Merge bitcoin/bitcoin#25594: refactor: Return BResult from restoreWallet

fa475e9c7977a952617738f2ee8cf600c07d4df8 refactor: Return BResult from restoreWallet (MacroFake)
fa8de09edc9ec4e6d171df80f746174a0ec58afb Prepare BResult for non-copyable types (MacroFake)

Pull request description:

  This avoids the `error` in-out param (and if `warnings` is added to `BResult`, it will avoid passing that in-out param as well).

  Also, as it is needed for this change, prepare `BResult` for non-copyable types.

ACKs for top commit:
  w0xlt:
    reACK https://github.com/bitcoin/bitcoin/pull/25594/commits/fa475e9c7977a952617738f2ee8cf600c07d4df8
  ryanofsky:
    Code review ACK fa475e9c7977a952617738f2ee8cf600c07d4df8. Changes since last review were replacing auto with explicit type and splitting commits

Tree-SHA512: 46350883572f13721ddd198f5dfb88d2fa58ebcbda416f74da3563ea15c920fb1e6ff30558526a4ac91c36c21e6afe27751a4e51b7b8bcbcbe805209f4e9014b

4 files changed, 18 insertions, 9 deletions

diff --git a/src/interfaces/wallet.h b/src/interfaces/wallet.h
index fe198c999b..e29fefae56 100644
--- a/src/interfaces/wallet.h
+++ b/src/interfaces/wallet.h
@@ -329,7 +329,7 @@ public:
    virtual std::string getWalletDir() = 0;
 
    //! Restore backup wallet
-   virtual std::unique_ptr<Wallet> restoreWallet(const fs::path& backup_file, const std::string& wallet_name, bilingual_str& error, std::vector<bilingual_str>& warnings) = 0;
+   virtual BResult<std::unique_ptr<Wallet>> restoreWallet(const fs::path& backup_file, const std::string& wallet_name, std::vector<bilingual_str>& warnings) = 0;
 
    //! Return available wallets in wallet directory.
    virtual std::vector<std::string> listWalletDir() = 0;
diff --git a/src/qt/walletcontroller.cpp b/src/qt/walletcontroller.cpp
index 11140c5da9..01d84624e8 100644
--- a/src/qt/walletcontroller.cpp
+++ b/src/qt/walletcontroller.cpp
@@ -391,9 +391,10 @@ void RestoreWalletActivity::restore(const fs::path& backup_file, const std::stri
         tr("Restoring Wallet <b>%1</b>…").arg(name.toHtmlEscaped()));
 
     QTimer::singleShot(0, worker(), [this, backup_file, wallet_name] {
-        std::unique_ptr<interfaces::Wallet> wallet = node().walletLoader().restoreWallet(backup_file, wallet_name, m_error_message, m_warning_message);
+        auto wallet{node().walletLoader().restoreWallet(backup_file, wallet_name, m_warning_message)};
 
-        if (wallet) m_wallet_model = m_wallet_controller->getOrCreateWallet(std::move(wallet));
+        m_error_message = wallet ? bilingual_str{} : wallet.GetError();
+        if (wallet) m_wallet_model = m_wallet_controller->getOrCreateWallet(wallet.ReleaseObj());
 
         QTimer::singleShot(0, this, &RestoreWalletActivity::finish);
     });
diff --git a/src/util/result.h b/src/util/result.h
index dcf5edaa5b..2f586a4c9b 100644
--- a/src/util/result.h
+++ b/src/util/result.h
@@ -6,6 +6,7 @@
 #define BITCOIN_UTIL_RESULT_H
 
 #include <util/translation.h>
+
 #include <variant>
 
 /*
@@ -18,9 +19,9 @@ private:
     std::variant<bilingual_str, T> m_variant;
 
 public:
-    BResult() : m_variant(Untranslated("")) {}
-    BResult(const T& _obj) : m_variant(_obj) {}
-    BResult(const bilingual_str& error) : m_variant(error) {}
+    BResult() : m_variant{Untranslated("")} {}
+    BResult(T obj) : m_variant{std::move(obj)} {}
+    BResult(bilingual_str error) : m_variant{std::move(error)} {}
 
     /* Whether the function succeeded or not */
     bool HasRes() const { return std::holds_alternative<T>(m_variant); }
@@ -30,6 +31,11 @@ public:
         assert(HasRes());
         return std::get<T>(m_variant);
     }
+    T ReleaseObj()
+    {
+        assert(HasRes());
+        return std::move(std::get<T>(m_variant));
+    }
 
     /* In case of failure, the error cause */
     const bilingual_str& GetError() const {
diff --git a/src/wallet/interfaces.cpp b/src/wallet/interfaces.cpp
index 547d972c8d..23f91d9b3a 100644
--- a/src/wallet/interfaces.cpp
+++ b/src/wallet/interfaces.cpp
@@ -569,11 +569,13 @@ public:
         options.require_existing = true;
         return MakeWallet(m_context, LoadWallet(m_context, name, true /* load_on_start */, options, status, error, warnings));
     }
-    std::unique_ptr<Wallet> restoreWallet(const fs::path& backup_file, const std::string& wallet_name, bilingual_str& error, std::vector<bilingual_str>& warnings) override
+    BResult<std::unique_ptr<Wallet>> restoreWallet(const fs::path& backup_file, const std::string& wallet_name, std::vector<bilingual_str>& warnings) override
     {
         DatabaseStatus status;
-
-        return MakeWallet(m_context, RestoreWallet(m_context, backup_file, wallet_name, /*load_on_start=*/true, status, error, warnings));
+        bilingual_str error;
+        BResult<std::unique_ptr<Wallet>> wallet{MakeWallet(m_context, RestoreWallet(m_context, backup_file, wallet_name, /*load_on_start=*/true, status, error, warnings))};
+        if (!wallet) return error;
+        return wallet;
     }
     std::string getWalletDir() override
     {