fix(IoT): Fixing a potential race condition in the timer ring queue

AWSIoT/Internal/MQTTSDK/AWSMQTTSession.m

@@ -19,6 +19,7 @@
 #import "AWSMQTTEncoder.h"
 #import "AWSMQttTxFlow.h"
 #import "AWSIoTMessage.h"
+#import "AWSMQTTTimerRing.h"
 #import "AWSIoTMessage+AWSMQTTMessage.h"
 
 @interface AWSMQTTSession () <AWSMQTTDecoderDelegate,AWSMQTTEncoderDelegate>  {
@@ -58,7 +59,7 @@ - (void)send:(AWSMQTTMessage*)msg;
 - (UInt16)nextMsgId;
 
 @property (strong,atomic) NSMutableArray* queue; //Queue to temporarily hold messages if encoder is busy sending another message
-@property (strong,atomic) NSMutableArray* timerRing; // circular array of 60. Each element is a set that contains the messages that need to be retried.
+@property (strong,atomic) AWSMQTTTimerRing* timerRing; // A collection of messages that need to be retried.
 @property (nonatomic, strong) dispatch_queue_t drainSenderSerialQueue;
 @property (nonatomic, strong) AWSMQTTEncoder* encoder; //Low level protocol handler that converts a message into out bound network data
 @property (nonatomic, strong) AWSMQTTDecoder* decoder; //Low level protocol handler that converts in bound network data into a Message
@@ -103,11 +104,7 @@ - (id)initWithClientId:(NSString*)theClientId
         txMsgId = 1;
         txFlows = [[NSMutableDictionary alloc] init];
         rxFlows = [[NSMutableDictionary alloc] init];
-        self.timerRing = [[NSMutableArray alloc] initWithCapacity:60];
-        int i;
-        for (i = 0; i < 60; i++) {
-            [self.timerRing addObject:[NSMutableSet new]];
-        }
+        self.timerRing = [[AWSMQTTTimerRing alloc] init];
         serialQueue = dispatch_queue_create("com.amazon.aws.iot.test-queue", DISPATCH_QUEUE_SERIAL);
         ticks = 0;
         status = AWSMQTTSessionStatusCreated;
@@ -233,7 +230,7 @@ - (UInt16)publishDataAtLeastOnce:(NSData*)data
     AWSMQttTxFlow *flow = [AWSMQttTxFlow flowWithMsg:msg
                                             deadline:deadline];
     [txFlows setObject:flow forKey:[NSNumber numberWithUnsignedInt:msgId]];
-    [[self.timerRing objectAtIndex:([flow deadline] % 60)] addObject:[NSNumber numberWithUnsignedInt:msgId]];
+    [self.timerRing addMsgId:[NSNumber numberWithUnsignedInt:msgId] atTick:[flow deadline]];
     AWSDDLogDebug(@"Published message %hu for QOS 1", msgId);
     [self send:msg];
     return msgId;
@@ -267,7 +264,7 @@ - (UInt16)publishDataExactlyOnce:(NSData*)data
     AWSMQttTxFlow *flow = [AWSMQttTxFlow flowWithMsg:msg
                                             deadline:(ticks + 60)];
     [txFlows setObject:flow forKey:[NSNumber numberWithUnsignedInt:msgId]];
-    [[self.timerRing objectAtIndex:([flow deadline] % 60)] addObject:[NSNumber numberWithUnsignedInt:msgId]];
+    [self.timerRing addMsgId:[NSNumber numberWithUnsignedInt:msgId] atTick:[flow deadline]];
     [self send:msg];
     return msgId;
 }
@@ -299,7 +296,7 @@ - (void)timerHandler:(NSTimer*)theTimer {
     dispatch_sync(serialQueue, ^{
         ticks++;
     });
-    NSEnumerator *e = [[[self.timerRing objectAtIndex:(ticks % 60)] allObjects] objectEnumerator];
+    NSEnumerator *e = [[self.timerRing allMsgIdsAtTick:ticks] objectEnumerator];
     id msgId;
 
     //Stay under the throttle here and move the work to the next tick if throttle is breached.
@@ -321,8 +318,8 @@ - (void)timerHandler:(NSTimer*)theTimer {
     while ((msgId = [e nextObject])) {
         AWSMQttTxFlow *flow = [txFlows objectForKey:msgId];
         [flow setDeadline:((ticks +1) %  60)];
-        [[self.timerRing objectAtIndex:((ticks + 1) % 60)] addObject:msgId];
-        [[self.timerRing objectAtIndex:(ticks % 60)] removeObject:msgId];
+        [self.timerRing addMsgId:msgId atTick:(ticks + 1)];
+        [self.timerRing removeMsgId:msgId atTick:ticks];
     }
 
     if (count > 0 ) {
@@ -567,8 +564,8 @@ - (void)handlePuback:(AWSMQTTMessage*)msg {
     if ([[flow msg] type] != AWSMQTTPublish || [[flow msg] qos] != 1) {
         return;
     }
-    
-    [[self.timerRing objectAtIndex:([flow deadline] % 60)] removeObject:msgId];
+
+    [self.timerRing removeMsgId:msgId atTick:[flow deadline]];
     [txFlows removeObjectForKey:msgId];
     AWSDDLogDebug(@"Removing msgID %@ from internal store for QOS1 guarantee", msgId);
     [self.delegate session:self newAckForMessageId:msgId.unsignedShortValue];
@@ -594,10 +591,10 @@ - (void)handlePubrec:(AWSMQTTMessage*)msg {
     }
     msg = [AWSMQTTMessage pubrelMessageWithMessageId:[msgId unsignedIntValue]];
     [flow setMsg:msg];
-    [[self.timerRing objectAtIndex:([flow deadline] % 60)] removeObject:msgId];
+    [self.timerRing removeMsgId:msgId atTick:[flow deadline]];
     [flow setDeadline:(ticks + 60)];
-    [[self.timerRing objectAtIndex:([flow deadline] % 60)] addObject:msgId];
-    
+    [self.timerRing addMsgId:msgId atTick:[flow deadline]];
+
     [self send:msg];
 }
 
@@ -638,8 +635,8 @@ - (void)handlePubcomp:(AWSMQTTMessage*)msg {
     if (flow == nil || [[flow msg] type] != AWSMQTTPubrel) {
         return;
     }
-    
-    [[self.timerRing objectAtIndex:([flow deadline] % 60)] removeObject:msgId];
+
+    [self.timerRing removeMsgId:msgId atTick:[flow deadline]];
     [txFlows removeObjectForKey:msgId];
 
     AWSDDLogDebug(@"Removing msgID %@ from internal store for QOS2 guarantee", msgId);

AWSIoT/Internal/MQTTSDK/AWSMQTTTimerRing.h

@@ -0,0 +1,30 @@
+//
+// Copyright 2010-2024 Amazon.com, Inc. or its affiliates. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License").
+// You may not use this file except in compliance with the License.
+// A copy of the License is located at
+//
+// http://aws.amazon.com/apache2.0
+//
+// or in the "license" file accompanying this file. This file is distributed
+// on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
+// express or implied. See the License for the specific language governing
+// permissions and limitations under the License.
+//
+
+#import <Foundation/Foundation.h>
+
+NS_ASSUME_NONNULL_BEGIN
+
+/// A circular collection containing the messages that need to be retried at a given clock tick.
+/// The maximum number of ticks is 60
+@interface AWSMQTTTimerRing: NSObject
+
+- (void)addMsgId:(NSNumber *)msgId atTick:(NSUInteger)tick;
+- (void)removeMsgId:(NSNumber *)msgId atTick:(NSUInteger)tick;
+- (NSArray<NSNumber *> *)allMsgIdsAtTick:(NSUInteger)tick;
+
+@end
+
+NS_ASSUME_NONNULL_END

AWSIoT/Internal/MQTTSDK/AWSMQTTTimerRing.m

@@ -0,0 +1,60 @@
+//
+// Copyright 2010-2024 Amazon.com, Inc. or its affiliates. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License").
+// You may not use this file except in compliance with the License.
+// A copy of the License is located at
+//
+// http://aws.amazon.com/apache2.0
+//
+// or in the "license" file accompanying this file. This file is distributed
+// on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
+// express or implied. See the License for the specific language governing
+// permissions and limitations under the License.
+//
+
+#import "AWSMQTTTimerRing.h"
+
+@interface AWSMQTTTimerRing()
+
+@property (nonatomic, strong) NSLock *lock;
+// Array of 60, with each index being a tick, and its value a set containing the messages that need to be retried.
+@property (strong,atomic) NSMutableArray<NSMutableSet *>* timerRing;
+
+@end
+
+@implementation AWSMQTTTimerRing
+
+- (instancetype)init
+{
+    self = [super init];
+    if (self) {
+        _lock = [[NSLock alloc] init];
+        _timerRing = [[NSMutableArray alloc] initWithCapacity:60];
+        int i;
+        for (i = 0; i < 60; i++) {
+            [_timerRing addObject:[NSMutableSet new]];
+        }
+    }
+    return self;
+}
+- (void)addMsgId:(NSNumber *)msgId atTick:(NSUInteger)tick {
+    [self.lock lock];
+    [[self.timerRing objectAtIndex:(tick % 60)] addObject:msgId];
+    [self.lock unlock];
+}
+
+- (void)removeMsgId:(NSNumber *)msgId atTick:(NSUInteger)tick {
+    [self.lock lock];
+    [[self.timerRing objectAtIndex:(tick % 60)] removeObject:msgId];
+    [self.lock unlock];
+}
+
+- (NSArray<NSNumber *> *)allMsgIdsAtTick:(NSUInteger)tick {
+    [self.lock lock];
+    NSArray<NSNumber *> *result = [[self.timerRing objectAtIndex:(tick % 60)] allObjects];
+    [self.lock unlock];
+    return result;
+}
+
+@end

AWSiOSSDKv2.xcodeproj/project.pbxproj

@@ -598,6 +598,8 @@
 		5C1590172755727C00F88085 /* AWSCore.framework in Frameworks */ = {isa = PBXBuildFile; fileRef = CE0D416D1C6A66E5006B91B5 /* AWSCore.framework */; };
 		5C1978DD2702364800F9C11E /* AWSLocationTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 5C1978DC2702364800F9C11E /* AWSLocationTests.swift */; };
 		5C71F33F295672B8001183A4 /* guten_tag.wav in Resources */ = {isa = PBXBuildFile; fileRef = 5C71F33E295672B8001183A4 /* guten_tag.wav */; };
+		685AA2112CDA7843008EFC7B /* AWSMQTTTimerRing.h in Headers */ = {isa = PBXBuildFile; fileRef = 685AA20F2CDA7843008EFC7B /* AWSMQTTTimerRing.h */; };
+		685AA2122CDA7843008EFC7B /* AWSMQTTTimerRing.m in Sources */ = {isa = PBXBuildFile; fileRef = 685AA2102CDA7843008EFC7B /* AWSMQTTTimerRing.m */; };
 		687952932B8FE2C5001E8990 /* AWSDDLog+Optional.swift in Sources */ = {isa = PBXBuildFile; fileRef = 687952922B8FE2C5001E8990 /* AWSDDLog+Optional.swift */; };
 		6883619E2B72D1C200D74FF4 /* AWSS3PreSignedURLBuilderUnitTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 6883619D2B72D1C200D74FF4 /* AWSS3PreSignedURLBuilderUnitTests.swift */; };
 		688361A12B73D25B00D74FF4 /* AWSIoTStreamThreadTests.m in Sources */ = {isa = PBXBuildFile; fileRef = 688361A02B73D25B00D74FF4 /* AWSIoTStreamThreadTests.m */; };
@@ -3215,6 +3217,8 @@
 		5C1978DB2702364800F9C11E /* AWSLocationTests-Bridging-Header.h */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.h; path = "AWSLocationTests-Bridging-Header.h"; sourceTree = "<group>"; };
 		5C1978DC2702364800F9C11E /* AWSLocationTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = AWSLocationTests.swift; sourceTree = "<group>"; };
 		5C71F33E295672B8001183A4 /* guten_tag.wav */ = {isa = PBXFileReference; lastKnownFileType = audio.wav; path = guten_tag.wav; sourceTree = "<group>"; };
+		685AA20F2CDA7843008EFC7B /* AWSMQTTTimerRing.h */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.h; path = AWSMQTTTimerRing.h; sourceTree = "<group>"; };
+		685AA2102CDA7843008EFC7B /* AWSMQTTTimerRing.m */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.objc; path = AWSMQTTTimerRing.m; sourceTree = "<group>"; };
 		687952922B8FE2C5001E8990 /* AWSDDLog+Optional.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = "AWSDDLog+Optional.swift"; sourceTree = "<group>"; };
 		6883619D2B72D1C200D74FF4 /* AWSS3PreSignedURLBuilderUnitTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = AWSS3PreSignedURLBuilderUnitTests.swift; sourceTree = "<group>"; };
 		688361A02B73D25B00D74FF4 /* AWSIoTStreamThreadTests.m */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.objc; path = AWSIoTStreamThreadTests.m; sourceTree = "<group>"; };
@@ -7244,6 +7248,8 @@
 				CE9DE6461C6A78D70060793F /* AWSMQTTSession.m */,
 				CE9DE6471C6A78D70060793F /* AWSMQttTxFlow.h */,
 				CE9DE6481C6A78D70060793F /* AWSMQttTxFlow.m */,
+				685AA20F2CDA7843008EFC7B /* AWSMQTTTimerRing.h */,
+				685AA2102CDA7843008EFC7B /* AWSMQTTTimerRing.m */,
 			);
 			path = MQTTSDK;
 			sourceTree = "<group>";
@@ -8455,6 +8461,7 @@
 			files = (
 				CE9DE6521C6A78D70060793F /* AWSIoTDataResources.h in Headers */,
 				CE9DE65A1C6A78D70060793F /* AWSIoTResources.h in Headers */,
+				685AA2112CDA7843008EFC7B /* AWSMQTTTimerRing.h in Headers */,
 				68EE1A6C2B713D8100B7CF41 /* AWSIoTStreamThread.h in Headers */,
 				CE9DE6231C6A78AF0060793F /* AWSIoT.h in Headers */,
 				CE9DE6561C6A78D70060793F /* AWSIoTManager.h in Headers */,
@@ -13398,6 +13405,7 @@
 				CE9DE66D1C6A78D70060793F /* AWSMQTTSession.m in Sources */,
 				CE9DE6551C6A78D70060793F /* AWSIoTDataService.m in Sources */,
 				0342776A269D185200379263 /* AWSIoTMessage+AWSMQTTMessage.m in Sources */,
+				685AA2122CDA7843008EFC7B /* AWSMQTTTimerRing.m in Sources */,
 				CE9DE66B1C6A78D70060793F /* AWSMQTTMessage.m in Sources */,
 				CE9DE65D1C6A78D70060793F /* AWSIoTService.m in Sources */,
 				68DD11872C5AF52B004E1C37 /* AWSIoTAtomicDictionary.m in Sources */,

CHANGELOG.md

@@ -2,7 +2,10 @@
 
 ## Unreleased
 
--Features for next release
+### Bug Fixes
+
+- **AWSIoT** 
+  - Fixing a potential race condition in the timer ring queue (#5461)
 
 ## 2.38.0