Fix flushing behavior for gzip_ng_threaded

Author: rhpvorderman

CHANGELOG.rst

@@ -11,6 +11,10 @@ version develop
 -----------------
 + Python 3.14 is supported.
 + Python 3.8 and 3.9 are no longer supported.
++ Fix an issue where flushing using igzip_threaded caused a gzip end of stream
+  and started a new gzip stream. In essence creating a concatenated gzip
+  stream. Now it is in concordance with how single threaded gzip streams
+  are flushed using Z_SYNC_FLUSH.
 + Switched to setuptools-scm for building the package rather than versioningit.
 + Test files are added to the source distribution.
 

src/zlib_ng/gzip_ng_threaded.py

@@ -321,30 +321,25 @@ def write(self, b) -> int:
         self.input_queues[worker_index].put((data, zdict))
         return len(data)
 
-    def _end_gzip_stream(self):
+    def flush(self):
         self._check_closed()
         # Wait for all data to be compressed
         for in_q in self.input_queues:
             in_q.join()
         # Wait for all data to be written
         for out_q in self.output_queues:
             out_q.join()
-        # Write an empty deflate block with a lost block marker.
+        self.raw.flush()
+
+    def close(self):
+        if self._closed:
+            return
         self.raw.write(zlib_ng.compress(b"", wbits=-15))
         trailer = struct.pack("<II", self._crc, self._size & 0xFFFFFFFF)
         self.raw.write(trailer)
         self._crc = 0
         self._size = 0
         self.raw.flush()
-
-    def flush(self):
-        self._end_gzip_stream()
-        self._write_gzip_header()
-
-    def close(self) -> None:
-        if self._closed:
-            return
-        self._end_gzip_stream()
         self.stop()
         if self.exception:
             self.raw.close()