key-wallet: improve FFI safety in Transaction and add helpers

- Use stable C strings for output addresses and free them to avoid dangling pointers
- Switch to typed withUnsafeBytes pointers for clarity and safety
- Add buildAndSign(managedWallet:wallet:...) convenience API
- Add getTxid(from:) helper to extract transaction ID

Author: PastaPastaPasta

packages/swift-sdk/Sources/SwiftDashSDK/KeyWallet/Transaction.swift

@@ -1,4 +1,5 @@
 import Foundation
+import Darwin
 import DashSDKFFI
 
 /// Transaction utilities for wallet operations
@@ -39,10 +40,23 @@ public class Transaction {
         var error = FFIError()
         var txBytesPtr: UnsafeMutablePointer<UInt8>?
         var txLen: size_t = 0
-        
-        // Convert outputs to FFI format
-        let ffiOutputs = outputs.map { $0.toFFI() }
-        
+        // Convert outputs to FFI format with stable C strings
+        var cStrings: [UnsafeMutablePointer<CChar>] = []
+        cStrings.reserveCapacity(outputs.count)
+        var ffiOutputs: [FFITxOutput] = []
+        ffiOutputs.reserveCapacity(outputs.count)
+
+        for output in outputs {
+            let cstr = output.address.withCString { strdup($0) }
+            guard let cstr else {
+                // Free any previously allocated strings before throwing
+                for ptr in cStrings { free(ptr) }
+                throw KeyWalletError.invalidInput("Failed to allocate C string for address")
+            }
+            cStrings.append(cstr)
+            ffiOutputs.append(FFITxOutput(address: UnsafePointer(cstr), amount: output.amount))
+        }
+
         let success = ffiOutputs.withUnsafeBufferPointer { outputsPtr in
             wallet_build_transaction(
                 wallet.ffiHandle,
@@ -57,6 +71,8 @@ public class Transaction {
         }
 
         defer {
+            // Free allocated C strings
+            for ptr in cStrings { free(ptr) }
             if error.message != nil {
                 error_message_free(error.message)
             }
@@ -89,8 +105,7 @@ public class Transaction {
         var signedTxPtr: UnsafeMutablePointer<UInt8>?
         var signedLen: size_t = 0
 
-        let success = transactionData.withUnsafeBytes { txBytes in
-            let txPtr = txBytes.bindMemory(to: UInt8.self).baseAddress
+        let success = transactionData.withUnsafeBytes { (txPtr: UnsafePointer<UInt8>) in
             return wallet_sign_transaction(
                 wallet.ffiHandle,
                 NetworkSet(wallet.network).ffiNetworks,
@@ -113,10 +128,122 @@ public class Transaction {
 
         // Copy the signed transaction data before freeing
         let signedData = Data(bytes: ptr, count: signedLen)
-        
+
         return signedData
     }
-    
+
+    /// Build and sign a transaction in one step using managed wallet
+    /// - Parameters:
+    ///   - managedWallet: The managed wallet with UTXO information
+    ///   - wallet: The wallet with private keys for signing
+    ///   - accountIndex: The account index to use
+    ///   - outputs: The transaction outputs
+    ///   - feePerKB: Fee per kilobyte in satoshis
+    ///   - currentHeight: Current blockchain height for UTXO selection
+    /// - Returns: The signed transaction bytes ready for broadcast
+    public static func buildAndSign(managedWallet: ManagedWallet,
+                                   wallet: Wallet,
+                                   accountIndex: UInt32 = 0,
+                                   outputs: [Output],
+                                   feePerKB: UInt64,
+                                   currentHeight: UInt32) throws -> Data {
+        guard !outputs.isEmpty else {
+            throw KeyWalletError.invalidInput("Transaction must have at least one output")
+        }
+
+        guard !wallet.isWatchOnly else {
+            throw KeyWalletError.invalidState("Cannot sign with watch-only wallet")
+        }
+
+        var error = FFIError()
+        var txBytesPtr: UnsafeMutablePointer<UInt8>?
+        var txLen: size_t = 0
+
+        // Get managed wallet handle
+        guard let managedHandle = managedWallet.getInfoHandle() else {
+            throw KeyWalletError.invalidState("Failed to get managed wallet handle")
+        }
+
+        // Convert outputs to FFI format with stable C strings
+        var cStrings: [UnsafeMutablePointer<CChar>] = []
+        cStrings.reserveCapacity(outputs.count)
+        var ffiOutputs: [FFITxOutput] = []
+        ffiOutputs.reserveCapacity(outputs.count)
+
+        for output in outputs {
+            let cstr = output.address.withCString { strdup($0) }
+            guard let cstr else {
+                for ptr in cStrings { free(ptr) }
+                throw KeyWalletError.invalidInput("Failed to allocate C string for address")
+            }
+            cStrings.append(cstr)
+            ffiOutputs.append(FFITxOutput(address: UnsafePointer(cstr), amount: output.amount))
+        }
+
+        let success = ffiOutputs.withUnsafeBufferPointer { outputsPtr in
+            wallet_build_and_sign_transaction(
+                managedHandle,
+                wallet.ffiHandle,
+                wallet.network.ffiValue,
+                accountIndex,
+                outputsPtr.baseAddress,
+                outputs.count,
+                feePerKB,
+                currentHeight,
+                &txBytesPtr,
+                &txLen,
+                &error)
+        }
+
+        defer {
+            for ptr in cStrings { free(ptr) }
+            if error.message != nil {
+                error_message_free(error.message)
+            }
+            if let ptr = txBytesPtr {
+                transaction_bytes_free(ptr)
+            }
+        }
+
+        guard success, let ptr = txBytesPtr else {
+            throw KeyWalletError(ffiError: error)
+        }
+
+        // Copy the transaction data before freeing
+        let txData = Data(bytes: ptr, count: txLen)
+
+        return txData
+    }
+
+    /// Extract TXID from raw transaction bytes
+    /// - Parameter transactionData: The transaction bytes
+    /// - Returns: The transaction ID as a hex string
+    public static func getTxid(from transactionData: Data) throws -> String {
+        var error = FFIError()
+        var txidPtr: UnsafeMutablePointer<CChar>?
+
+        let success = transactionData.withUnsafeBytes { txBytes in
+            let txPtr = txBytes.bindMemory(to: UInt8.self).baseAddress
+            txidPtr = transaction_get_txid_from_bytes(txPtr, transactionData.count, &error)
+            return txidPtr != nil
+        }
+
+        defer {
+            if error.message != nil {
+                error_message_free(error.message)
+            }
+            if let ptr = txidPtr {
+                string_free(ptr)
+            }
+        }
+
+        guard success, let ptr = txidPtr else {
+            throw KeyWalletError(ffiError: error)
+        }
+
+        return String(cString: ptr)
+    }
+
     /// Check if a transaction belongs to a wallet
     /// - Parameters:
     ///   - wallet: The wallet to check against
@@ -137,12 +264,10 @@ public class Transaction {
         var error = FFIError()
         var result = FFITransactionCheckResult()
 
-        let success = transactionData.withUnsafeBytes { txBytes in
-            let txPtr = txBytes.bindMemory(to: UInt8.self).baseAddress
+        let success = transactionData.withUnsafeBytes { (txPtr: UnsafePointer<UInt8>) in
 
             if let hash = blockHash {
-                return hash.withUnsafeBytes { hashBytes in
-                    let hashPtr = hashBytes.bindMemory(to: UInt8.self).baseAddress
+                return hash.withUnsafeBytes { (hashPtr: UnsafePointer<UInt8>) in
 
                     return wallet_check_transaction(
                         wallet.ffiHandle,
@@ -181,8 +306,7 @@ public class Transaction {
     public static func classify(_ transactionData: Data) throws -> String {
         var error = FFIError()
 
-        let classificationPtr = transactionData.withUnsafeBytes { txBytes in
-            let txPtr = txBytes.bindMemory(to: UInt8.self).baseAddress
+        let classificationPtr = transactionData.withUnsafeBytes { (txPtr: UnsafePointer<UInt8>) in
             return transaction_classify(txPtr, transactionData.count, &error)
         }
 
@@ -201,4 +325,4 @@ public class Transaction {
 
         return classification
     }
-}
+}