Refresh OAuth token using Reactivecocoa - objective-c

Question

So what I'm trying to do is this.
Make a HTTP request including a header which specifies the authorization token.
If the response is a 401 Unauthorized I wish to use my refresh token to obtain a new access token and a new refresh token.
When I have the new token pair I wish to retry the failed request using the new tokens.
I got this working by looking at this article. They solve it like this.
- (RACSignal *)doRequestAndRefreshTokenIfNecessary:(RACSignal *)requestSignal {
return [requestSignal catch:^(NSError *error) {
// Catch the error, refresh the token, and then do the request again.
BOOL hasRefreshToken = [UserManager sharedInstance].refreshToken != nil;
BOOL httpCode401AccessDenied = error.code == -1011;
if (httpCode401AccessDenied && hasRefreshToken) {
return [[[self refreshToken] ignoreValues] concat:requestSignal];
}
return requestSignal;
}];
}
But here's the catch. If I fire multiple requests with an expired access token I sometimes end up with a race condition where there are multiple refreshes.
So I thought that I could save the refresh signal as an instance variable and just "chain" all subsequent failed requests to that. But the result is that the refresh signal is done multiple times.
- (RACSignal *) refreshToken {
NSDictionary *params = #{
#"client_id": [Config clientId],
#"client_secret": [Config clientSecret],
#"grant_type": #"refresh",
#"refresh_token": [[Tokens sharedTokens] refreshToken]
};
if (self.activeRefreshSignal == nil) {
#weakify(self)
self.activeRefreshSignal = [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
#strongify(self)
[[RestAPI sharedClient] POST:#"/oauth/token"
parameters:params
success:^(NSURLSessionDataTask *operation, id responseObject) {
[self extractTokensFromResponseObject:responseObject];
[subscriber sendNext:nil];
[subscriber sendCompleted];
self.activeRefreshSignal = nil;
}
failure:^(NSURLSessionDataTask *operation, NSError *error) {
[[Tokens sharedTokens] clearTokens];
[subscriber sendError:error];
self.activeRefreshSignal = nil;
}];
return nil;
}];
}
return self.activeRefreshSignal;
}
Any ideas how to avoid the race condition and only do one refresh?