Disallow more unsafe string->path conversions allowed by path append operators

Add more fs::path operator/ and operator+ overloads to prevent unsafe
string->path conversions on Windows that would cause strings to be
decoded according to the current Windows locale & code page instead of
the correct string encoding.

Update application code to deal with loss of implicit string->path
conversions by calling fs::u8path or fs::PathFromString explicitly, or
by just changing variable types from std::string to fs::path to avoid
conversions altoghther, or make them happen earlier.

In all cases, there's no change in behavior either (1) because strings
only contained ASCII characters and would be decoded the same regardless
of what encoding was used, or (2) because of the 1:1 mapping between
paths and strings using the PathToString and PathFromString functions.

Co-authored-by: Hennadii Stepanov <32963518+hebasto@users.noreply.github.com>

src/wallet/test/db_tests.cpp

@@ -15,22 +15,22 @@
 namespace wallet {
 BOOST_FIXTURE_TEST_SUITE(db_tests, BasicTestingSetup)
 
-static std::shared_ptr<BerkeleyEnvironment> GetWalletEnv(const fs::path& path, std::string& database_filename)
+static std::shared_ptr<BerkeleyEnvironment> GetWalletEnv(const fs::path& path, fs::path& database_filename)
 {
     fs::path data_file = BDBDataFile(path);
-    database_filename = fs::PathToString(data_file.filename());
+    database_filename = data_file.filename();
     return GetBerkeleyEnv(data_file.parent_path(), false);
 }
 
 BOOST_AUTO_TEST_CASE(getwalletenv_file)
 {
-    std::string test_name = "test_name.dat";
+    fs::path test_name = "test_name.dat";
     const fs::path datadir = m_args.GetDataDirNet();
     fs::path file_path = datadir / test_name;
     std::ofstream f{file_path};
     f.close();
 
-    std::string filename;
+    fs::path filename;
     std::shared_ptr<BerkeleyEnvironment> env = GetWalletEnv(file_path, filename);
     BOOST_CHECK_EQUAL(filename, test_name);
     BOOST_CHECK_EQUAL(env->Directory(), datadir);
@@ -38,10 +38,10 @@ BOOST_AUTO_TEST_CASE(getwalletenv_file)
 
 BOOST_AUTO_TEST_CASE(getwalletenv_directory)
 {
-    std::string expected_name = "wallet.dat";
+    fs::path expected_name = "wallet.dat";
     const fs::path datadir = m_args.GetDataDirNet();
 
-    std::string filename;
+    fs::path filename;
     std::shared_ptr<BerkeleyEnvironment> env = GetWalletEnv(datadir, filename);
     BOOST_CHECK_EQUAL(filename, expected_name);
     BOOST_CHECK_EQUAL(env->Directory(), datadir);
@@ -51,7 +51,7 @@ BOOST_AUTO_TEST_CASE(getwalletenv_g_dbenvs_multiple)
 {
     fs::path datadir = m_args.GetDataDirNet() / "1";
     fs::path datadir_2 = m_args.GetDataDirNet() / "2";
-    std::string filename;
+    fs::path filename;
 
     std::shared_ptr<BerkeleyEnvironment> env_1 = GetWalletEnv(datadir, filename);
     std::shared_ptr<BerkeleyEnvironment> env_2 = GetWalletEnv(datadir, filename);
@@ -65,7 +65,7 @@ BOOST_AUTO_TEST_CASE(getwalletenv_g_dbenvs_free_instance)
 {
     fs::path datadir = gArgs.GetDataDirNet() / "1";
     fs::path datadir_2 = gArgs.GetDataDirNet() / "2";
-    std::string filename;
+    fs::path filename;
 
     std::shared_ptr <BerkeleyEnvironment> env_1_a = GetWalletEnv(datadir, filename);
     std::shared_ptr <BerkeleyEnvironment> env_2_a = GetWalletEnv(datadir_2, filename);

src/wallet/test/fuzz/notifications.cpp

@@ -64,7 +64,7 @@ struct FuzzedWallet {
         assert(RemoveWallet(context, wallet, load_on_start, warnings));
         assert(warnings.empty());
         UnloadWallet(std::move(wallet));
-        fs::remove_all(GetWalletDir() / name);
+        fs::remove_all(GetWalletDir() / fs::PathFromString(name));
     }
     CScript GetScriptPubKey(FuzzedDataProvider& fuzzed_data_provider)
     {

src/wallet/test/init_test_fixture.cpp

@@ -17,8 +17,7 @@ InitWalletDirTestingSetup::InitWalletDirTestingSetup(const std::string& chainNam
 {
     m_wallet_loader = MakeWalletLoader(*m_node.chain, m_args);
 
-    std::string sep;
-    sep += fs::path::preferred_separator;
+    const auto sep = fs::path::preferred_separator;
 
     m_datadir = m_args.GetDataDirNet();
     m_cwd = fs::current_path();
@@ -27,8 +26,8 @@ InitWalletDirTestingSetup::InitWalletDirTestingSetup(const std::string& chainNam
     m_walletdir_path_cases["custom"] = m_datadir / "my_wallets";
     m_walletdir_path_cases["nonexistent"] = m_datadir / "path_does_not_exist";
     m_walletdir_path_cases["file"] = m_datadir / "not_a_directory.dat";
-    m_walletdir_path_cases["trailing"] = m_datadir / ("wallets" + sep);
-    m_walletdir_path_cases["trailing2"] = m_datadir / ("wallets" + sep + sep);
+    m_walletdir_path_cases["trailing"] = (m_datadir / "wallets") + sep;
+    m_walletdir_path_cases["trailing2"] = (m_datadir / "wallets") + sep + sep;
 
     fs::current_path(m_datadir);
     m_walletdir_path_cases["relative"] = "wallets";