Merge bitcoin/bitcoin#30621: wallet: fix blank legacy detection

6ed424f wallet: fix, detect blank legacy wallets in IsLegacy (furszy)

Pull request description:

  Blank legacy wallets do not have active SPKM. They can only be
  detected by checking the descriptors' flag or the db format.

  This enables the migration of blank legacy wallets in the GUI.

  To test this:
  1) Create a blank legacy wallet.
  2) Try to migrate it using the GUI's toolbar "Migrate Wallet" button.
      -> In master: The button will be disabled because `CWallet::IsLegacy()` returns false for blank legacy wallet.
      -> In this PR: the button will be enabled, allowing the migration of legacy wallets.

ACKs for top commit:
  achow101:
    ACK 6ed424f
  tdb3:
    ACK 6ed424f
  glozow:
    ACK 6ed424f

Tree-SHA512: c06c4c4c2e546ccb033287b9aa3aee4ca36b47aeb2fac6fbed5de774b65caef9c818fc8dfdaac6ce78839b2d5d642a5632a5b44c5e889ea169ced80ed50501a7

src/wallet/wallet.cpp

@@ -1038,22 +1038,22 @@ bool CWallet::IsSpentKey(const CScript& scriptPubKey) const
     if (IsAddressPreviouslySpent(dest)) {
         return true;
     }
-    if (IsLegacy()) {
-        LegacyScriptPubKeyMan* spk_man = GetLegacyScriptPubKeyMan();
-        assert(spk_man != nullptr);
-        for (const auto& keyid : GetAffectedKeys(scriptPubKey, *spk_man)) {
-            WitnessV0KeyHash wpkh_dest(keyid);
-            if (IsAddressPreviouslySpent(wpkh_dest)) {
-                return true;
-            }
-            ScriptHash sh_wpkh_dest(GetScriptForDestination(wpkh_dest));
-            if (IsAddressPreviouslySpent(sh_wpkh_dest)) {
-                return true;
-            }
-            PKHash pkh_dest(keyid);
-            if (IsAddressPreviouslySpent(pkh_dest)) {
-                return true;
-            }
+
+    LegacyScriptPubKeyMan* spk_man = GetLegacyScriptPubKeyMan();
+    if (!spk_man) return false;
+
+    for (const auto& keyid : GetAffectedKeys(scriptPubKey, *spk_man)) {
+        WitnessV0KeyHash wpkh_dest(keyid);
+        if (IsAddressPreviouslySpent(wpkh_dest)) {
+            return true;
+        }
+        ScriptHash sh_wpkh_dest(GetScriptForDestination(wpkh_dest));
+        if (IsAddressPreviouslySpent(sh_wpkh_dest)) {
+            return true;
+        }
+        PKHash pkh_dest(keyid);
+        if (IsAddressPreviouslySpent(pkh_dest)) {
+            return true;
         }
     }
     return false;
@@ -1626,7 +1626,9 @@ isminetype CWallet::IsMine(const CScript& script) const
     }
 
     // Legacy wallet
-    if (IsLegacy()) return GetLegacyScriptPubKeyMan()->IsMine(script);
+    if (LegacyScriptPubKeyMan* spkm = GetLegacyScriptPubKeyMan()) {
+        return spkm->IsMine(script);
+    }
 
     return ISMINE_NO;
 }
@@ -3559,7 +3561,8 @@ std::set<ScriptPubKeyMan*> CWallet::GetScriptPubKeyMans(const CScript& script) c
     Assume(std::all_of(spk_mans.begin(), spk_mans.end(), [&script, &sigdata](ScriptPubKeyMan* spkm) { return spkm->CanProvide(script, sigdata); }));
 
     // Legacy wallet
-    if (IsLegacy() && GetLegacyScriptPubKeyMan()->CanProvide(script, sigdata)) spk_mans.insert(GetLegacyScriptPubKeyMan());
+    LegacyScriptPubKeyMan* spkm = GetLegacyScriptPubKeyMan();
+    if (spkm && spkm->CanProvide(script, sigdata)) spk_mans.insert(spkm);
 
     return spk_mans;
 }
@@ -3589,7 +3592,8 @@ std::unique_ptr<SigningProvider> CWallet::GetSolvingProvider(const CScript& scri
     }
 
     // Legacy wallet
-    if (IsLegacy() && GetLegacyScriptPubKeyMan()->CanProvide(script, sigdata)) return GetLegacyScriptPubKeyMan()->GetSolvingProvider(script);
+    LegacyScriptPubKeyMan* spkm = GetLegacyScriptPubKeyMan();
+    if (spkm && spkm->CanProvide(script, sigdata)) return spkm->GetSolvingProvider(script);
 
     return nullptr;
 }
@@ -3846,11 +3850,7 @@ void CWallet::DeactivateScriptPubKeyMan(uint256 id, OutputType type, bool intern
 
 bool CWallet::IsLegacy() const
 {
-    if (m_internal_spk_managers.count(OutputType::LEGACY) == 0) {
-        return false;
-    }
-    auto spk_man = dynamic_cast<LegacyScriptPubKeyMan*>(m_internal_spk_managers.at(OutputType::LEGACY));
-    return spk_man != nullptr;
+    return !IsWalletFlagSet(WALLET_FLAG_DESCRIPTORS);
 }
 
 DescriptorScriptPubKeyMan* CWallet::GetDescriptorScriptPubKeyMan(const WalletDescriptor& desc) const