ios: unblock readBlocking when the peripheral disconnects

The readBlocking loop would hang forever waiting for a semaphore,
which is never signalled. We unblock it by signalling it upon
disconnect.

The current connection context now also has an identifier. This
mitigates race conditions where e.g. a new connection is made before
readBlocking resumes executing (to exit the loop).

Author: benma

frontends/ios/BitBoxApp/BitBoxApp/Bluetooth.swift

@@ -52,6 +52,7 @@ var pairedDeviceIdentifiers: Set<String> {
 }
 
 class BLEConnectionContext {
+    let identifier = UUID()
     let semaphore = DispatchSemaphore(value: 0)
     var readBuffer = Data()
     var readBufferLock = NSLock()
@@ -294,9 +295,6 @@ class BluetoothManager: NSObject, ObservableObject, CBCentralManagerDelegate, CB
     }
 
     func handleDisconnect() {
-        currentContextLock.lock()
-        currentContext = nil
-        currentContextLock.unlock()
         connectedPeripheral = nil
         pReader = nil
         pWriter = nil
@@ -309,6 +307,12 @@ class BluetoothManager: NSObject, ObservableObject, CBCentralManagerDelegate, CB
         // Otherwise there would be up to a second of delay (the backend device manager scan interval).
         MobileserverUsbUpdate()
 
+        // Unblock a pending readBlocking() call if there is one.
+        currentContextLock.lock()
+        currentContext?.semaphore.signal()
+        currentContext = nil
+        currentContextLock.unlock()
+
         restartScan()
     }
 
@@ -318,25 +322,42 @@ class BluetoothManager: NSObject, ObservableObject, CBCentralManagerDelegate, CB
         handleDisconnect()
     }
 
-    func readBlocking(length: Int) -> Data? {
+    struct ReadError: Error {
+        let message: String
+    }
+    
+    func readBlocking(length: Int) throws -> Data {
         if !isConnected() {
-            return nil
+            throw ReadError(message: "not connected")
         }
         print("BLE: wants to read \(length)")
 
         currentContextLock.lock()
         guard let ctx = currentContext else {
             currentContextLock.unlock()
-            return nil
+            throw ReadError(message: "no connection context")
         }
         currentContextLock.unlock()
+        
+        let currentID = ctx.identifier;
 
         var data = Data()
 
         // Loop until we've read the required amount of data
         while data.count < length {
-            // Block until BLE reader callback notifies us
+            // Block until BLE reader callback notifies us or the peripheral is disconnected.
             ctx.semaphore.wait()
+            
+            if !isConnected() {
+                throw ReadError(message: "the peripheral has disconnected while reading")
+            }
+            currentContextLock.lock()
+            let exit = currentContext?.identifier != currentID
+            currentContextLock.unlock()
+            if exit {
+                throw ReadError(message: "the peripheral has disconnected while reading")
+            }
+            
             ctx.readBufferLock.lock()
             data.append(ctx.readBuffer.prefix(64))
             ctx.readBuffer = ctx.readBuffer.advanced(by: 64)
@@ -485,7 +506,7 @@ class BluetoothReadWriteCloser: NSObject, MobileserverGoReadWriteCloserInterface
     }
 
     func read(_ n: Int) throws -> Data {
-        return bluetoothManager.readBlocking(length: n)!
+        try bluetoothManager.readBlocking(length: n)
     }
 
     func write(_ data: Data?, n: UnsafeMutablePointer<Int>?) throws {