Merge bitcoin/bitcoin#22937: refactor: Forbid calling unsafe fs::path(std::string) constructor and fs::path::string() method

6544ea5035268025207d2402db2f7d90fde947a6 refactor: Block unsafe fs::path std::string conversion calls (Russell Yanofsky)
b39a477ec69a51b2016d3a8c70c0c77670f87f2b refactor: Add fs::PathToString, fs::PathFromString, u8string, u8path functions (Russell Yanofsky)

Pull request description:

  The `fs::path` class has a `std::string` constructor which will implicitly convert from strings. Implicit conversions like this are not great in general because they can hide complexity and inefficiencies in the code, but this case is especially bad, because after the transition from `boost::filesystem` to `std::filesystem` in #20744 the behavior of this constructor on windows will be more complicated and can mangle path strings. The `fs::path` class also has a `.string()` method which is inverse of the constructor and has the same problems.

  Fix this by replacing the unsafe method calls with `PathToString` and `PathFromString` function calls, and by forbidding unsafe method calls in the future.

ACKs for top commit:
  kiminuo:
    ACK 6544ea5035268025207d2402db2f7d90fde947a6
  laanwj:
    Code review ACK 6544ea5035268025207d2402db2f7d90fde947a6
  hebasto:
    re-ACK 6544ea5035268025207d2402db2f7d90fde947a6, only added `fsbridge_stem` test case, updated comment, and rebased since my [previous](https://github.com/bitcoin/bitcoin/pull/22937#pullrequestreview-765503126) review. Verified with the following command:

Tree-SHA512: c36324740eb4ee55151146626166c00d5ccc4b6f3df777e75c112bcb4d1db436c1d9cc8c29a1e7fb96051457d317961ab42e6c380c3be2771d135771b2b49fa0

1 files changed, 9 insertions, 9 deletions

diff --git a/src/dbwrapper.cpp b/src/dbwrapper.cpp
index bcaf746167..2fdc54464a 100644
--- a/src/dbwrapper.cpp
+++ b/src/dbwrapper.cpp
@@ -115,7 +115,7 @@ static leveldb::Options GetOptions(size_t nCacheSize)
 }
 
 CDBWrapper::CDBWrapper(const fs::path& path, size_t nCacheSize, bool fMemory, bool fWipe, bool obfuscate)
-    : m_name{path.stem().string()}
+    : m_name{fs::PathToString(path.stem())}
 {
     penv = nullptr;
     readoptions.verify_checksums = true;
@@ -129,21 +129,21 @@ CDBWrapper::CDBWrapper(const fs::path& path, size_t nCacheSize, bool fMemory, bo
         options.env = penv;
     } else {
         if (fWipe) {
-            LogPrintf("Wiping LevelDB in %s\n", path.string());
-            leveldb::Status result = leveldb::DestroyDB(path.string(), options);
+            LogPrintf("Wiping LevelDB in %s\n", fs::PathToString(path));
+            leveldb::Status result = leveldb::DestroyDB(fs::PathToString(path), options);
             dbwrapper_private::HandleError(result);
         }
         TryCreateDirectories(path);
-        LogPrintf("Opening LevelDB in %s\n", path.string());
+        LogPrintf("Opening LevelDB in %s\n", fs::PathToString(path));
     }
-    leveldb::Status status = leveldb::DB::Open(options, path.string(), &pdb);
+    leveldb::Status status = leveldb::DB::Open(options, fs::PathToString(path), &pdb);
     dbwrapper_private::HandleError(status);
     LogPrintf("Opened LevelDB successfully\n");
 
     if (gArgs.GetBoolArg("-forcecompactdb", false)) {
-        LogPrintf("Starting database compaction of %s\n", path.string());
+        LogPrintf("Starting database compaction of %s\n", fs::PathToString(path));
         pdb->CompactRange(nullptr, nullptr);
-        LogPrintf("Finished database compaction of %s\n", path.string());
+        LogPrintf("Finished database compaction of %s\n", fs::PathToString(path));
     }
 
     // The base-case obfuscation key, which is a noop.
@@ -160,10 +160,10 @@ CDBWrapper::CDBWrapper(const fs::path& path, size_t nCacheSize, bool fMemory, bo
         Write(OBFUSCATE_KEY_KEY, new_key);
         obfuscate_key = new_key;
 
-        LogPrintf("Wrote new obfuscate key for %s: %s\n", path.string(), HexStr(obfuscate_key));
+        LogPrintf("Wrote new obfuscate key for %s: %s\n", fs::PathToString(path), HexStr(obfuscate_key));
     }
 
-    LogPrintf("Using obfuscation key for %s: %s\n", path.string(), HexStr(obfuscate_key));
+    LogPrintf("Using obfuscation key for %s: %s\n", fs::PathToString(path), HexStr(obfuscate_key));
 }
 
 CDBWrapper::~CDBWrapper()