Merge bitcoin/bitcoin#23348: rpc, wallet: Do not return "keypoololdest" for blank descriptor wallets

ee03c782ba61993d9e95fa499546cd14cee35445 wallet: Make GetOldestKeyPoolTime return nullopt for blank wallets (Hennadii Stepanov)
3e4f069d23cd2ea5de8fa3c4b1a761ab097ad56f wallet, refactor: Make GetOldestKeyPoolTime return type std::optional (Hennadii Stepanov)

Pull request description:

  The "keypoololdest" field in the `getwalletinfo` RPC response should be used for legacy wallets only.

  Th current implementation (04437ee721e66a7b76bef5ec2f88dd1efcd03b84) assumes that `CWallet::GetOldestKeyPoolTime()` always return `0` for descriptor wallets. This assumption is wrong for _blank_ descriptor wallets, when `m_spk_managers` is empty. As a result:
  ```
  $ src/bitcoin-cli -signet -rpcwallet=211024-d-DPK getwalletinfo
  {
    "walletname": "211024-d-DPK",
    "walletversion": 169900,
    "format": "sqlite",
    "balance": 0.00000000,
    "unconfirmed_balance": 0.00000000,
    "immature_balance": 0.00000000,
    "txcount": 0,
    "keypoololdest": 9223372036854775807,
    "keypoolsize": 0,
    "keypoolsize_hd_internal": 0,
    "paytxfee": 0.00000000,
    "private_keys_enabled": false,
    "avoid_reuse": false,
    "scanning": false,
    "descriptors": true
  }
  ```

  This PR fixes this issue with direct checking of the `WALLET_FLAG_DESCRIPTORS` flag.

ACKs for top commit:
  lsilva01:
    re-ACK ee03c78
  stratospher:
    ACK ee03c78.
  meshcollider:
    Code review ACK ee03c782ba61993d9e95fa499546cd14cee35445

Tree-SHA512: 9852f9f8ed5c08c07507274d7714f039bbfda66da6df65cf98f67bf11a600167d0f7f872680c95775399477f4df9ba9fce80ec0cbe0adb7f2bb33c3bd65b15df

5 files changed, 19 insertions, 15 deletions

diff --git a/src/wallet/rpcwallet.cpp b/src/wallet/rpcwallet.cpp
index babb61b03a..e24d1111c7 100644
--- a/src/wallet/rpcwallet.cpp
+++ b/src/wallet/rpcwallet.cpp
@@ -2522,7 +2522,6 @@ static RPCHelpMan getwalletinfo()
 
     size_t kpExternalSize = pwallet->KeypoolCountExternalKeys();
     const auto bal = GetBalance(*pwallet);
-    int64_t kp_oldest = pwallet->GetOldestKeyPoolTime();
     obj.pushKV("walletname", pwallet->GetName());
     obj.pushKV("walletversion", pwallet->GetVersion());
     obj.pushKV("format", pwallet->GetDatabase().Format());
@@ -2530,8 +2529,9 @@ static RPCHelpMan getwalletinfo()
     obj.pushKV("unconfirmed_balance", ValueFromAmount(bal.m_mine_untrusted_pending));
     obj.pushKV("immature_balance", ValueFromAmount(bal.m_mine_immature));
     obj.pushKV("txcount",       (int)pwallet->mapWallet.size());
-    if (kp_oldest > 0) {
-        obj.pushKV("keypoololdest", kp_oldest);
+    const auto kp_oldest = pwallet->GetOldestKeyPoolTime();
+    if (kp_oldest.has_value()) {
+        obj.pushKV("keypoololdest", kp_oldest.value());
     }
     obj.pushKV("keypoolsize", (int64_t)kpExternalSize);
 
diff --git a/src/wallet/scriptpubkeyman.cpp b/src/wallet/scriptpubkeyman.cpp
index 9173c790d4..faeb6068db 100644
--- a/src/wallet/scriptpubkeyman.cpp
+++ b/src/wallet/scriptpubkeyman.cpp
@@ -528,7 +528,7 @@ static int64_t GetOldestKeyTimeInPool(const std::set<int64_t>& setKeyPool, Walle
     return keypool.nTime;
 }
 
-int64_t LegacyScriptPubKeyMan::GetOldestKeyPoolTime() const
+std::optional<int64_t> LegacyScriptPubKeyMan::GetOldestKeyPoolTime() const
 {
     LOCK(cs_KeyStore);
 
@@ -1970,11 +1970,10 @@ bool DescriptorScriptPubKeyMan::HavePrivateKeys() const
     return m_map_keys.size() > 0 || m_map_crypted_keys.size() > 0;
 }
 
-int64_t DescriptorScriptPubKeyMan::GetOldestKeyPoolTime() const
+std::optional<int64_t> DescriptorScriptPubKeyMan::GetOldestKeyPoolTime() const
 {
     // This is only used for getwalletinfo output and isn't relevant to descriptor wallets.
-    // The magic number 0 indicates that it shouldn't be displayed so that's what we return.
-    return 0;
+    return std::nullopt;
 }
 
 
diff --git a/src/wallet/scriptpubkeyman.h b/src/wallet/scriptpubkeyman.h
index ef74638751..2d447f1d67 100644
--- a/src/wallet/scriptpubkeyman.h
+++ b/src/wallet/scriptpubkeyman.h
@@ -19,6 +19,7 @@
 
 #include <boost/signals2/signal.hpp>
 
+#include <optional>
 #include <unordered_map>
 
 enum class OutputType;
@@ -203,7 +204,7 @@ public:
     //! The action to do when the DB needs rewrite
     virtual void RewriteDB() {}
 
-    virtual int64_t GetOldestKeyPoolTime() const { return GetTime(); }
+    virtual std::optional<int64_t> GetOldestKeyPoolTime() const { return GetTime(); }
 
     virtual unsigned int GetKeyPoolSize() const { return 0; }
 
@@ -371,7 +372,7 @@ public:
 
     void RewriteDB() override;
 
-    int64_t GetOldestKeyPoolTime() const override;
+    std::optional<int64_t> GetOldestKeyPoolTime() const override;
     size_t KeypoolCountExternalKeys() const;
     unsigned int GetKeyPoolSize() const override;
 
@@ -577,7 +578,7 @@ public:
 
     bool HavePrivateKeys() const override;
 
-    int64_t GetOldestKeyPoolTime() const override;
+    std::optional<int64_t> GetOldestKeyPoolTime() const override;
     unsigned int GetKeyPoolSize() const override;
 
     int64_t GetTimeFirstKey() const override;
diff --git a/src/wallet/wallet.cpp b/src/wallet/wallet.cpp
index 7f60dd6906..e586785c24 100644
--- a/src/wallet/wallet.cpp
+++ b/src/wallet/wallet.cpp
@@ -2169,14 +2169,18 @@ bool CWallet::GetNewChangeDestination(const OutputType type, CTxDestination& des
     return true;
 }
 
-int64_t CWallet::GetOldestKeyPoolTime() const
+std::optional<int64_t> CWallet::GetOldestKeyPoolTime() const
 {
     LOCK(cs_wallet);
-    int64_t oldestKey = std::numeric_limits<int64_t>::max();
+    if (m_spk_managers.empty()) {
+        return std::nullopt;
+    }
+
+    std::optional<int64_t> oldest_key{std::numeric_limits<int64_t>::max()};
     for (const auto& spk_man_pair : m_spk_managers) {
-        oldestKey = std::min(oldestKey, spk_man_pair.second->GetOldestKeyPoolTime());
+        oldest_key = std::min(oldest_key, spk_man_pair.second->GetOldestKeyPoolTime());
     }
-    return oldestKey;
+    return oldest_key;
 }
 
 void CWallet::MarkDestinationsDirty(const std::set<CTxDestination>& destinations) {
diff --git a/src/wallet/wallet.h b/src/wallet/wallet.h
index 3855ad821d..bab8314c76 100644
--- a/src/wallet/wallet.h
+++ b/src/wallet/wallet.h
@@ -632,7 +632,7 @@ public:
     size_t KeypoolCountExternalKeys() const EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
     bool TopUpKeyPool(unsigned int kpSize = 0);
 
-    int64_t GetOldestKeyPoolTime() const;
+    std::optional<int64_t> GetOldestKeyPoolTime() const;
 
     std::set<CTxDestination> GetLabelAddresses(const std::string& label) const EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);