gh-155286: Write a valid empty zstd archive

Author: RealBhupesh

Lib/compression/zstd/_zstdfile.py

@@ -54,6 +54,7 @@ def __init__(self, file, /, mode='r', *,
         self._close_fp = False
         self._mode = _MODE_CLOSED
         self._buffer = None
+        self._write_started = False
 
         if not isinstance(mode, str):
             raise ValueError('mode must be a str')
@@ -68,6 +69,9 @@ def __init__(self, file, /, mode='r', *,
             if level is not None and not isinstance(level, int):
                 raise TypeError('level must be int or None')
             self._mode = _MODE_WRITE
+            # Do not add an empty frame when closing an existing archive in
+            # append mode without writing anything.
+            self._write_started = mode == 'a'
             self._compressor = ZstdCompressor(level=level, options=options,
                                               zstd_dict=zstd_dict)
             self._pos = 0
@@ -131,6 +135,7 @@ def write(self, data, /):
         length = _nbytes(data)
 
         compressed = self._compressor.compress(data)
+        self._write_started = True
         self._fp.write(compressed)
         self._pos += length
         return length
@@ -153,10 +158,11 @@ def flush(self, mode=FLUSH_BLOCK):
             raise ValueError('Invalid mode argument, expected either '
                              'ZstdFile.FLUSH_FRAME or '
                              'ZstdFile.FLUSH_BLOCK')
-        if self._compressor.last_mode == mode:
+        if self._compressor.last_mode == mode and self._write_started:
             return
         # Flush zstd block/frame, and write.
         data = self._compressor.flush(mode)
+        self._write_started = True
         self._fp.write(data)
         if hasattr(self._fp, 'flush'):
             self._fp.flush()

Lib/test/test_zstd.py

@@ -2120,16 +2120,16 @@ def test_write_empty_frame(self):
         self.assertNotEqual(c.flush(c.FLUSH_FRAME), b'')
         self.assertNotEqual(c.flush(c.FLUSH_FRAME), b'')
 
-        # don't generate empty content frame
+        # generate an empty content frame when the file is closed
         bo = io.BytesIO()
         with ZstdFile(bo, 'w') as f:
             pass
-        self.assertEqual(bo.getvalue(), b'')
+        self.assertEqual(decompress(bo.getvalue()), b'')
 
         bo = io.BytesIO()
         with ZstdFile(bo, 'w') as f:
             f.flush(f.FLUSH_FRAME)
-        self.assertEqual(bo.getvalue(), b'')
+        self.assertEqual(decompress(bo.getvalue()), b'')
 
         # if .write(b''), generate empty content frame
         bo = io.BytesIO()

Misc/NEWS.d/next/Library/2026-08-07-00-00-00.gh-issue-155286.empty-zstd-file.rst

@@ -0,0 +1,2 @@
+Fix :class:`~compression.zstd.ZstdFile` creating an invalid zero-byte archive
+when an output file is closed without any writes.