Fix possible race condition when writing user file

While a user file write operation #1 was queued in the _write_loop,
it was possible meanwhile for another function to read the file
(the current old version on disk), add some data to it,
add submit the write (operation #2).
The modifications from operatio #1 were then completly lost at best.

src/backend/user_files.py

@@ -65,7 +65,10 @@ class DataFile:
 
 
     async def read(self):
-        """Return content of the existing file on disk, or default content."""
+        """Return future, existing or default content for the file on disk."""
+
+        if self._to_write is not None:
+            return self._to_write
 
         try:
             return self.path.read_text()
@@ -125,7 +128,7 @@ class JSONDataFile(DataFile):
 
 
     async def read(self) -> JsonData:
-        """Return content of the existing file on disk, or default content.
+        """Return future, existing or default content for the file on disk.
 
         If the file has missing keys, the missing data will be merged and
         written to disk before returning.
@@ -134,6 +137,9 @@ class JSONDataFile(DataFile):
         created.
         """
 
+        if self._to_write is not None:
+            return json.loads(self._to_write)
+
         try:
             data = json.loads(self.path.read_text())
         except FileNotFoundError: