Use os.fdatasync before replacing temp files (#261)

* Use `os.fdatasync` before replacing temp files

According to https://stackoverflow.com/a/67905617, flushing/closing may
not be enough.

* changelog

Author: epwalsh

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+### Fixed
+
+- Ensure filesystem syncs data from temp file before replacing with permanent file.
+
 ## [v1.8.1](https://github.com/allenai/cached_path/releases/tag/v1.8.1) - 2025-12-15
 
 - Allow `google-cloud-storage` version 3.x.

cached_path/cache_file.py

@@ -31,16 +31,25 @@ def __enter__(self):
         return self.temp_file
 
     def __exit__(self, exc_type, exc_value, traceback):
-        self.temp_file.close()
+        del exc_type, traceback
+
         if exc_value is None:
+            # Ensure all data is written to disk.
+            self.temp_file.flush()
+            if hasattr(os, "fdatasync"):  # only available on linux
+                os.fdatasync(self.temp_file)  # type: ignore
+            self.temp_file.close()
+
             # Success.
             logger.debug(
                 "Renaming temp file %s to cache at %s", self.temp_file.name, self.cache_filename
             )
             # Rename the temp file to the actual cache filename.
             os.replace(self.temp_file.name, self.cache_filename)
             return True
-        # Something went wrong, remove the temp file.
-        logger.debug("removing temp file %s", self.temp_file.name)
-        os.remove(self.temp_file.name)
-        return False
+        else:
+            # Something went wrong, remove the temp file.
+            logger.debug("removing temp file %s", self.temp_file.name)
+            self.temp_file.close()
+            os.remove(self.temp_file.name)
+            return False