If an input already used in a payjoin is reused in another, we should not attempt to broadcast the original transaction.

BTCPayServer.Tests/PayJoinTests.cs

@@ -87,6 +87,19 @@ namespace BTCPayServer.Tests
                 Assert.True(await repo.TryLock(outpoint));
                 Assert.True(await repo.TryUnlock(outpoint));
                 Assert.False(await repo.TryUnlock(outpoint));
+
+                // Make sure that if any can't be locked, all are not locked
+                var outpoint1 = RandomOutpoint();
+                var outpoint2 = RandomOutpoint();
+                Assert.True(await repo.TryLockInputs(new[] { outpoint1 }));
+                Assert.False(await repo.TryLockInputs(new[] { outpoint1, outpoint2 }));
+                Assert.True(await repo.TryLockInputs(new[] { outpoint2 }));
+
+                outpoint1 = RandomOutpoint();
+                outpoint2 = RandomOutpoint();
+                Assert.True(await repo.TryLockInputs(new[] { outpoint1 }));
+                Assert.False(await repo.TryLockInputs(new[] { outpoint2, outpoint1 }));
+                Assert.True(await repo.TryLockInputs(new[] { outpoint2 }));
             }
         }
 
@@ -901,10 +914,6 @@ retry:
                     .SendEstimatedFees(new FeeRate(100m))
                     .BuildTransaction(true);
 
-                //Attempt 1: Send a signed tx to invoice 1 that does not pay the invoice at all 
-                //Result: reject
-                // Assert.False((await tester.PayTester.HttpClient.PostAsync(endpoint,
-                //     new StringContent(Invoice2Coin1.ToHex(), Encoding.UTF8, "text/plain"))).IsSuccessStatusCode);
 
                 //Attempt 2: Create two transactions using different inputs and send them to the same invoice. 
                 //Result: Second Tx should be rejected. 

BTCPayServer/Payments/PayJoin/PayJoinEndpointController.cs

@@ -83,35 +83,30 @@ namespace BTCPayServer.Payments.PayJoin
         private readonly BTCPayNetworkProvider _btcPayNetworkProvider;
         private readonly InvoiceRepository _invoiceRepository;
         private readonly ExplorerClientProvider _explorerClientProvider;
-        private readonly StoreRepository _storeRepository;
         private readonly BTCPayWalletProvider _btcPayWalletProvider;
         private readonly PayJoinRepository _payJoinRepository;
         private readonly EventAggregator _eventAggregator;
         private readonly NBXplorerDashboard _dashboard;
         private readonly DelayedTransactionBroadcaster _broadcaster;
-        private readonly WalletRepository _walletRepository;
         private readonly BTCPayServerEnvironment _env;
 
         public PayJoinEndpointController(BTCPayNetworkProvider btcPayNetworkProvider,
             InvoiceRepository invoiceRepository, ExplorerClientProvider explorerClientProvider,
-            StoreRepository storeRepository, BTCPayWalletProvider btcPayWalletProvider,
+            BTCPayWalletProvider btcPayWalletProvider,
             PayJoinRepository payJoinRepository,
             EventAggregator eventAggregator,
             NBXplorerDashboard dashboard,
             DelayedTransactionBroadcaster broadcaster,
-            WalletRepository walletRepository,
             BTCPayServerEnvironment env)
         {
             _btcPayNetworkProvider = btcPayNetworkProvider;
             _invoiceRepository = invoiceRepository;
             _explorerClientProvider = explorerClientProvider;
-            _storeRepository = storeRepository;
             _btcPayWalletProvider = btcPayWalletProvider;
             _payJoinRepository = payJoinRepository;
             _eventAggregator = eventAggregator;
             _dashboard = dashboard;
             _broadcaster = broadcaster;
-            _walletRepository = walletRepository;
             _env = env;
         }
 
@@ -285,6 +280,8 @@ namespace BTCPayServer.Payments.PayJoin
 
                 if (!await _payJoinRepository.TryLockInputs(ctx.OriginalTransaction.Inputs.Select(i => i.PrevOut).ToArray()))
                 {
+                    // We do not broadcast, since we might double spend a delayed transaction of a previous payjoin
+                    ctx.DoNotBroadcast();
                     return CreatePayjoinErrorAndLog(503, PayjoinReceiverWellknownErrors.Unavailable, "Some of those inputs have already been used to make another payjoin transaction");
                 }
 

BTCPayServer/Payments/PayJoin/PayJoinRepository.cs

@@ -1,3 +1,4 @@
+using System.Collections.Generic;
 using System.Threading.Tasks;
 using BTCPayServer.Data;
 using Microsoft.EntityFrameworkCore;