Common/FileUtil: Add CreateTempFileForAtomicWrite function.

Source/Core/Common/FileUtil.cpp

@@ -25,6 +25,7 @@
 #include "Common/CommonFuncs.h"
 #include "Common/CommonPaths.h"
 #include "Common/CommonTypes.h"
+#include "Common/DirectIOFile.h"
 #ifdef __APPLE__
 #include "Common/DynamicLibrary.h"
 #endif
@@ -668,13 +669,42 @@ std::string CreateTempDir()
 #endif
 }
 
-std::string GetTempFilenameForAtomicWrite(std::string path)
+static auto TryToGetAbsolutePath(std::string path)
 {
   std::error_code error;
   auto absolute_path = fs::absolute(StringToPath(path), error);
   if (!error)
     path = PathToString(absolute_path);
-  return std::move(path) + ".xxx";
+  return path;
+}
+
+std::string GetTempFilenameForAtomicWrite(std::string path)
+{
+  return TryToGetAbsolutePath(std::move(path)) + ".xxx";
+}
+
+std::string CreateTempFileForAtomicWrite(std::string path)
+{
+  path = TryToGetAbsolutePath(std::move(path));
+  while (true)
+  {
+    DirectIOFile file;
+
+    // e.g. "/dir/file.txt" -> "/dir/file.txt.189234789.tmp"
+    const auto timestamp = Clock::now().time_since_epoch().count();
+    std::string tmp_path = fmt::format("{}.{}.tmp", path, timestamp);
+
+    const auto open_result = file.Open(tmp_path, AccessMode::Write, OpenMode::Create);
+    if (open_result.Succeeded())
+      return tmp_path;
+
+    // In the very unlikely case that the file already exists, we will try again.
+    if (open_result.Error() == File::OpenError::AlreadyExists)
+      continue;
+
+    // Failure.
+    return {};
+  }
 }
 
 #if defined(__APPLE__)

Source/Core/Common/FileUtil.h

@@ -223,6 +223,10 @@ std::string CreateTempDir();
 // Get a filename that can hopefully be atomically renamed to the given path.
 std::string GetTempFilenameForAtomicWrite(std::string path);
 
+// Creates and returns the path to a newly created temporary file next to the given path.
+// Returns an empty string on error, generally caused by lack of write permissions.
+std::string CreateTempFileForAtomicWrite(std::string path);
+
 // Gets a set user directory path
 // Don't call prior to setting the base user directory
 const std::string& GetUserPath(unsigned int dir_index);

Source/UnitTests/Common/FileUtilTest.cpp

@@ -154,6 +154,17 @@ TEST_F(FileUtilTest, CreateFullPath)
   EXPECT_TRUE(File::IsFile(p3file));
 }
 
+TEST_F(FileUtilTest, CreateTempFileForAtomicWrite)
+{
+  EXPECT_TRUE(File::Exists(File::CreateTempFileForAtomicWrite(m_file_path)));
+
+#if defined(_WIN32)
+  EXPECT_FALSE(File::Exists(File::CreateTempFileForAtomicWrite("C:/con/cant_write_here.txt")));
+#else
+  EXPECT_FALSE(File::Exists(File::CreateTempFileForAtomicWrite("/dev/null/cant_write_here.txt")));
+#endif
+}
+
 TEST_F(FileUtilTest, DirectIOFile)
 {
   static constexpr std::array<u8, 3> u8_test_data = {42, 7, 99};