From 1dba05e7f6b188e2fb4183037aaf27cfb1bff916 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?L=C5=91rinc?= <pap.lorinc@gmail.com>
Date: Wed, 6 May 2026 14:27:13 +0200
Subject: [PATCH] wallet: use outpoint when estimating input size

`CalculateMaximumSignedInputSize()` is passed the outpoint being sized, but that context was not used when estimating the signed input size.
Pass the outpoint through so externally selected inputs are not underestimated.

Co-authored-by: Antoine Poinsot <darosior@protonmail.com>

Github-Pull: #35228
Rebased-From: cd8d3bd937b5515ea000408eb07d2ae3cd1aa417
---
 src/wallet/spend.cpp            |  2 +-
 src/wallet/test/spend_tests.cpp | 17 +++++++++++++++++
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/src/wallet/spend.cpp b/src/wallet/spend.cpp
index dc34060d729..9524e7a0bec 100644
--- a/src/wallet/spend.cpp
+++ b/src/wallet/spend.cpp
@@ -94,7 +94,7 @@ int CalculateMaximumSignedInputSize(const CTxOut& txout, const COutPoint outpoin
     if (!provider) return -1;
 
     if (const auto desc = InferDescriptor(txout.scriptPubKey, *provider)) {
-        if (const auto weight = MaxInputWeight(*desc, {}, coin_control, true, can_grind_r)) {
+        if (const auto weight = MaxInputWeight(*desc, CTxIn{outpoint}, coin_control, true, can_grind_r)) {
             return static_cast<int>(GetVirtualTransactionSize(*weight, 0, 0));
         }
     }
diff --git a/src/wallet/test/spend_tests.cpp b/src/wallet/test/spend_tests.cpp
index 963c0f838b1..ac76fe4dda0 100644
--- a/src/wallet/test/spend_tests.cpp
+++ b/src/wallet/test/spend_tests.cpp
@@ -3,6 +3,7 @@
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include <consensus/amount.h>
+#include <key.h>
 #include <policy/fees.h>
 #include <script/solver.h>
 #include <validation.h>
@@ -16,6 +17,22 @@
 namespace wallet {
 BOOST_FIXTURE_TEST_SUITE(spend_tests, WalletTestingSetup)
 
+BOOST_AUTO_TEST_CASE(max_signed_input_size_uses_external_outpoint)
+{
+    const CKey key{GenerateRandomKey()};
+    FillableSigningProvider provider;
+    BOOST_REQUIRE(provider.AddKey(key));
+
+    const CTxOut txout{COIN, GetScriptForDestination(PKHash{key.GetPubKey()})};
+    const COutPoint outpoint{Txid{}, 0};
+    CCoinControl coin_control;
+    coin_control.Select(outpoint).SetTxOut(txout);
+
+    const int low_r{CalculateMaximumSignedInputSize(txout, COutPoint{}, &provider, /*can_grind_r=*/true, &coin_control)};
+    const int high_r{CalculateMaximumSignedInputSize(txout, outpoint, &provider, /*can_grind_r=*/true, &coin_control)};
+    BOOST_CHECK_EQUAL(high_r, low_r + 1);
+}
+
 BOOST_FIXTURE_TEST_CASE(SubtractFee, TestChain100Setup)
 {
     CreateAndProcessBlock({}, GetScriptForRawPubKey(coinbaseKey.GetPubKey()));