wallet: accurately account for the size of the witness stack

When estimating the maximum size of an input, we were assuming the
number of elements on the witness stack could be encode in a single
byte. This is a valid approximation for all the descriptors we support
(including P2WSH Miniscript ones), but may not hold anymore once we
support Miniscript within Taproot descriptors (since the max standard
witness stack size of 100 gets lifted).

It's a low-hanging fruit to account for it correctly, so just do it now.

1 files changed, 14 insertions, 13 deletions

diff --git a/src/wallet/spend.cpp b/src/wallet/spend.cpp
index 81dd033177..750b6c100b 100644
--- a/src/wallet/spend.cpp
+++ b/src/wallet/spend.cpp
@@ -59,19 +59,20 @@ static std::optional<int64_t> MaxInputWeight(const Descriptor& desc, const std::
                                              const CCoinControl* coin_control, const bool tx_is_segwit,
                                              const bool can_grind_r) {
     if (const auto sat_weight = desc.MaxSatisfactionWeight(!can_grind_r || UseMaxSig(txin, coin_control))) {
-        const bool is_segwit = IsSegwit(desc);
-        // Account for the size of the scriptsig and the number of elements on the witness stack. Note
-        // that if any input in the transaction is spending a witness program, we need to specify the
-        // witness stack size for every input regardless of whether it is segwit itself.
-        // NOTE: this also works in case of mixed scriptsig-and-witness such as in p2sh-wrapped segwit v0
-        // outputs. In this case the size of the scriptsig length will always be one (since the redeemScript
-        // is always a push of the witness program in this case, which is smaller than 253 bytes).
-        const int64_t scriptsig_len = is_segwit ? 1 : GetSizeOfCompactSize(*sat_weight / WITNESS_SCALE_FACTOR);
-        // FIXME: use the real number of stack elements instead of the "1" placeholder.
-        const int64_t witstack_len = is_segwit ? GetSizeOfCompactSize(1) : (tx_is_segwit ? 1 : 0);
-        // previous txid + previous vout + sequence + scriptsig len + witstack size + scriptsig or witness
-        // NOTE: sat_weight already accounts for the witness discount accordingly.
-        return (32 + 4 + 4 + scriptsig_len) * WITNESS_SCALE_FACTOR + witstack_len + *sat_weight;
+        if (const auto elems_count = desc.MaxSatisfactionElems()) {
+            const bool is_segwit = IsSegwit(desc);
+            // Account for the size of the scriptsig and the number of elements on the witness stack. Note
+            // that if any input in the transaction is spending a witness program, we need to specify the
+            // witness stack size for every input regardless of whether it is segwit itself.
+            // NOTE: this also works in case of mixed scriptsig-and-witness such as in p2sh-wrapped segwit v0
+            // outputs. In this case the size of the scriptsig length will always be one (since the redeemScript
+            // is always a push of the witness program in this case, which is smaller than 253 bytes).
+            const int64_t scriptsig_len = is_segwit ? 1 : GetSizeOfCompactSize(*sat_weight / WITNESS_SCALE_FACTOR);
+            const int64_t witstack_len = is_segwit ? GetSizeOfCompactSize(*elems_count) : (tx_is_segwit ? 1 : 0);
+            // previous txid + previous vout + sequence + scriptsig len + witstack size + scriptsig or witness
+            // NOTE: sat_weight already accounts for the witness discount accordingly.
+            return (32 + 4 + 4 + scriptsig_len) * WITNESS_SCALE_FACTOR + witstack_len + *sat_weight;
+        }
     }
 
     return {};