fix folly::fs::lexically_normal

Summary:
Make the implementation of `folly::fs::lexically_normal` match C++17 `std::filesystem::path::lexically_normal`. The implementation from `boost::filesystem::path::lexically_normal` is easy to fall back on but has some differences.

The implementation is copied mutatis mutandis from https://github.com/gulrak/filesystem/ at tag v1.5.0, in which `ghc::filesystem` is a complete portable standalone implementation of the `std::filesystem` spec and released under the MIT license.

Reviewed By: Orvid

Differential Revision: D26572520

fbshipit-source-id: 5ce03f37c078ee4f62e3480d4abc259199867c8e

folly/portability/Filesystem.cpp

@@ -16,12 +16,8 @@
 
 #include <folly/portability/Filesystem.h>
 
-#include <boost/filesystem.hpp>
-
 namespace folly::fs {
 
-namespace boost_fs = boost::filesystem;
-
 #if __cpp_lib_filesystem >= 201703
 
 path lexically_normal_fn::operator()(path const& p) const {
@@ -30,8 +26,38 @@ path lexically_normal_fn::operator()(path const& p) const {
 
 #elif __cpp_lib_experimental_filesystem >= 201406
 
+//  mimic: std::filesystem::path::lexically_normal, C++17
+//  from: https://github.com/gulrak/filesystem/tree/v1.5.0, MIT
 path lexically_normal_fn::operator()(path const& p) const {
-  return path(boost_fs::path(p.native()).lexically_normal().native());
+  path dest;
+  bool lastDotDot = false;
+  for (path::string_type s : p) {
+    if (s == ".") {
+      dest /= "";
+      continue;
+    } else if (s == ".." && !dest.empty()) {
+      auto root = p.root_path();
+      if (dest == root) {
+        continue;
+      } else if (*(--dest.end()) != "..") {
+        auto drepr = dest.native();
+        if (drepr.back() == path::preferred_separator) {
+          drepr.pop_back();
+          dest = std::move(drepr);
+        }
+        dest.remove_filename();
+        continue;
+      }
+    }
+    if (!(s.empty() && lastDotDot)) {
+      dest /= s;
+    }
+    lastDotDot = s == "..";
+  }
+  if (dest.empty()) {
+    dest = ".";
+  }
+  return dest;
 }
 
 #else

folly/portability/test/FilesystemTest.cpp

@@ -24,14 +24,12 @@ using namespace testing;
 class FilesystemTest : public Test {};
 
 TEST_F(FilesystemTest, lexically_normal) {
-  // cases from cppreference.com
-  // implementation either std::filesystem or boost::filesystem
-  auto gold = folly::fs::which == folly::fs::which_enum::std;
-  auto sep = std::string({folly::fs::path::preferred_separator});
+  //  from: https://en.cppreference.com/w/cpp/filesystem/path/lexically_normal,
+  //      CC-BY-SA, GFDL
   EXPECT_THAT(
       folly::fs::lexically_normal("foo/./bar/..").native(),
-      Eq(folly::fs::path(gold ? "foo" + sep : "foo").native()));
+      Eq(folly::fs::path("foo/").make_preferred().native()));
   EXPECT_THAT(
       folly::fs::lexically_normal("foo/.///bar/../").native(),
-      Eq(folly::fs::path(gold ? "foo" + sep : "foo" + sep + ".").native()));
+      Eq(folly::fs::path("foo/").make_preferred().native()));
 }