[RC] Fix experiment state races during database loading

FirebaseRemoteConfig/CHANGELOG.md

@@ -1,3 +1,9 @@
+# Unreleased
+- [fixed] Fixed data races that could mix stale and current Remote Config
+  experiment state during database loading and activation. (#16303)
+- [fixed] Made experiment payload replacement atomic and delayed A/B Testing
+  updates until the queued active-experiment database replacement completes.
+
 # 12.17.0
 - [fixed] Fixed a memory leak in Remote Config where `activateWithCompletion:`
   retained the `FIRRemoteConfig` instance indefinitely (#16413).

FirebaseRemoteConfig/Sources/RCNConfigDBManager.h

@@ -95,6 +95,15 @@ typedef void (^RCNDBLoadCompletion)(BOOL success,
                                value:(NSData *)value
                    completionHandler:(RCNDBCompletion)handler;
 
+/// Atomically replaces all experiment records for `key`.
+/// @param key The experiment data key, as defined in `RCNConfigDefines.h`.
+/// @param values The serialized experiment values to persist.
+/// @param handler The callback, invoked on the main queue after the replacement finishes. It is an
+/// ordering barrier for the queued operation, not a durability guarantee.
+- (void)replaceExperimentTableWithKey:(NSString *)key
+                               values:(NSArray<NSData *> *)values
+                    completionHandler:(RCNDBCompletion)handler;
+
 - (void)updateMetadataWithOption:(RCNUpdateOption)option
                        namespace:(NSString *)namespace
                           values:(NSArray *)values

FirebaseRemoteConfig/Sources/RCNConfigDBManager.m

@@ -41,6 +41,15 @@
 /// Introduce a dedicated serial queue for gIsNewDatabase access.
 static dispatch_queue_t gIsNewDatabaseQueue;
 
+static dispatch_queue_t RCNIsNewDatabaseQueue(void) {
+  static dispatch_once_t onceToken;
+  dispatch_once(&onceToken, ^{
+    gIsNewDatabaseQueue = dispatch_queue_create("com.google.FirebaseRemoteConfig.gIsNewDatabase",
+                                                DISPATCH_QUEUE_SERIAL);
+  });
+  return gIsNewDatabaseQueue;
+}
+
 /// Remote Config database path for deprecated V0 version.
 static NSString *RemoteConfigPathForOldDatabaseV0(void) {
   NSArray *dirPaths =
@@ -85,7 +94,7 @@ static BOOL RemoteConfigCreateFilePathIfNotExist(NSString *filePath) {
   }
   NSFileManager *fileManager = [NSFileManager defaultManager];
   if (![fileManager fileExistsAtPath:filePath]) {
-    dispatch_sync(gIsNewDatabaseQueue, ^{
+    dispatch_sync(RCNIsNewDatabaseQueue(), ^{
       gIsNewDatabase = YES;
     });
     NSError *error;
@@ -124,8 +133,6 @@ + (instancetype)sharedInstance {
   static dispatch_once_t onceToken;
   static RCNConfigDBManager *sharedInstance;
   dispatch_once(&onceToken, ^{
-    gIsNewDatabaseQueue = dispatch_queue_create("com.google.FirebaseRemoteConfig.gIsNewDatabase",
-                                                DISPATCH_QUEUE_SERIAL);
     sharedInstance = [[RCNConfigDBManager alloc] init];
   });
   return sharedInstance;
@@ -139,6 +146,7 @@ + (NSString *)remoteConfigPathForDatabase {
 - (instancetype)init {
   self = [super init];
   if (self) {
+    (void)RCNIsNewDatabaseQueue();
     _databaseOperationQueue =
         dispatch_queue_create("com.google.GoogleConfigService.database", DISPATCH_QUEUE_SERIAL);
     [self createOrOpenDatabase];
@@ -485,6 +493,41 @@ - (void)insertExperimentTableWithKey:(NSString *)key
   });
 }
 
+- (void)replaceExperimentTableWithKey:(NSString *)key
+                               values:(NSArray<NSData *> *)values
+                    completionHandler:(RCNDBCompletion)handler {
+  NSString *keyCopy = [key copy];
+  NSArray<NSData *> *valuesCopy = [values copy];
+  dispatch_async(_databaseOperationQueue, ^{
+    BOOL transactionStarted = [self executeQuery:"BEGIN IMMEDIATE TRANSACTION"];
+    BOOL success = transactionStarted;
+    if (success) {
+      const char *deleteSQL = "DELETE FROM " RCNTableNameExperiment " WHERE key = ?";
+      success = [self executeQuery:deleteSQL withParams:@[ keyCopy ]];
+    }
+
+    for (NSData *value in valuesCopy) {
+      if (!success) {
+        break;
+      }
+      success = [self insertExperimentTableWithKey:keyCopy value:value];
+    }
+
+    if (success) {
+      success = [self executeQuery:"COMMIT TRANSACTION"];
+    }
+    if (!success && transactionStarted) {
+      [self executeQuery:"ROLLBACK TRANSACTION"];
+    }
+
+    if (handler) {
+      dispatch_async(dispatch_get_main_queue(), ^{
+        handler(success, nil);
+      });
+    }
+  });
+}
+
 - (BOOL)insertExperimentTableWithKey:(NSString *)key value:(NSData *)dataValue {
   if ([key isEqualToString:@RCNExperimentTableKeyMetadata]) {
     return [self updateExperimentMetadata:dataValue];
@@ -1227,7 +1270,7 @@ - (BOOL)logErrorWithSQL:(const char *)SQL
 
 - (BOOL)isNewDatabase {
   __block BOOL isNew;
-  dispatch_sync(gIsNewDatabaseQueue, ^{
+  dispatch_sync(RCNIsNewDatabaseQueue(), ^{
     isNew = gIsNewDatabase;
   });
   return isNew;