "turn on lock-free reads.");
+jsTest.log("Starting server as a replica set member with featureFlagLockFreeReads=true: this " +
+           "should turn on lock-free reads.");
 
-conn = MongoRunner.runMongod({replSet: replSetName, setParameter: "disableLockFreeReads=false"});
+conn = MongoRunner.runMongod({replSet: replSetName, setParameter: "featureFlagLockFreeReads=true"});
 assert(conn);
 checkLog.containsJson(conn, 4788403);  // Logging that lock-free reads is enabled.
 MongoRunner.stopMongod(conn);
 
-jsTest.log("Starting server as a replica set member with disableLockFreeReads=true: this is the " +
-           "default and nothing should happen.");
+jsTest.log(
+    "Starting server as a replica set member with featureFlagLockFreeReads=false: this is the " +
+    "default and nothing should happen.");
 
-conn = MongoRunner.runMongod({replSet: replSetName, setParameter: "disableLockFreeReads=true"});
+conn =
+    MongoRunner.runMongod({replSet: replSetName, setParameter: "featureFlagLockFreeReads=false"});
 assert(conn);
 assert(!checkLog.checkContainsOnce(conn, "disabling lock-free reads"));
 checkLog.containsJson(conn, 4788402);  // Logging that lock-free reads is disabled.

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',

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 */

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.

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