Use filesystem of C++17

[nguyenpham]

Use filesystem of C++17 to improve Lc0 filesystem functions:

• CreateDirectory
• GetFileList
• GetFileSize
• GetFileTime

This PR also fixed issue #831 since the improved function GetFileSize returns 0 instead of throwing an exception.

Tested with macOS. Need verifies with other OSs.

[nguyenpham]

The test build system (build clang) printed out some similar blames such as:

/root/project/build-clang/../src/utils/filesystem.cc:37: undefined reference to std::filesystem::create_directory(std::filesystem::__cxx11::path const&)

I guess cxx11 is C++11 and I don't understand why it uses cxx11 for testing but not cxx17? Can someone help?

[ddobbelaere]

The C++ 17 filesystem functionality is poorly supported on some platforms up to the point that it is close to unusable for our cross-platform engine. See e.g. the discussion in #1276 .

[borg323]

The following works around the issue (tested with g++-7), but there are still issues with windows and android builds:

diff --git a/meson.build b/meson.build
index 19fe77c..42dfc74 100644
--- a/meson.build
+++ b/meson.build
@@ -501,6 +501,19 @@ endif
 
 
 #############################################################################
+## Workarounds
+#############################################################################
+if not cc.has_header('filesystem')
+  code = '''#include <experimental/filesystem>
+    int main() {return std::experimental::filesystem::exists(".");}'''
+  # Need more here
+  if cc.links(code, args : '-lstdc++fs', name : '<experimental/filesystem> test')
+    add_project_link_arguments(['-lstdc++fs'], language : 'cpp')
+  endif
+endif
+
+
+#############################################################################
 ## Main Executable
 #############################################################################
 
diff --git a/src/utils/filesystem.cc b/src/utils/filesystem.cc
index c59e8d9..128d0b8 100644
--- a/src/utils/filesystem.cc
+++ b/src/utils/filesystem.cc
@@ -28,7 +28,15 @@
 #include "utils/exception.h"
 #include "utils/filesystem.h"
 
+#if __has_include(<filesystem>)
 #include <filesystem>
+#else
+#include <experimental/filesystem>
+// This works for the compilers we care about.
+namespace std {
+namespace filesystem = experimental::filesystem;
+}
+#endif
 
 namespace lczero {
 
@@ -46,7 +54,7 @@ std::vector<std::string> GetFileList(const std::string& directory) {
     const std::filesystem::path p(directory);
     if (std::filesystem::exists(p)) {
       for (const auto & entry : std::filesystem::directory_iterator(directory)) {
-        if (!entry.is_symlink()) {
+        if (!std::filesystem::is_symlink(entry.path())) {
           result.push_back(entry.path().filename());
         }
       }

[borg323]

Unfortunately, it seems library support for on android was merged less than a month ago, and will be available in the forthcoming NDK R22: android/ndk#609.

There is no point continuing work on this for the time being.

[nguyenpham]

@borg323: Thanks for your effort. Let me try some workaround first, say create one version for Android, and one for the rest, in the hope the app for other OSs can get some benefit from the C++17 library.

[nguyenpham]

I am going to close this PR. Doable but not nice and more trouble to create three versions, one for Win, one for Android and one for the rest.

[borg323]

We can revisit this in a few months when android NDK R22 is released.