ArgsManager: return path by value from GetBlocksDirPath()

`ArgsManager::m_cached_blocks_path` is protected by
`ArgsManager::cs_args` and returning a reference to it after releasing
the mutex is unsafe.

To resolve this, return a copy of the path. This has some performance
penalty which is presumably ok, given that paths are a few 100s bytes
at most and `GetBlocksDirPath()` is not called often.

This silences the following (clang 18):

```
common/args.cpp:288:31: error: returning variable 'm_cached_blocks_path' by reference requires holding mutex 'cs_args' [-Werror,-Wthread-safety-reference-return]
  288 |     if (!path.empty()) return path;
      |                               ^
```

Do the same with
`ArgsManager::GetDataDir()`,
`ArgsManager::GetDataDirBase()` and
`ArgsManager::GetDataDirNet()`.

Author: vasild

src/common/args.cpp

@@ -277,7 +277,7 @@ fs::path ArgsManager::GetPathArg(std::string arg, const fs::path& default_value)
     return result.has_filename() ? result : result.parent_path();
 }
 
-const fs::path& ArgsManager::GetBlocksDirPath() const
+fs::path ArgsManager::GetBlocksDirPath() const
 {
     LOCK(cs_args);
     fs::path& path = m_cached_blocks_path;
@@ -302,7 +302,7 @@ const fs::path& ArgsManager::GetBlocksDirPath() const
     return path;
 }
 
-const fs::path& ArgsManager::GetDataDir(bool net_specific) const
+fs::path ArgsManager::GetDataDir(bool net_specific) const
 {
     LOCK(cs_args);
     fs::path& path = net_specific ? m_cached_network_datadir_path : m_cached_datadir_path;

src/common/args.h

@@ -215,21 +215,21 @@ class ArgsManager
      *
      * @return Blocks path which is network specific
      */
-    const fs::path& GetBlocksDirPath() const;
+    fs::path GetBlocksDirPath() const;
 
     /**
      * Get data directory path
      *
      * @return Absolute path on success, otherwise an empty path when a non-directory path would be returned
      */
-    const fs::path& GetDataDirBase() const { return GetDataDir(false); }
+    fs::path GetDataDirBase() const { return GetDataDir(false); }
 
     /**
      * Get data directory path with appended network identifier
      *
      * @return Absolute path on success, otherwise an empty path when a non-directory path would be returned
      */
-    const fs::path& GetDataDirNet() const { return GetDataDir(true); }
+    fs::path GetDataDirNet() const { return GetDataDir(true); }
 
     /**
      * Clear cached directory paths
@@ -420,7 +420,7 @@ class ArgsManager
      * @param net_specific Append network identifier to the returned path
      * @return Absolute path on success, otherwise an empty path when a non-directory path would be returned
      */
-    const fs::path& GetDataDir(bool net_specific) const;
+    fs::path GetDataDir(bool net_specific) const;
 
     /**
      * Return -regtest/-signet/-testnet/-chain= setting as a ChainType enum if a