ios: put peripheral context in a struct for clarity

Author: benma

frontends/ios/BitBoxApp/BitBoxApp/Bluetooth.swift

@@ -51,6 +51,12 @@ var pairedDeviceIdentifiers: Set<String> {
     }
 }
 
+class BLEConnectionContext {
+    let semaphore = DispatchSemaphore(value: 0)
+    var readBuffer = Data()
+    var readBufferLock = NSLock()
+}
+
 class BluetoothManager: NSObject, ObservableObject, CBCentralManagerDelegate, CBPeripheralDelegate {
     private var state: State = State(
         bluetoothAvailable: false,
@@ -70,9 +76,11 @@ class BluetoothManager: NSObject, ObservableObject, CBCentralManagerDelegate, CB
     // This is for failed connections to not enter an infinite connect loop.
     private var dontAutoConnectSet: Set<UUID> = []
 
-    private var readBuffer = Data()
-    private let readBufferLock = NSLock()  // Ensure thread-safe buffer access
-    private let semaphore = DispatchSemaphore(value: 0)
+    private var currentContext: BLEConnectionContext?
+    // Locks access to the `currentContext` var only, not to its contents. This is important, as
+    // one can't keep the context locked while waiting for the semaphore, which would lead to a
+    // deadlock.
+    private let currentContextLock = NSLock()
 
     override init() {
         super.init()
@@ -93,6 +101,9 @@ class BluetoothManager: NSObject, ObservableObject, CBCentralManagerDelegate, CB
         state.discoveredPeripherals[peripheralID] = metadata
         state.scanning = false
         updateBackendState()
+        currentContextLock.lock()
+        currentContext = BLEConnectionContext()
+        currentContextLock.unlock()
         centralManager.connect(metadata.peripheral, options: nil)
     }
 
@@ -246,17 +257,24 @@ class BluetoothManager: NSObject, ObservableObject, CBCentralManagerDelegate, CB
             return
         }
 
+        currentContextLock.lock()
+        guard let ctx = currentContext else {
+            currentContextLock.unlock()
+            return
+        }
+        currentContextLock.unlock()
+
         if characteristic == pReader, let data = characteristic.value {
             if data.count != 64 {
                 print("BLE: ERROR, expected 64 bytes")
             }
             print("BLE: received data: \(data.hexEncodedString())")
-            readBufferLock.lock()
-            readBuffer.append(data)
-            readBufferLock.unlock()
+            ctx.readBufferLock.lock()
+            ctx.readBuffer.append(data)
+            ctx.readBufferLock.unlock()
 
             // Signal the semaphore to unblock `readBlocking`
-            semaphore.signal()
+            ctx.semaphore.signal()
         }
         if characteristic == pProduct {
             print("BLE: product changed: \(String(describing: parseProduct()))")
@@ -276,12 +294,15 @@ class BluetoothManager: NSObject, ObservableObject, CBCentralManagerDelegate, CB
     }
 
     func handleDisconnect() {
+        currentContextLock.lock()
+        currentContext = nil
+        currentContextLock.unlock()
         connectedPeripheral = nil
         pReader = nil
         pWriter = nil
         pProduct = nil
         state.discoveredPeripherals.removeAll()
-       isPaired = false
+        isPaired = false
         updateBackendState()
 
         // Have the backend scan right away, which will make it detect that we disconnected.
@@ -303,16 +324,23 @@ class BluetoothManager: NSObject, ObservableObject, CBCentralManagerDelegate, CB
         }
         print("BLE: wants to read \(length)")
 
+        currentContextLock.lock()
+        guard let ctx = currentContext else {
+            currentContextLock.unlock()
+            return nil
+        }
+        currentContextLock.unlock()
+
         var data = Data()
 
         // Loop until we've read the required amount of data
         while data.count < length {
             // Block until BLE reader callback notifies us
-            semaphore.wait()
-            readBufferLock.lock()
-            data.append(readBuffer.prefix(64))
-            readBuffer = readBuffer.advanced(by: 64)
-            readBufferLock.unlock()
+            ctx.semaphore.wait()
+            ctx.readBufferLock.lock()
+            data.append(ctx.readBuffer.prefix(64))
+            ctx.readBuffer = ctx.readBuffer.advanced(by: 64)
+            ctx.readBufferLock.unlock()
         }
         print("BLE: got \(data.count)")