Prevents simultaneous database access from multiple AltStores

AltStore now sends a “WillAccessDatabase” notification before loading the persistent store, which causes other AltStore instances in memory to exit (either immediately, or upon returning to foreground).

This prevents multiple AltStore instances from simultaneously accessing the same database, which could result in corrupted data (especially if they used different database model versions).

AltStoreCore/Model/DatabaseManager.swift

@@ -11,6 +11,15 @@ import CoreData
 import AltSign
 import Roxas
 
+extension CFNotificationName
+{
+    fileprivate static let willAccessDatabase = CFNotificationName("com.rileytestut.AltStore.WillAccessDatabase" as CFString)
+}
+
+private let ReceivedWillAccessDatabaseNotification: @convention(c) (CFNotificationCenter?, UnsafeMutableRawPointer?, CFNotificationName?, UnsafeRawPointer?, CFDictionary?) -> Void = { (center, observer, name, object, userInfo) in
+    DatabaseManager.shared.receivedWillAccessDatabaseNotification()
+}
+
 fileprivate class PersistentContainer: RSTPersistentContainer
 {
     override class func defaultDirectoryURL() -> URL
@@ -43,10 +52,15 @@ public class DatabaseManager
     private let coordinator = NSFileCoordinator()
     private let coordinatorQueue = OperationQueue()
     
+    private var ignoreWillAccessDatabaseNotification = false
+    
     private init()
     {
         self.persistentContainer = PersistentContainer(name: "AltStore", bundle: Bundle(for: DatabaseManager.self))
         self.persistentContainer.preferredMergePolicy = MergePolicy()
+        
+        let observer = Unmanaged.passUnretained(self).toOpaque()
+        CFNotificationCenterAddObserver(CFNotificationCenterGetDarwinNotifyCenter(), observer, ReceivedWillAccessDatabaseNotification, CFNotificationName.willAccessDatabase.rawValue, nil, .deliverImmediately)
     }
 }
 
@@ -73,6 +87,10 @@ public extension DatabaseManager
             
             guard !self.isStarted else { return finish(nil) }
             
+            // Quit any other running AltStore processes to prevent concurrent database access during and after migration.
+            self.ignoreWillAccessDatabaseNotification = true
+            CFNotificationCenterPostNotification(CFNotificationCenterGetDarwinNotifyCenter(), .willAccessDatabase, nil, nil, true)
+            
             self.migrateDatabaseToAppGroupIfNeeded { (result) in
                 switch result
                 {
@@ -398,4 +416,14 @@ private extension DatabaseManager
             }
         }
     }
+    
+    func receivedWillAccessDatabaseNotification()
+    {
+        defer { self.ignoreWillAccessDatabaseNotification = false }
+        
+        // Ignore notifications sent by the current process.
+        guard !self.ignoreWillAccessDatabaseNotification else { return }
+        
+        exit(104)
+    }
 }