wallet: keep track of when the passphrase is needed when rescanning

Wallet passphrases are needed to top up the keypool during a
rescan. The following RPCs need the passphrase when rescanning:
    - `importdescriptors`
    - `rescanblockchain`

The following RPCs use the information about whether or not the
passphrase is being used to ensure that full rescans are able to
take place:
    - `walletlock`
    - `encryptwallet`
    - `walletpassphrasechange`

src/wallet/rpc/backup.cpp

@@ -1651,7 +1651,7 @@ RPCHelpMan importdescriptors()
     }
 
     WalletRescanReserver reserver(*pwallet);
-    if (!reserver.reserve()) {
+    if (!reserver.reserve(/*with_passphrase=*/true)) {
         throw JSONRPCError(RPC_WALLET_ERROR, "Wallet is currently rescanning. Abort existing rescan or wait.");
     }
 

src/wallet/rpc/encrypt.cpp

@@ -128,6 +128,10 @@ RPCHelpMan walletpassphrasechange()
         throw JSONRPCError(RPC_WALLET_WRONG_ENC_STATE, "Error: running with an unencrypted wallet, but walletpassphrasechange was called.");
     }
 
+    if (pwallet->IsScanningWithPassphrase()) {
+        throw JSONRPCError(RPC_WALLET_ERROR, "Error: the wallet is currently being used to rescan the blockchain for related transactions. Please call `abortrescan` before changing the passphrase.");
+    }
+
     // TODO: get rid of these .c_str() calls by implementing SecureString::operator=(std::string)
     // Alternately, find a way to make request.params[0] mlock()'d to begin with.
     SecureString strOldWalletPass;
@@ -181,6 +185,10 @@ RPCHelpMan walletlock()
         throw JSONRPCError(RPC_WALLET_WRONG_ENC_STATE, "Error: running with an unencrypted wallet, but walletlock was called.");
     }
 
+    if (pwallet->IsScanningWithPassphrase()) {
+        throw JSONRPCError(RPC_WALLET_ERROR, "Error: the wallet is currently being used to rescan the blockchain for related transactions. Please call `abortrescan` before locking the wallet.");
+    }
+
     pwallet->Lock();
     pwallet->nRelockTime = 0;
 
@@ -229,6 +237,10 @@ RPCHelpMan encryptwallet()
         throw JSONRPCError(RPC_WALLET_WRONG_ENC_STATE, "Error: running with an encrypted wallet, but encryptwallet was called.");
     }
 
+    if (pwallet->IsScanningWithPassphrase()) {
+        throw JSONRPCError(RPC_WALLET_ERROR, "Error: the wallet is currently being used to rescan the blockchain for related transactions. Please call `abortrescan` before encrypting the wallet.");
+    }
+
     // TODO: get rid of this .c_str() by implementing SecureString::operator=(std::string)
     // Alternately, find a way to make request.params[0] mlock()'d to begin with.
     SecureString strWalletPass;

src/wallet/rpc/transactions.cpp

@@ -872,15 +872,17 @@ RPCHelpMan rescanblockchain()
     wallet.BlockUntilSyncedToCurrentChain();
 
     WalletRescanReserver reserver(*pwallet);
-    if (!reserver.reserve()) {
+    if (!reserver.reserve(/*with_passphrase=*/true)) {
         throw JSONRPCError(RPC_WALLET_ERROR, "Wallet is currently rescanning. Abort existing rescan or wait.");
     }
 
     int start_height = 0;
     std::optional<int> stop_height;
     uint256 start_block;
+
     {
         LOCK(pwallet->cs_wallet);
+        EnsureWalletIsUnlocked(*pwallet);
         int tip_height = pwallet->GetLastBlockHeight();
 
         if (!request.params[0].isNull()) {

src/wallet/wallet.h

@@ -243,6 +243,7 @@ private:
     std::atomic<bool> fAbortRescan{false};
     std::atomic<bool> fScanningWallet{false}; // controlled by WalletRescanReserver
     std::atomic<bool> m_attaching_chain{false};
+    std::atomic<bool> m_scanning_with_passphrase{false};
     std::atomic<int64_t> m_scanning_start{0};
     std::atomic<double> m_scanning_progress{0};
     friend class WalletRescanReserver;
@@ -467,6 +468,7 @@ public:
     void AbortRescan() { fAbortRescan = true; }
     bool IsAbortingRescan() const { return fAbortRescan; }
     bool IsScanning() const { return fScanningWallet; }
+    bool IsScanningWithPassphrase() const { return m_scanning_with_passphrase; }
     int64_t ScanningDuration() const { return fScanningWallet ? GetTimeMillis() - m_scanning_start : 0; }
     double ScanningProgress() const { return fScanningWallet ? (double) m_scanning_progress : 0; }
 
@@ -960,12 +962,13 @@ private:
 public:
     explicit WalletRescanReserver(CWallet& w) : m_wallet(w) {}
 
-    bool reserve()
+    bool reserve(bool with_passphrase = false)
     {
         assert(!m_could_reserve);
         if (m_wallet.fScanningWallet.exchange(true)) {
             return false;
         }
+        m_wallet.m_scanning_with_passphrase.exchange(with_passphrase);
         m_wallet.m_scanning_start = GetTimeMillis();
         m_wallet.m_scanning_progress = 0;
         m_could_reserve = true;
@@ -985,6 +988,7 @@ public:
     {
         if (m_could_reserve) {
             m_wallet.fScanningWallet = false;
+            m_wallet.m_scanning_with_passphrase = false;
         }
     }
 };