Merge bitcoin/bitcoin#26418: Fix signing of multi_a and rawtr scripts with wallets that only have corresponding keys

0de30ed509a9969cb254e00097671625c9e107d2 tests: Test Taproot PSBT signing with keys in other descriptor (Andrew Chow)
6efcdf6b7f6daa83b5937aa630fce358fdaed333 tests: Use new wallets for each test in wallet_taproot.py (Andrew Chow)
8781a1b6bbd0af3cfdf1421fd18de5432494619a psbt: Include output pubkey in additional pubkeys to sign (Andrew Chow)
323890d0d7db2628f9dc6eaeba6e99ce0a12e1f5 sign: Fill in taproot pubkey info for all script path sigs (Andrew Chow)

Pull request description:

  A user reported on [stackexchange](https://bitcoin.stackexchange.com/q/115742/48884) that they were unable to sign for a `multi_a` script using a wallet that only had the corresponding keys (i.e. it did not have the `multi_a()` descriptor). This PR fixes this issue.

  Additionally, `wallet_taproot.py` is modified to test for this scenario by having another wallet in `do_test_psbt` which contains descriptors that only have the keys involved in the descriptor being tested. `wallet_taproot.py` was also modified to create new wallets for each test case rather than sharing wallets throughout as the sharing could result in the signing wallet having the keys in a different descriptor and accidentally result in failing to detect a test failure.

  The changes to the test also revealed a similar issue with `rawtr()` descriptors, which has also been fixed by checking if a descriptor can produce a `SigningProvider` for the Taproot output pubkey.

ACKs for top commit:
  instagibbs:
    crACK https://github.com/bitcoin/bitcoin/pull/26418/commits/0de30ed509a9969cb254e00097671625c9e107d2
  darosior:
    ACK 0de30ed509a9969cb254e00097671625c9e107d2

Tree-SHA512: 12e131dd8afd93da7b1288c9054de2415a228d4477b97102da3ee4e82ce9de20b186260c3085a4b7b067bd8b74400751dcadf153f113db83abc59e7466e69f14

2 files changed, 34 insertions, 22 deletions

diff --git a/src/script/sign.cpp b/src/script/sign.cpp
index 5da0d076d8..0d74a661a5 100644
--- a/src/script/sign.cpp
+++ b/src/script/sign.cpp
@@ -146,6 +146,16 @@ static bool CreateSig(const BaseSignatureCreator& creator, SignatureData& sigdat
 
 static bool CreateTaprootScriptSig(const BaseSignatureCreator& creator, SignatureData& sigdata, const SigningProvider& provider, std::vector<unsigned char>& sig_out, const XOnlyPubKey& pubkey, const uint256& leaf_hash, SigVersion sigversion)
 {
+    KeyOriginInfo info;
+    if (provider.GetKeyOriginByXOnly(pubkey, info)) {
+        auto it = sigdata.taproot_misc_pubkeys.find(pubkey);
+        if (it == sigdata.taproot_misc_pubkeys.end()) {
+            sigdata.taproot_misc_pubkeys.emplace(pubkey, std::make_pair(std::set<uint256>({leaf_hash}), info));
+        } else {
+            it->second.first.insert(leaf_hash);
+        }
+    }
+
     auto lookup_key = std::make_pair(pubkey, leaf_hash);
     auto it = sigdata.taproot_script_sigs.find(lookup_key);
     if (it != sigdata.taproot_script_sigs.end()) {
@@ -170,17 +180,6 @@ static bool SignTaprootScript(const SigningProvider& provider, const BaseSignatu
     // <xonly pubkey> OP_CHECKSIG
     if (script.size() == 34 && script[33] == OP_CHECKSIG && script[0] == 0x20) {
         XOnlyPubKey pubkey{Span{script}.subspan(1, 32)};
-
-        KeyOriginInfo info;
-        if (provider.GetKeyOriginByXOnly(pubkey, info)) {
-            auto it = sigdata.taproot_misc_pubkeys.find(pubkey);
-            if (it == sigdata.taproot_misc_pubkeys.end()) {
-                sigdata.taproot_misc_pubkeys.emplace(pubkey, std::make_pair(std::set<uint256>({leaf_hash}), info));
-            } else {
-                it->second.first.insert(leaf_hash);
-            }
-        }
-
         std::vector<unsigned char> sig;
         if (CreateTaprootScriptSig(creator, sigdata, provider, sig, pubkey, leaf_hash, sigversion)) {
             result = Vector(std::move(sig));
diff --git a/src/wallet/scriptpubkeyman.cpp b/src/wallet/scriptpubkeyman.cpp
index 4c534d64ec..896ade77dd 100644
--- a/src/wallet/scriptpubkeyman.cpp
+++ b/src/wallet/scriptpubkeyman.cpp
@@ -2502,14 +2502,23 @@ TransactionError DescriptorScriptPubKeyMan::FillPSBT(PartiallySignedTransaction&
             keys->Merge(std::move(*script_keys));
         } else {
             // Maybe there are pubkeys listed that we can sign for
-            script_keys = std::make_unique<FlatSigningProvider>();
-            for (const auto& pk_pair : input.hd_keypaths) {
-                const CPubKey& pubkey = pk_pair.first;
-                std::unique_ptr<FlatSigningProvider> pk_keys = GetSigningProvider(pubkey);
-                if (pk_keys) {
-                    keys->Merge(std::move(*pk_keys));
-                }
+            std::vector<CPubKey> pubkeys;
+
+            // ECDSA Pubkeys
+            for (const auto& [pk, _] : input.hd_keypaths) {
+                pubkeys.push_back(pk);
+            }
+
+            // Taproot output pubkey
+            std::vector<std::vector<unsigned char>> sols;
+            if (Solver(script, sols) == TxoutType::WITNESS_V1_TAPROOT) {
+                sols[0].insert(sols[0].begin(), 0x02);
+                pubkeys.emplace_back(sols[0]);
+                sols[0][0] = 0x03;
+                pubkeys.emplace_back(sols[0]);
             }
+
+            // Taproot pubkeys
             for (const auto& pk_pair : input.m_tap_bip32_paths) {
                 const XOnlyPubKey& pubkey = pk_pair.first;
                 for (unsigned char prefix : {0x02, 0x03}) {
@@ -2517,10 +2526,14 @@ TransactionError DescriptorScriptPubKeyMan::FillPSBT(PartiallySignedTransaction&
                     std::copy(pubkey.begin(), pubkey.end(), b + 1);
                     CPubKey fullpubkey;
                     fullpubkey.Set(b, b + 33);
-                    std::unique_ptr<FlatSigningProvider> pk_keys = GetSigningProvider(fullpubkey);
-                    if (pk_keys) {
-                        keys->Merge(std::move(*pk_keys));
-                    }
+                    pubkeys.push_back(fullpubkey);
+                }
+            }
+
+            for (const auto& pubkey : pubkeys) {
+                std::unique_ptr<FlatSigningProvider> pk_keys = GetSigningProvider(pubkey);
+                if (pk_keys) {
+                    keys->Merge(std::move(*pk_keys));
                 }
             }
         }