JsonIO atomic write improvements (#2132)

Use `async with Lock` instead of deprecated `with await lock` usage.

Forces a file fsync prior and a directory fsync (where available) after rename to prevent issues with left behind temp files.

Also should clarify: this is not threadsafe. Comments were clarified, function names remain misleading.

redbot/core/json_io.py

@@ -5,8 +5,8 @@ import asyncio
 import logging
 from uuid import uuid4
 
-# This is basically our old DataIO, except that it's now threadsafe
+# This is basically our old DataIO and just a base for much more elaborate classes
-# and just a base for much more elaborate classes
+# This still isn't completely threadsafe, (do not use config in threads)
 from pathlib import Path
 
 log = logging.getLogger("red")
@@ -27,18 +27,50 @@ class JsonIO:
 
     # noinspection PyUnresolvedReferences
     def _save_json(self, data, settings=PRETTY):
+        """
+        This fsync stuff here is entirely neccessary. 
+        
+        On windows, it is not available in entirety.
+        If a windows user ends up with tons of temp files, they should consider hosting on
+        something POSIX compatible, or using the mongo backend instead.
+
+        Most users wont encounter this issue, but with high write volumes, 
+        without the fsync on both the temp file, and after the replace on the directory,
+        There's no real durability or atomicity guarantee from the filesystem.
+
+        In depth overview of underlying reasons why this is needed:
+            https://lwn.net/Articles/457667/
+
+        Also see:  
+            http://man7.org/linux/man-pages/man2/open.2.html#NOTES (synchronous I/O section)
+        And:
+            https://www.mjmwired.net/kernel/Documentation/filesystems/ext4.txt#310
+        """
         log.debug("Saving file {}".format(self.path))
         filename = self.path.stem
         tmp_file = "{}-{}.tmp".format(filename, uuid4().fields[0])
         tmp_path = self.path.parent / tmp_file
         with tmp_path.open(encoding="utf-8", mode="w") as f:
             json.dump(data, f, **settings)
+            f.flush()  # This does get closed on context exit, ...
+            os.fsync(f.fileno())  #  but that needs to happen prior to this line
+
         tmp_path.replace(self.path)
 
+        # pylint: disable=E1101
+        try:
+            fd = os.open(self.path.parent, os.O_DIRECTORY)
+            os.fsync(fd)
+        except AttributeError:
+            fd = None
+        finally:
+            if fd is not None:
+                os.close(fd)
+
     async def _threadsafe_save_json(self, data, settings=PRETTY):
         loop = asyncio.get_event_loop()
         func = functools.partial(self._save_json, data, settings)
-        with await self._lock:
+        async with self._lock:
             await loop.run_in_executor(None, func)
 
     # noinspection PyUnresolvedReferences
@@ -51,6 +83,5 @@ class JsonIO:
     async def _threadsafe_load_json(self, path):
         loop = asyncio.get_event_loop()
         func = functools.partial(self._load_json, path)
-        task = loop.run_in_executor(None, func)
+        async with self._lock:
-        with await self._lock:
+            return await loop.run_in_executor(None, func)
-            return await asyncio.wait_for(task)