Verifies source’s identifier doesn’t match existing sources when adding

AltStore/Managing Apps/AppManager.swift

@@ -365,8 +365,12 @@ extension AppManager
         let action = await UIAlertAction(title: NSLocalizedString("Add Source", comment: ""), style: .default)
         try await presentingViewController.presentConfirmationAlert(title: title, message: message, primaryAction: action)
 
-        // Wait for fetch to finish before saving context.
-        _ = try await fetchedSource
+        // Wait for fetch to finish before saving context to make
+        // sure there isn't already a source with this identifier.
+        let sourceExists = try await fetchedSource.isAdded
+        
+        // This is just a sanity check, so pass nil for previousSourceName to keep code simple.
+        guard !sourceExists else { throw SourceError.duplicate(source, previousSourceName: nil) }
         
         try await context.performAsync {
             try context.save()

AltStore/Operations/FetchSourceOperation.swift

@@ -23,6 +23,7 @@ extension SourceError
         case duplicateVersion
         
         case changedID
+        case duplicate
     }
     
     static func unsupported(_ source: Source) -> SourceError { SourceError(code: .unsupported, source: source) }
@@ -30,6 +31,7 @@ extension SourceError
     static func duplicateVersion(_ version: String, for app: StoreApp, source: Source) -> SourceError { SourceError(code: .duplicateVersion, source: source, app: app, version: version) }
     
     static func changedID(_ identifier: String, previousID: String, source: Source) -> SourceError { SourceError(code: .changedID, source: source, sourceID: identifier, previousSourceID: previousID) }
+    static func duplicate(_ source: Source, previousSourceName: String?) -> SourceError { SourceError(code: .duplicate, source: source, previousSourceName: previousSourceName) }
 }
 
 struct SourceError: ALTLocalizedError
@@ -43,6 +45,8 @@ struct SourceError: ALTLocalizedError
     var bundleID: String?
     var version: String?
     
+    @UserInfoValue var previousSourceName: String?
+    
     // Store in userInfo so they can be viewed from Error Log.
     @UserInfoValue var sourceID: String?
     @UserInfoValue var previousSourceID: String?
@@ -79,6 +83,13 @@ struct SourceError: ALTLocalizedError
         case .changedID:
             let failureReason = String(format: NSLocalizedString("The identifier of the source “%@” has changed.", comment: ""), self.$source.name)
             return failureReason
+            
+        case .duplicate:
+            let baseMessage = String(format: NSLocalizedString("A source with the identifier '%@' already exists", comment: ""), self.$source.identifier)
+            guard let previousSourceName else { return baseMessage + "." }
+            
+            let failureReason = baseMessage + " (“\(previousSourceName)”)."
+            return failureReason
         }
     }
     
@@ -86,6 +97,10 @@ struct SourceError: ALTLocalizedError
         switch self.code
         {
         case .changedID: return NSLocalizedString("A source cannot change its identifier once added. This source can no longer be updated.", comment: "")
+        case .duplicate:
+            let failureReason = NSLocalizedString("Please remove the existing source in order to add this one.", comment: "")
+            return failureReason
+            
         default: return nil
         }
     }

AltStore/Sources/SourcesViewController.swift

@@ -283,13 +283,31 @@ private extension SourcesViewController
         AppManager.shared.fetchSource(sourceURL: url, dependencies: dependencies) { (result) in
             do
             {
+                // Use @Managed before calling perform() to keep
+                // strong reference to source.managedObjectContext.
                 @Managed var source = try result.get()
                 
-                DispatchQueue.main.async {
-                    self.showSourceDetails(for: source)
-                }
+                let backgroundContext = DatabaseManager.shared.persistentContainer.newBackgroundContext()
+                backgroundContext.perform {
+                    do
+                    {
+                        let predicate = NSPredicate(format: "%K == %@", #keyPath(Source.identifier), $source.identifier)
+                        if let existingSource = Source.first(satisfying: predicate, in: backgroundContext)
+                        {
+                            throw SourceError.duplicate(source, previousSourceName: existingSource.name)
+                        }
                         
-                finish(.success(()))
+                        DispatchQueue.main.async {
+                            self.showSourceDetails(for: source)
+                        }
+                        
+                        finish(.success(()))
+                    }
+                    catch
+                    {
+                        finish(.failure(error))
+                    }
+                }
             }
             catch
             {