Merge bitcoin#26005: Wallet: Fix error handling (copy_file failure in RestoreWallet, and in general via interfaces)

c3e5365 Bugfix: Wallet: Return util::Error rather than non-error nullptr when CreateWallet/LoadWallet/RestoreWallet fail (Luke Dashjr)
335ff98 Bugfix: Wallet: Wrap RestoreWallet content in a try block to ensure exceptions become returned errors and incomplete wallet directory is removed (Luke Dashjr)

Pull request description:

  Bug 1: `copy_file` can throw exceptions, but `RestoreWallet` is expected to return a nullptr with a populated `errors` parameter. This is fixed by wrapping `copy_file` and `LoadWallet` (for good measure) in a `try` block, and converting any exceptions to the intended return style.

  Bug 2: `util::Result` turns what would have been a `false` unique_ptr into a `true` nullptr result, which leads to nullptr dereferences in at least the 3 cases of wallet creation/loading/restoring. This is fixed by keeping the pointer as a plain `std::unique_ptr` until actually returning it (ie, after the nullptr check).

  Fixes bitcoin-core/gui#661

ACKs for top commit:
  achow101:
    ACK c3e5365

Tree-SHA512: 4291b3dbbb147acea2e63a704324c9371bc16ecb4237f8753729b0b0a6e55c9758ad61bfe8bd432fd7b0bae95d8b63a9831e61ac8b8d5c0197b550a2e0f4a105

Author: fanquake

src/wallet/interfaces.cpp

@@ -560,8 +560,12 @@ class WalletLoaderImpl : public WalletLoader
         options.create_flags = wallet_creation_flags;
         options.create_passphrase = passphrase;
         bilingual_str error;
-        util::Result<std::unique_ptr<Wallet>> wallet{MakeWallet(m_context, CreateWallet(m_context, name, /*load_on_start=*/true, options, status, error, warnings))};
-        return wallet ? std::move(wallet) : util::Error{error};
+        std::unique_ptr<Wallet> wallet{MakeWallet(m_context, CreateWallet(m_context, name, /*load_on_start=*/true, options, status, error, warnings))};
+        if (wallet) {
+            return {std::move(wallet)};
+        } else {
+            return util::Error{error};
+        }
     }
     util::Result<std::unique_ptr<Wallet>> loadWallet(const std::string& name, std::vector<bilingual_str>& warnings) override
     {
@@ -570,15 +574,23 @@ class WalletLoaderImpl : public WalletLoader
         ReadDatabaseArgs(*m_context.args, options);
         options.require_existing = true;
         bilingual_str error;
-        util::Result<std::unique_ptr<Wallet>> wallet{MakeWallet(m_context, LoadWallet(m_context, name, /*load_on_start=*/true, options, status, error, warnings))};
-        return wallet ? std::move(wallet) : util::Error{error};
+        std::unique_ptr<Wallet> wallet{MakeWallet(m_context, LoadWallet(m_context, name, /*load_on_start=*/true, options, status, error, warnings))};
+        if (wallet) {
+            return {std::move(wallet)};
+        } else {
+            return util::Error{error};
+        }
     }
     util::Result<std::unique_ptr<Wallet>> restoreWallet(const fs::path& backup_file, const std::string& wallet_name, std::vector<bilingual_str>& warnings) override
     {
         DatabaseStatus status;
         bilingual_str error;
-        util::Result<std::unique_ptr<Wallet>> wallet{MakeWallet(m_context, RestoreWallet(m_context, backup_file, wallet_name, /*load_on_start=*/true, status, error, warnings))};
-        return wallet ? std::move(wallet) : util::Error{error};
+        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 {std::move(wallet)};
+        } else {
+            return util::Error{error};
+        }
     }
     std::string getWalletDir() override
     {

src/wallet/wallet.cpp

@@ -386,25 +386,31 @@ std::shared_ptr<CWallet> RestoreWallet(WalletContext& context, const fs::path& b
     ReadDatabaseArgs(*context.args, options);
     options.require_existing = true;
 
-    if (!fs::exists(backup_file)) {
-        error = Untranslated("Backup file does not exist");
-        status = DatabaseStatus::FAILED_INVALID_BACKUP_FILE;
-        return nullptr;
-    }
-
     const fs::path wallet_path = fsbridge::AbsPathJoin(GetWalletDir(), fs::u8path(wallet_name));
+    auto wallet_file = wallet_path / "wallet.dat";
+    std::shared_ptr<CWallet> wallet;
 
-    if (fs::exists(wallet_path) || !TryCreateDirectories(wallet_path)) {
-        error = Untranslated(strprintf("Failed to create database path '%s'. Database already exists.", fs::PathToString(wallet_path)));
-        status = DatabaseStatus::FAILED_ALREADY_EXISTS;
-        return nullptr;
-    }
+    try {
+        if (!fs::exists(backup_file)) {
+            error = Untranslated("Backup file does not exist");
+            status = DatabaseStatus::FAILED_INVALID_BACKUP_FILE;
+            return nullptr;
+        }
 
-    auto wallet_file = wallet_path / "wallet.dat";
-    fs::copy_file(backup_file, wallet_file, fs::copy_options::none);
+        if (fs::exists(wallet_path) || !TryCreateDirectories(wallet_path)) {
+            error = Untranslated(strprintf("Failed to create database path '%s'. Database already exists.", fs::PathToString(wallet_path)));
+            status = DatabaseStatus::FAILED_ALREADY_EXISTS;
+            return nullptr;
+        }
 
-    auto wallet = LoadWallet(context, wallet_name, load_on_start, options, status, error, warnings);
+        fs::copy_file(backup_file, wallet_file, fs::copy_options::none);
 
+        wallet = LoadWallet(context, wallet_name, load_on_start, options, status, error, warnings);
+    } catch (const std::exception& e) {
+        assert(!wallet);
+        if (!error.empty()) error += Untranslated("\n");
+        error += strprintf(Untranslated("Unexpected exception: %s"), e.what());
+    }
     if (!wallet) {
         fs::remove(wallet_file);
         fs::remove(wallet_path);