Merge bitcoin/bitcoin#29040: refactor: Remove pre-C++20 code, fs::path cleanup

6666713041 refactor: Rename fs::path::u8string() to fs::path::utf8string() (MarcoFalke)
856c88776f ArgsManager: return path by value from GetBlocksDirPath() (Vasil Dimov)
fa3d9304e8 refactor: Remove pre-C++20 fs code (MarcoFalke)
fa00098e1a Add tests for C++20 std::u8string (MarcoFalke)
fa2bac08c2 refactor: Avoid copy/move in fs.h (MarcoFalke)
faea30227b refactor: Use C++20 std::chrono::days (MarcoFalke)

Pull request description:

  This:

  * Removes dead code.
  * Avoids unused copies in some places.
  * Adds copies in other places for safety.

ACKs for top commit:
  achow101:
    ACK 6666713041
  ryanofsky:
    Code review ACK 6666713041. Just documentation change since last review.
  stickies-v:
    re-ACK 6666713041

Tree-SHA512: 6176e44f30b310d51632ec2d3827c3819905d0ddc6a4b57acfcb6cfa1f9735176da75ee8ed4a4abd1296cb0b83bee9374cc6f91ffac87c19b63c435eeadf3f46

src/util/fs.cpp

@@ -1,4 +1,4 @@
-// Copyright (c) 2017-2022 The Bitcoin Core developers
+// Copyright (c) 2017-present The Bitcoin Core developers
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
@@ -18,6 +18,7 @@
 #endif
 
 #include <cassert>
+#include <cerrno>
 #include <string>
 
 namespace fsbridge {
@@ -130,4 +131,4 @@ std::string get_filesystem_error_message(const fs::filesystem_error& e)
 #endif
 }
 
-} // fsbridge
+} // namespace fsbridge

src/util/fs.h

@@ -1,4 +1,4 @@
-// Copyright (c) 2017-2022 The Bitcoin Core developers
+// Copyright (c) 2017-present The Bitcoin Core developers
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
@@ -14,6 +14,8 @@
 #include <ios>
 #include <ostream>
 #include <string>
+#include <system_error>
+#include <type_traits>
 #include <utility>
 
 /** Filesystem operations and types */
@@ -35,7 +37,7 @@ public:
     // Allow path objects arguments for compatibility.
     path(std::filesystem::path path) : std::filesystem::path::path(std::move(path)) {}
     path& operator=(std::filesystem::path path) { std::filesystem::path::operator=(std::move(path)); return *this; }
-    path& operator/=(std::filesystem::path path) { std::filesystem::path::operator/=(path); return *this; }
+    path& operator/=(const std::filesystem::path& path) { std::filesystem::path::operator/=(path); return *this; }
 
     // Allow literal string arguments, which are safe as long as the literals are ASCII.
     path(const char* c) : std::filesystem::path(c) {}
@@ -52,12 +54,15 @@ public:
     // Disallow std::string conversion method to avoid locale-dependent encoding on windows.
     std::string string() const = delete;
 
-    std::string u8string() const
+    /**
+     * Return a UTF-8 representation of the path as a std::string, for
+     * compatibility with code using std::string. For code using the newer
+     * std::u8string type, it is more efficient to call the inherited
+     * std::filesystem::path::u8string method instead.
+     */
+    std::string utf8string() const
     {
-        const auto& utf8_str{std::filesystem::path::u8string()};
-        // utf8_str might either be std::string (C++17) or std::u8string
-        // (C++20). Convert both to std::string. This method can be removed
-        // after switching to C++20.
+        const std::u8string& utf8_str{std::filesystem::path::u8string()};
         return std::string{utf8_str.begin(), utf8_str.end()};
     }
 
@@ -69,11 +74,7 @@ public:
 
 static inline path u8path(const std::string& utf8_str)
 {
-#if __cplusplus < 202002L
-    return std::filesystem::u8path(utf8_str);
-#else
     return std::filesystem::path(std::u8string{utf8_str.begin(), utf8_str.end()});
-#endif
 }
 
 // Disallow implicit std::string conversion for absolute to avoid
@@ -97,9 +98,9 @@ static inline auto quoted(const std::string& s)
 }
 
 // Allow safe path append operations.
-static inline path operator/(path p1, path p2)
+static inline path operator/(path p1, const path& p2)
 {
-    p1 /= std::move(p2);
+    p1 /= p2;
     return p1;
 }
 static inline path operator/(path p1, const char* p2)
@@ -140,7 +141,7 @@ static inline bool copy_file(const path& from, const path& to, copy_options opti
  * Because \ref PathToString and \ref PathFromString functions don't specify an
  * encoding, they are meant to be used internally, not externally. They are not
  * appropriate to use in applications requiring UTF-8, where
- * fs::path::u8string() and fs::u8path() methods should be used instead. Other
+ * fs::path::u8string() / fs::path::utf8string() and fs::u8path() methods should be used instead. Other
  * applications could require still different encodings. For example, JSON, XML,
  * or URI applications might prefer to use higher-level escapes (\uXXXX or
  * &XXXX; or %XX) instead of multibyte encoding. Rust, Python, Java applications
@@ -154,13 +155,13 @@ static inline std::string PathToString(const path& path)
     // use here, because these methods encode the path using C++'s narrow
     // multibyte encoding, which on Windows corresponds to the current "code
     // page", which is unpredictable and typically not able to represent all
-    // valid paths. So fs::path::u8string() and
+    // valid paths. So fs::path::utf8string() and
     // fs::u8path() functions are used instead on Windows. On
-    // POSIX, u8string/u8path functions are not safe to use because paths are
+    // POSIX, u8string/utf8string/u8path functions are not safe to use because paths are
     // not always valid UTF-8, so plain string methods which do not transform
     // the path there are used.
 #ifdef WIN32
-    return path.u8string();
+    return path.utf8string();
 #else
     static_assert(std::is_same<path::string_type, std::string>::value, "PathToString not implemented on this platform");
     return path.std::filesystem::path::string();