Make "database view registration complete" check thread-safe.

// FREEBIE

SignalServiceKit/src/Storage/TSDatabaseView.h

@@ -23,6 +23,7 @@ extern NSString *const TSSecondaryDevicesDatabaseViewExtensionName;
 
 - (instancetype)init NS_UNAVAILABLE;
 
+// This method can be called from any thread.
 + (BOOL)hasPendingViewRegistrations;
 
 // This method must be called _AFTER_ registerThreadInteractionsDatabaseView.

SignalServiceKit/src/Storage/TSDatabaseView.m

@@ -64,13 +64,29 @@ NSString *const TSSecondaryDevicesDatabaseViewExtensionName = @"TSSecondaryDevic
     return self;
 }
 
-+ (BOOL)hasPendingViewRegistrations
+- (BOOL)hasPendingViewRegistrations
 {
-    OWSAssert([NSThread isMainThread]);
+    @synchronized(self)
+    {
+        return !self.areAllAsyncRegistrationsComplete;
+    }
+}
 
++ (BOOL)hasPendingViewRegistrations
+{
     return ![TSDatabaseView sharedInstance].areAllAsyncRegistrationsComplete;
 }
 
+- (void)setAreAllAsyncRegistrationsComplete
+{
+    @synchronized(self)
+    {
+        OWSAssert(!self.areAllAsyncRegistrationsComplete);
+
+        self.areAllAsyncRegistrationsComplete = YES;
+    }
+}
+
 + (void)registerMessageDatabaseViewWithName:(NSString *)viewName
                                viewGrouping:(YapDatabaseViewGrouping *)viewGrouping
                                     version:(NSString *)version
@@ -423,8 +439,9 @@ NSString *const TSSecondaryDevicesDatabaseViewExtensionName = @"TSSecondaryDevic
     // All async registrations are complete when writes are unblocked.
     [[TSStorageManager sharedManager].newDatabaseConnection
         asyncReadWriteWithBlock:^(YapDatabaseReadWriteTransaction *_Nonnull transaction) {
+            [TSDatabaseView.sharedInstance setAreAllAsyncRegistrationsComplete];
+
             dispatch_async(dispatch_get_main_queue(), ^{
-                TSDatabaseView.sharedInstance.areAllAsyncRegistrationsComplete = YES;
                 [[NSNotificationCenter defaultCenter]
                     postNotificationName:kNSNotificationName_DatabaseViewRegistrationComplete
                                   object:nil