Add own-instance API request retrying

Tusker/Controllers/MastodonController.swift

@@ -46,6 +46,9 @@ class MastodonController: ObservableObject {
     @Published private(set) var instance: Instance!
     private(set) var customEmojis: [Emoji]?
     
+    private var pendingOwnInstanceRequestCallbacks = [(Instance) -> Void]()
+    private var ownInstanceRequest: URLSessionTask?
+    
     var loggedIn: Bool {
         accountInfo != nil
     }
@@ -115,17 +118,56 @@ class MastodonController: ObservableObject {
         }
     }
     
-    // todo: this should dedup requests
     func getOwnInstance(completion: ((Instance) -> Void)? = nil) {
+        getOwnInstanceInternal(retryAttempt: 0, completion: completion)
+    }
+    
+    private func getOwnInstanceInternal(retryAttempt: Int, completion: ((Instance) -> Void)?) {
+        // this is main thread only to prevent concurrent access to ownInstanceRequest and pendingOwnInstanceRequestCallbacks
+        assert(Thread.isMainThread)
+        
         if let instance = self.instance {
             completion?(instance)
         } else {
-            let request = Client.getInstance()
-            run(request) { (response) in
-                guard case let .success(instance, _) = response else { fatalError() }
-                DispatchQueue.main.async {
-                    self.instance = instance
-                    completion?(instance)
+            if let completion = completion {
+                pendingOwnInstanceRequestCallbacks.append(completion)
+            }
+          
+            if ownInstanceRequest == nil {
+                let request = Client.getInstance()
+                ownInstanceRequest = run(request) { (response) in
+                    switch response {
+                    case .failure(_):
+                        let delay: DispatchTimeInterval
+                        switch retryAttempt {
+                        case 0:
+                            delay = .seconds(1)
+                        case 1:
+                            delay = .seconds(5)
+                        case 2:
+                            delay = .seconds(30)
+                        case 3:
+                            delay = .seconds(60)
+                        default:
+                            // if we've failed four times, just give up :/
+                            return
+                        }
+                        DispatchQueue.main.asyncAfter(deadline: .now() + delay) {
+                            // completion is nil because in this invocation of getOwnInstanceInternal we've already added it to the pending callbacks array
+                            self.getOwnInstanceInternal(retryAttempt: retryAttempt + 1, completion: nil)
+                        }
+
+                    case let .success(instance, _):
+                        DispatchQueue.main.async {
+                            self.ownInstanceRequest = nil
+                            self.instance = instance
+                            
+                            for completion in self.pendingOwnInstanceRequestCallbacks {
+                                completion(instance)
+                            }
+                            self.pendingOwnInstanceRequestCallbacks = []
+                        }
+                    }
                 }
             }
         }