Respect TMP environment variables for tmp::file on POSIX (#230)

Unfortunately std::tmpfile doesn't respect TMPDIR, TMP, and other environment variables for base tmp directories Implementation mirrors glibc's tmpfile but uses std::filesystem::temp_directory_path to get correct base path

---------

Co-authored-by: bugdea1er <bugdealer@icloud.com>

Author: IgnatSergeev

src/create_file.cpp

@@ -8,21 +8,73 @@
 #include <cerrno>
 #include <cstdio>
 #include <filesystem>
+#include <string>
 #include <system_error>
 
+#if __has_include(<unistd.h>)
+#include <sys/fcntl.h>
+#include <sys/stat.h>
+#include <unistd.h>
+#endif
+
 namespace tmp::detail {
 
 namespace fs = std::filesystem;
 
+namespace {
+
+#if __has_include(<unistd.h>)
+/// Creates and opens a binary temporary file as if by POSIX `mkstemp`
+/// @returns A file descriptor associated with the temporary file
+/// @throws fs::filesystem_error if cannot create a temporary file
+int create_file_descriptor() {
+  fs::path temp_dir = fs::temp_directory_path();
+  int descriptor    = int{};
+
+#ifdef __linux__
+  descriptor = open(temp_dir.c_str(), O_RDWR | O_TMPFILE, S_IRUSR | S_IWUSR);
+  if (descriptor >= 0) {
+    return descriptor;
+  }
+#endif
+
+  std::string path = temp_dir / "XXXXXX";
+
+  descriptor = mkstemp(path.data());
+  if (descriptor == -1) {
+    std::error_code ec = std::error_code(errno, std::system_category());
+    throw fs::filesystem_error("Cannot create a temporary file", ec);
+  }
+
+  unlink(path.data());
+
+  return descriptor;
+}
+#endif
+}    // namespace
+
 /// Creates and opens a binary temporary file as if by POSIX `tmpfile`
 /// @returns A pointer to the file stream associated with the temporary file
 /// @throws fs::filesystem_error if cannot create a temporary file
 std::FILE* create_file() {
+#if __has_include(<unistd.h>)
+  int descriptor = create_file_descriptor();
+
+  // TODO: let `filebuf` use the file descriptor without `FILE` wrapping
+  std::FILE* file = fdopen(descriptor, "wb+");
+  if (file == nullptr) {
+    std::error_code ec = std::error_code(errno, std::system_category());
+    close(descriptor);
+
+    throw fs::filesystem_error("Cannot create a temporary file", ec);
+  }
+#else
   std::FILE* file = std::tmpfile();
   if (file == nullptr) {
     std::error_code ec = std::error_code(errno, std::generic_category());
     throw fs::filesystem_error("Cannot create a temporary file", ec);
   }
+#endif
 
   return file;
 }

tests/directory.cpp

@@ -7,7 +7,6 @@
 
 #include <filesystem>
 #include <fstream>
-#include <ios>
 #include <stdexcept>
 #include <string>
 #include <string_view>

tests/file.cpp

@@ -11,7 +11,6 @@
 #include <istream>
 #include <iterator>
 #include <ostream>
-#include <system_error>
 #include <type_traits>
 #include <utility>
 
@@ -20,11 +19,14 @@
 #include <Windows.h>
 #else
 #include <fcntl.h>
+#include <sys/stat.h>
 #endif
 
 namespace tmp {
 namespace {
 
+namespace fs = std::filesystem;
+
 /// Test fixture for `basic_file` tests
 template<class charT> class file : public testing::Test {
 public:
@@ -85,6 +87,34 @@ TYPED_TEST(file, create) {
   basic_file<TypeParam> tmpfile = basic_file<TypeParam>();
   EXPECT_TRUE(TestFixture::is_open(tmpfile.rdbuf()));
   EXPECT_TRUE(TestFixture::is_open(tmpfile.native_handle()));
+
+  fs::path temp_dir = fs::temp_directory_path();
+  auto handle       = tmpfile.native_handle();
+
+#ifdef _WIN32
+  BY_HANDLE_FILE_INFORMATION file_info;
+  ASSERT_TRUE(GetFileInformationByHandle(handle, &file_info));
+
+  HANDLE dir_handle =
+      CreateFileW(temp_dir.c_str(), FILE_READ_ATTRIBUTES,
+                  FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE,
+                  nullptr, OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS, nullptr);
+  ASSERT_NE(dir_handle, INVALID_HANDLE_VALUE);
+
+  BY_HANDLE_FILE_INFORMATION dir_info;
+  ASSERT_TRUE(GetFileInformationByHandle(dir_handle, &dir_info));
+  CloseHandle(dir_handle);
+
+  EXPECT_EQ(file_info.dwVolumeSerialNumber, dir_info.dwVolumeSerialNumber);
+#else
+  struct stat file_stat {};
+  ASSERT_EQ(fstat(handle, &file_stat), 0);
+
+  struct stat dir_stat {};
+  ASSERT_EQ(stat(temp_dir.c_str(), &dir_stat), 0);
+
+  EXPECT_EQ(file_stat.st_dev, dir_stat.st_dev);
+#endif
 }
 
 /// Tests multiple file creation