Do not use file::path() in file::copy (#174)

This is the next step to removing the `path()` method of the temporary
file (#165): we now can create a
temporary copy without using the copy result path

Author: bugdea1er

include/tmp/file

@@ -54,6 +54,7 @@ public:
 
   /// Creates a unique temporary copy from the given path
   /// @note std::ios::in | std::ios::out are always added to the `mode`
+  /// @note after creating a copy, its I/O position indicator points to the end
   /// @param path A path to make a temporary copy from
   /// @param mode Specifies stream open mode
   /// @returns The new temporary file

src/file.cpp

@@ -4,6 +4,7 @@
 #include "create.hpp"
 #include "move.hpp"
 
+#include <array>
 #include <filesystem>
 #include <fstream>
 #include <ios>
@@ -12,7 +13,111 @@
 #include <system_error>
 #include <utility>
 
+#ifdef _WIN32
+#define NOMINMAX
+#define UNICODE
+#include <Windows.h>
+#else
+#include <cerrno>
+#include <fcntl.h>
+#include <unistd.h>
+#endif
+
 namespace tmp {
+namespace {
+
+/// A block size for file reading
+/// @note should always be less than INT_MAX
+constexpr std::size_t block_size = 4096;
+
+/// A type of buffer for I/O file operations
+using buffer_type = std::array<std::byte, block_size>;
+
+/// Opens a file in read-only mode
+/// @param[in]  path The path to the file to open
+/// @param[out] ec   Parameter for error reporting
+/// @returns A handle to the open file
+file::native_handle_type open(const fs::path& path,
+                              std::error_code& ec) noexcept {
+  ec.clear();
+
+#ifdef _WIN32
+  HANDLE handle =
+      CreateFile(path.c_str(), GENERIC_READ,
+                 FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE, NULL,
+                 OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);
+  if (handle == INVALID_HANDLE_VALUE) {
+    ec = std::error_code(GetLastError(), std::system_category());
+  }
+#else
+  int handle = ::open(path.c_str(), O_RDONLY | O_NONBLOCK);
+  if (handle == -1) {
+    ec = std::error_code(errno, std::system_category());
+  }
+#endif
+
+  return handle;
+}
+
+/// Closes the given handle, ignoring any errors
+/// @param[in] handle The handle to close
+void close(file::native_handle_type handle) noexcept {
+#ifdef _WIN32
+  CloseHandle(handle);
+#else
+  ::close(handle);
+#endif
+}
+
+/// Copies a file contents from the given path to the given file descriptor
+/// @param[in]  from The path to the source file
+/// @param[in]  to   The target file descriptor
+/// @param[out] ec   Parameter for error reporting
+void copy_file(const fs::path& from, file::native_handle_type to,
+               std::error_code& ec) noexcept {
+  // TODO: can be optimized using `sendfile`, `copyfile` or other system API
+  file::native_handle_type source = open(from, ec);
+  if (ec) {
+    return;
+  }
+
+  buffer_type buffer = buffer_type();
+  while (true) {
+#ifdef _WIN32
+    DWORD bytes_read;
+    if (!ReadFile(source, buffer.data(), block_size, &bytes_read, nullptr)) {
+      ec = std::error_code(GetLastError(), std::system_category());
+      break;
+    }
+#else
+    ssize_t bytes_read = read(source, buffer.data(), block_size);
+    if (bytes_read < 0) {
+      ec = std::error_code(errno, std::system_category());
+      break;
+    }
+#endif
+    if (bytes_read == 0) {
+      break;
+    }
+
+#ifdef _WIN32
+    DWORD written;
+    if (!WriteFile(to, buffer.data(), bytes_read, &written, nullptr)) {
+      ec = std::error_code(GetLastError(), std::system_category());
+      return;
+    }
+#else
+    ssize_t written = write(to, buffer.data(), bytes_read);
+    if (written < 0) {
+      ec = std::error_code(errno, std::system_category());
+      break;
+    }
+#endif
+  }
+
+  close(source);
+}
+}    // namespace
 
 file::file(std::pair<fs::path, filebuf> handle) noexcept
     : entry(std::move(handle.first)),
@@ -26,7 +131,7 @@ file file::copy(const fs::path& path, std::ios::openmode mode) {
   file tmpfile = file(mode);
 
   std::error_code ec;
-  fs::copy_file(path, tmpfile, fs::copy_options::overwrite_existing, ec);
+  copy_file(path, tmpfile.native_handle(), ec);
 
   if (ec) {
     throw fs::filesystem_error("Cannot create a temporary copy", path, ec);

tests/file.cpp

@@ -108,15 +108,29 @@ TEST(file, copy_file) {
 
   EXPECT_TRUE(fs::is_regular_file(tmpfile));
 
+  // Test get file pointer position after copying
+  std::streampos gstreampos = copy.tellg();
+  copy.seekg(0, std::ios::end);
+  EXPECT_EQ(gstreampos, copy.tellg());
+
+  // Test put file pointer position after copying
+  std::streampos pstreampos = copy.tellp();
+  copy.seekp(0, std::ios::end);
+  EXPECT_EQ(pstreampos, copy.tellp());
+
+  // Test file copy contents
   std::ifstream stream = std::ifstream(copy.path());
   std::string content = std::string(std::istreambuf_iterator(stream), {});
   EXPECT_EQ(content, "Hello, world!");
 }
 
-/// Tests creation of a temporary copy of a directory
-TEST(file, copy_directory) {
-  directory tmpdir = directory();
-  EXPECT_THROW(file::copy(tmpdir), fs::filesystem_error);
+/// Tests creation of copy errors
+TEST(file, copy_errors) {
+  // `file::copy` cannot copy a directory
+  EXPECT_THROW(file::copy(directory()), fs::filesystem_error);
+
+  // `file::copy` cannot copy a non-existent file
+  EXPECT_THROW(file::copy("nonexistent.txt"), fs::filesystem_error);
 }
 
 /// Tests that moving a temporary file to itself does nothing