feat(io): support Windows junctions

We support symlinks on Windows but not junctions (which are similar).
Teach canonicalize_path to follow junctions, handling them similarly to
symlinks. (The implementation is wrong, but wrong in some edge cases
(#1200) is perhaps a
bit better than consistently crashing (QLJS_UNIMPLEMENTED).)

src/quick-lint-js/io/file-canonical.cpp

@@ -694,9 +694,34 @@ class Windows_Path_Canonicalizer
     ::REPARSE_DATA_BUFFER *reparse_data =
         reinterpret_cast<::REPARSE_DATA_BUFFER *>(reparse_data_raw.data());
     switch (reparse_data->ReparseTag) {
-    case IO_REPARSE_TAG_MOUNT_POINT:
-      QLJS_UNIMPLEMENTED();
+    // NOTE(strager): Mount points are also known as junctions.
+    case IO_REPARSE_TAG_MOUNT_POINT: {
+      // TODO(#1200): Make '..' after following the junction escape the
+      // junction.
+
+      auto &mount_point = reparse_data->MountPointReparseBuffer;
+      std::wstring &new_readlink_buffer =
+          readlink_buffers_[1 - used_readlink_buffer_];
+      // NOTE[reparse-point-null-terminator]: The path is not null-terminated.
+      std::wstring_view target(
+          &mount_point
+               .PathBuffer[mount_point.SubstituteNameOffset / sizeof(wchar_t)],
+          mount_point.SubstituteNameLength / sizeof(wchar_t));
+
+      // NOTE(strager): Mount point targets are always absolute.
+      new_readlink_buffer.reserve(target.size() + path_to_process_.size());
+      new_readlink_buffer = target;
+      new_readlink_buffer += path_to_process_;
+      path_to_process_ = new_readlink_buffer;
+      // After assigning to path_to_process_,
+      // readlink_buffers_[used_readlink_buffer_] is no longer in use.
+      swap_readlink_buffers();
+
+      Result<void, Canonicalizing_Path_IO_Error> r = process_start_of_path();
+      if (!r.ok()) return r.propagate();
+
       break;
+    }
 
     case IO_REPARSE_TAG_SYMLINK: {
       auto &symlink = reparse_data->SymbolicLinkReparseBuffer;

src/quick-lint-js/io/file.cpp

@@ -11,6 +11,7 @@
 #include <cstring>
 #include <optional>
 #include <quick-lint-js/assert.h>
+#include <quick-lint-js/container/monotonic-allocator.h>
 #include <quick-lint-js/io/file-handle.h>
 #include <quick-lint-js/io/file.h>
 #include <quick-lint-js/port/char8.h>
@@ -40,6 +41,7 @@
 
 #if QLJS_HAVE_WINDOWS_H
 #include <quick-lint-js/port/windows.h>
+#include <winioctl.h>
 #endif
 
 #if QLJS_HAVE_WINDOWS_H
@@ -565,6 +567,80 @@ void delete_posix_symbolic_link_or_exit(const char *path) {
     result.error().print_and_exit();
   }
 }
+
+#if defined(QLJS_FILE_WINDOWS)
+void create_windows_junction_or_exit(const char *path, const char *target) {
+  Monotonic_Allocator memory("create_windows_junction");
+
+  std::optional<std::wstring> wpath = mbstring_to_wstring(path);
+  std::optional<std::wstring> wtarget = mbstring_to_wstring(target);
+  if (!wpath.has_value() || !wtarget.has_value()) {
+    std::fprintf(stderr, "fatal: cannot convert path to wide string\n");
+    std::abort();
+  }
+  if (!::CreateDirectoryW(wpath->c_str(), /*lpSecurityAttributes=*/nullptr)) {
+    std::fprintf(stderr, "fatal: CreateDirectoryW failed\n");
+    std::abort();
+  }
+  Windows_Handle_File file(
+      ::CreateFileW(wpath->c_str(), GENERIC_WRITE,
+                    FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE,
+                    /*lpSecurityAttributes=*/nullptr, OPEN_EXISTING,
+                    FILE_FLAG_BACKUP_SEMANTICS | FILE_FLAG_OPEN_REPARSE_POINT,
+                    /*hTemplateFile=*/nullptr));
+
+  std::size_t reparse_data_data_size =
+      offsetof(
+          decltype(
+              std::declval<::REPARSE_DATA_BUFFER>().MountPointReparseBuffer),
+          PathBuffer) +
+      // SubsituteName. +1 for null terminator.
+      (wtarget->size() + 1) * sizeof(wchar_t) +
+      // PrintName. Empty string with null terminator.
+      sizeof(wchar_t);
+  std::size_t reparse_data_total_size =
+      offsetof(::REPARSE_DATA_BUFFER, MountPointReparseBuffer) +
+      reparse_data_data_size;
+  ::REPARSE_DATA_BUFFER *reparse_data =
+      reinterpret_cast<::REPARSE_DATA_BUFFER *>(memory.allocate(
+          reparse_data_total_size, alignof(::REPARSE_DATA_BUFFER)));
+  reparse_data->ReparseTag = IO_REPARSE_TAG_MOUNT_POINT;
+  reparse_data->ReparseDataLength = reparse_data_data_size;
+  reparse_data->Reserved = 0;
+
+  // Write the null-terminated SubstituteName (*wtarget) followed by the
+  // null-terminated PrintName (L"").
+  {
+    wchar_t *path_buffer = reparse_data->MountPointReparseBuffer.PathBuffer;
+    auto current_offset = [&]() -> ::USHORT {
+      return narrow_cast<::USHORT>(
+          (reinterpret_cast<char *>(path_buffer) -
+           reinterpret_cast<char *>(
+               reparse_data->MountPointReparseBuffer.PathBuffer)));
+    };
+    reparse_data->MountPointReparseBuffer.SubstituteNameOffset =
+        current_offset();
+    reparse_data->MountPointReparseBuffer.SubstituteNameLength =
+        wtarget->size() * sizeof(wchar_t);
+    path_buffer = std::copy(wtarget->begin(), wtarget->end(), path_buffer);
+    *path_buffer++ = L'\0';
+
+    reparse_data->MountPointReparseBuffer.PrintNameOffset = current_offset();
+    reparse_data->MountPointReparseBuffer.PrintNameLength = 0;
+    *path_buffer++ = L'\0';
+  }
+
+  if (!::DeviceIoControl(file.get(), FSCTL_SET_REPARSE_POINT, reparse_data,
+                         narrow_cast<::DWORD>(reparse_data_total_size),
+                         /*lpOutBuffer=*/nullptr,
+                         /*nOutBufferSize=*/0,
+                         /*lpBytesReturned=*/nullptr,
+                         /*lpOverlapped=*/nullptr)) {
+    std::fprintf(stderr, "fatal: DeviceIoControl failed\n");
+    std::abort();
+  }
+}
+#endif
 }
 
 #endif

src/quick-lint-js/io/file.h

@@ -115,6 +115,10 @@ void create_posix_file_symbolic_link_or_exit(const char *path,
 
 Result<void, Delete_File_IO_Error> delete_posix_symbolic_link(const char *path);
 void delete_posix_symbolic_link_or_exit(const char *path);
+
+// target must begin with \??\.
+void create_windows_junction_or_exit(const char *path, const char *target);
+
 }
 
 #endif

test/test-file-canonical.cpp

@@ -723,6 +723,102 @@ TEST_F(Test_File_Canonical,
   EXPECT_SAME_FILE(canonical->path(), input_path);
 }
 
+#if defined(_WIN32)
+TEST_F(Test_File_Canonical, canonical_path_resolves_directory_junctions) {
+  std::string temp_dir = this->make_temporary_directory();
+  create_directory_or_exit(temp_dir + "/realdir");
+  create_windows_junction_or_exit(
+      (temp_dir + "/linkdir").c_str(),
+      ("\\??\\" + temp_dir + QLJS_PREFERRED_PATH_DIRECTORY_SEPARATOR "realdir")
+          .c_str());
+  write_file_or_exit(temp_dir + "/realdir/temp.js", u8""_sv);
+
+  std::string input_path = temp_dir + "/linkdir/temp.js";
+  Result<Canonical_Path_Result, Canonicalize_Path_IO_Error> canonical =
+      canonicalize_path(input_path);
+  ASSERT_TRUE(canonical.ok()) << canonical.error().to_string();
+
+  EXPECT_THAT(std::string(canonical->path()),
+              AnyOf(HasSubstr("/realdir/"), HasSubstr("\\realdir\\")));
+  EXPECT_THAT(std::string(canonical->path()), Not(HasSubstr("/linkdir/")));
+  EXPECT_THAT(std::string(canonical->path()), Not(HasSubstr("\\linkdir\\")));
+  EXPECT_SAME_FILE(canonical->path(), temp_dir + "/realdir/temp.js");
+  EXPECT_SAME_FILE(canonical->path(), input_path);
+}
+#endif
+
+#if defined(_WIN32)
+TEST_F(Test_File_Canonical, symlink_with_dot_dot_escapes_symlinked_dir) {
+  // $temp_dir\dir [directory]
+  // $temp_dir\dir\file.txt [file]
+  // $temp_dir\dir\subdir [directory]
+  // $temp_dir\dir\subdir\filelink.txt [symlink] -> ..\file.txt
+  // $temp_dir\linkdir [junction] -> $temp_dir\dir\subdir
+  // $temp_dir\file.txt [file]
+  //
+  // $temp_dir\linkdir\filelink.txt
+  // is the same as $temp_dir\dir\subdir\..\file.txt
+  // is the same as $temp_dir\dir\file.txt
+
+  std::string temp_dir = this->make_temporary_directory();
+  create_directory_or_exit(temp_dir + "/dir");
+  write_file_or_exit(temp_dir + "/dir/file.txt", u8"$tempdir/dir/file.txt");
+  create_directory_or_exit(temp_dir + "/dir/subdir");
+  create_posix_file_symbolic_link_or_exit(
+      (temp_dir + "/dir/subdir/filelink.txt").c_str(), "..\\file.txt");
+  create_posix_directory_symbolic_link(
+      (temp_dir + "/linkdir").c_str(),
+      "dir" QLJS_PREFERRED_PATH_DIRECTORY_SEPARATOR "subdir");
+  write_file_or_exit(temp_dir + "/file.txt", u8"$tempdir/file.txt");
+
+  std::string input_path = temp_dir + "/linkdir/filelink.txt";
+  Result<Canonical_Path_Result, Canonicalize_Path_IO_Error> canonical =
+      canonicalize_path(input_path);
+  ASSERT_TRUE(canonical.ok()) << canonical.error().to_string();
+
+  EXPECT_SAME_FILE(canonical->path(), temp_dir + "/dir/subdir/../file.txt");
+  EXPECT_SAME_FILE(canonical->path(), temp_dir + "/dir/file.txt");
+}
+#endif
+
+#if defined(_WIN32)
+// TODO(#1200): Fix canonicalize_file when it encounters junctions.
+TEST_F(Test_File_Canonical, DISABLED_symlink_with_dot_dot_escapes_junction) {
+  // $temp_dir\dir [directory]
+  // $temp_dir\dir\file.txt [file]
+  // $temp_dir\dir\subdir [directory]
+  // $temp_dir\dir\subdir\filelink.txt [symlink] -> ..\file.txt
+  // $temp_dir\linkdir [junction] -> $temp_dir\dir\subdir
+  // $temp_dir\file.txt [file]
+  //
+  // $temp_dir\linkdir\filelink.txt
+  // is the same as $temp_dir\linkdir\..\file.txt
+  // is the same as $temp_dir\file.txt
+
+  std::string temp_dir = this->make_temporary_directory();
+  create_directory_or_exit(temp_dir + "/dir");
+  write_file_or_exit(temp_dir + "/dir/file.txt", u8"$tempdir/dir/file.txt");
+  create_directory_or_exit(temp_dir + "/dir/subdir");
+  create_posix_file_symbolic_link_or_exit(
+      (temp_dir + "/dir/subdir/filelink.txt").c_str(), "..\\file.txt");
+  create_windows_junction_or_exit(
+      (temp_dir + "/linkdir").c_str(),
+      ("\\??\\" + temp_dir +
+       QLJS_PREFERRED_PATH_DIRECTORY_SEPARATOR
+       "dir" QLJS_PREFERRED_PATH_DIRECTORY_SEPARATOR "subdir")
+          .c_str());
+  write_file_or_exit(temp_dir + "/file.txt", u8"$tempdir/file.txt");
+
+  std::string input_path = temp_dir + "/linkdir/filelink.txt";
+  Result<Canonical_Path_Result, Canonicalize_Path_IO_Error> canonical =
+      canonicalize_path(input_path);
+  ASSERT_TRUE(canonical.ok()) << canonical.error().to_string();
+
+  EXPECT_SAME_FILE(canonical->path(), temp_dir + "/linkdir/../file.txt");
+  EXPECT_SAME_FILE(canonical->path(), temp_dir + "/file.txt");
+}
+#endif
+
 TEST_F(Test_File_Canonical,
        canonical_path_resolves_directory_relative_symlinks) {
   std::string temp_dir = this->make_temporary_directory();