Apply assert to ensure singletons are only created once.

// FREEBIE

src/Messages/Attachments/TSAttachmentPointer.h

@@ -17,11 +17,13 @@ typedef NS_ENUM(NSUInteger, TSAttachmentPointerState) {
  */
 @interface TSAttachmentPointer : TSAttachment
 
+- (instancetype)init NS_UNAVAILABLE;
+
 - (instancetype)initWithServerId:(UInt64)serverId
                              key:(NSData *)key
                           digest:(nullable NSData *)digest
                      contentType:(NSString *)contentType
-                           relay:(NSString *)relay NS_DESIGNATED_INITIALIZER;
+                           relay:(NSString *)relay;
 
 @property (nonatomic, readonly) NSString *relay;
 @property (atomic) TSAttachmentPointerState state;

src/Messages/TSBlockingManager.m

@@ -66,6 +66,8 @@ NSString *const kTSStorageManager_SyncedBlockedPhoneNumbersKey = @"kTSStorageMan
     _storageManager = storageManager;
     _messageSender = messageSender;
 
+    OWSSingletonAssert();
+
     return self;
 }
 

src/Messages/TSMessagesManager.m

@@ -106,6 +106,8 @@ NS_ASSUME_NONNULL_BEGIN
     _incomingMessageFinder = [[OWSIncomingMessageFinder alloc] initWithDatabase:storageManager.database];
     _blockingManager = [TSBlockingManager sharedManager];
 
+    OWSSingletonAssert();
+
     return self;
 }
 

src/Network/WebSockets/TSSocketManager.h

@@ -1,9 +1,5 @@
 //
-//  TSSocketManager.h
-//  TextSecureiOS
-//
-//  Created by Frederic Jacobs on 17/05/14.
-//  Copyright (c) 2014 Open Whisper Systems. All rights reserved.
+//  Copyright (c) 2017 Open Whisper Systems. All rights reserved.
 //
 
 #import <Foundation/Foundation.h>
@@ -23,6 +19,8 @@ extern NSString *const SocketConnectingNotification;
 
 @interface TSSocketManager : NSObject <SRWebSocketDelegate>
 
+- (instancetype)init NS_UNAVAILABLE;
+
 + (void)becomeActiveFromForeground;
 + (void)becomeActiveFromBackgroundExpectMessage:(BOOL)expected;
 

src/Network/WebSockets/TSSocketManager.m

@@ -61,6 +61,8 @@ NSString *const SocketConnectingNotification = @"SocketConnectingNotification";
 
     [self addObserver:self forKeyPath:@"status" options:0 context:kSocketStatusObservationContext];
 
+    OWSSingletonAssert();
+
     return self;
 }
 

src/Storage/TSStorageManager.h

@@ -16,6 +16,8 @@ extern NSString *const TSUIDatabaseConnectionDidUpdateNotification;
 
 @interface TSStorageManager : NSObject
 
+- (instancetype)init NS_UNAVAILABLE;
+
 + (instancetype)sharedManager;
 
 /**

src/Storage/TSStorageManager.m

@@ -129,6 +129,8 @@ static NSString *keychainDBPassAccount    = @"TSDatabasePass";
 
             [NSException raise:TSStorageManagerExceptionNameNoDatabase format:@"Failed to initialize database."];
         }
+
+        OWSSingletonAssert();
     }
 
     return self;

src/Util/Asserts.h

@@ -35,3 +35,37 @@ NSCAssert(0, @"Assertion failed: %s", CONVERT_EXPR_TO_STRING(X)); \
 #endif
 
 #endif
+
+#pragma mark - Singleton Asserts
+
+// The "singleton asserts" can be used to ensure
+// that we only create a singleton once.
+//
+// The simplest way to use them is the OWSSingletonAssert() macro.
+// It is intended to be used inside the singleton's initializer.
+//
+// If, however, a singleton has multiple possible initializers,
+// you need to:
+//
+// 1. Use OWSSingletonAssertFlag() outside the class definition.
+// 2. Use OWSSingletonAssertInit() in each initializer.
+#ifdef DEBUG
+
+#define OWSSingletonAssertFlag() static BOOL _isSingletonCreated = NO;
+
+#define OWSSingletonAssertInit()                                                                                       \
+    @synchronized([self class])                                                                                        \
+    {                                                                                                                  \
+        OWSAssert(!_isSingletonCreated);                                                                               \
+        _isSingletonCreated = YES;                                                                                     \
+    }
+
+#define OWSSingletonAssert() OWSSingletonAssertFlag() OWSSingletonAssertInit()
+
+#else
+
+#define OWSSingletonAssertFlag()
+#define OWSSingletonAssertInit()
+#define OWSSingletonAssert()
+
+#endif