Validate buttonless DFU advertising name length

Library/Classes/Implementation/DFUServiceDelegate.swift

@@ -220,6 +220,8 @@ internal enum DFURemoteError : Int {
     /// Error raised when the CRC reported by the remote device does not match.
     /// Service has done 3 attempts to send the data.
     case crcError                             = 309
+    /// The requested advertising name is too long for the current ATT MTU.
+    case invalidAdvertisementName             = 310
     /// The service went into an invalid state. The service will try to close
     /// without crashing. Recovery to a know state is not possible.
     case invalidInternalState                 = 500

Library/Classes/Implementation/DFUServiceInitiator.swift

@@ -281,8 +281,14 @@ import CoreBluetooth
      If ``alternativeAdvertisingNameEnabled`` is `true` then this specifies the
      alternative name to use. If `nil` (default) then a random name is generated.
 
-     The maximum length of the alternative advertising name is 20 bytes.
-     Longer name will be truncated. UTF-8 characters can be cut in the middle.
+     The maximum length of the alternative advertising name depends on the
+     current ATT MTU, and can be up to 20 bytes. To work with the default
+     ATT MTU, keep the name to 18 UTF-8 bytes or fewer. Longer names may
+     work after a larger ATT MTU has been negotiated.
+
+     The library validates the name before writing it to the device and fails
+     with ``DFUError/invalidAdvertisementName`` if it is too long for the
+     current connection.
      */
     @objc public var alternativeAdvertisingName: String? = nil
 

Library/Classes/Implementation/SecureDFU/Characteristics/ButtonlessDFU.swift

@@ -99,7 +99,10 @@ extension ButtonlessDFUResultCode : CustomStringConvertible {
 internal enum ButtonlessDFURequest {
     case enterBootloader
     case set(name: String)
-
+
+    private static let setNameRequestOverhead = 2
+    private static let maximumAdvertisingNameLength = 20
+
     var data: Data {
         switch self {
         case .enterBootloader:
@@ -111,6 +114,28 @@ internal enum ButtonlessDFURequest {
             return data
         }
     }
+
+    func maximumPayloadLength(for peripheral: CBPeripheral) -> Int {
+        let writePayloadLength = peripheral.maximumWriteValueLength(for: .withResponse)
+        switch self {
+        case .enterBootloader:
+            return writePayloadLength
+        case .set:
+            return min(
+                Self.maximumAdvertisingNameLength,
+                max(0, writePayloadLength - Self.setNameRequestOverhead)
+            )
+        }
+    }
+
+    var payloadLength: Int {
+        switch self {
+        case .enterBootloader:
+            return data.count
+        case .set(let name):
+            return name.lengthOfBytes(using: .utf8)
+        }
+    }
 }
 
 extension ButtonlessDFURequest : CustomStringConvertible {
@@ -270,6 +295,13 @@ internal class ButtonlessDFU : NSObject, CBPeripheralDelegate, DFUCharacteristic
         peripheral.delegate = self
 
         let buttonlessUUID = characteristic.uuid.uuidString
+        let maximumPayloadLength = request.maximumPayloadLength(for: peripheral)
+        guard request.payloadLength <= maximumPayloadLength else {
+            logger.e("\(request) exceeds maximum payload length \(maximumPayloadLength)")
+            report?(.invalidAdvertisementName,
+                    "Alternative advertising name is too long. Maximum length is \(maximumPayloadLength) bytes.")
+            return
+        }
 
         logger.v("Writing to characteristic \(buttonlessUUID)...")
         logger.d("peripheral.writeValue(0x\(request.data.hexString), for: \(buttonlessUUID), type: .withResponse)")

Library/Classes/Implementation/SecureDFU/Peripheral/SecureDFUPeripheral.swift

@@ -65,7 +65,7 @@ internal class SecureDFUPeripheral : BaseCommonDFUPeripheral<SecureDFUExecutor,
 
     override init(_ initiator: DFUServiceInitiator, _ logger: LoggerHelper) {
         self.alternativeAdvertisingNameEnabled = initiator.alternativeAdvertisingNameEnabled
-        self.alternativeAdvertisingName = initiator.alternativeAdvertisingName.map { String($0.prefix(20)) }
+        self.alternativeAdvertisingName = initiator.alternativeAdvertisingName
         super.init(initiator, logger)
     }