[utils] locked_file: Do not truncate files before locking (#2994)

Authored by: jakeogh, pukkandan

yt_dlp/utils.py

@@ -2222,10 +2222,23 @@ class locked_file(object):
     locked = False
 
     def __init__(self, filename, mode, block=True, encoding=None):
-        assert mode in {'r', 'rb', 'a', 'ab', 'w', 'wb'}
-        self.f = open(filename, mode, encoding=encoding)
-        self.mode = mode
-        self.block = block
+        if mode not in {'r', 'rb', 'a', 'ab', 'w', 'wb'}:
+            raise NotImplementedError(mode)
+        self.mode, self.block = mode, block
+
+        writable = any(f in mode for f in 'wax+')
+        readable = any(f in mode for f in 'r+')
+        flags = functools.reduce(operator.ior, (
+            getattr(os, 'O_CLOEXEC', 0),  # UNIX only
+            getattr(os, 'O_BINARY', 0),  # Windows only
+            getattr(os, 'O_NOINHERIT', 0),  # Windows only
+            os.O_CREAT if writable else 0,  # O_TRUNC only after locking
+            os.O_APPEND if 'a' in mode else 0,
+            os.O_EXCL if 'x' in mode else 0,
+            os.O_RDONLY if not writable else os.O_RDWR if readable else os.O_WRONLY,
+        ))
+
+        self.f = os.fdopen(os.open(filename, flags), mode, encoding=encoding)
 
     def __enter__(self):
         exclusive = 'r' not in self.mode
@@ -2235,6 +2248,8 @@ class locked_file(object):
         except IOError:
             self.f.close()
             raise
+        if 'w' in self.mode:
+            self.f.truncate()
         return self
 
     def unlock(self):