pycompression · rhpvorderman · Jun 24, 2025 · Jun 24, 2025 · Jun 24, 2025 · Jun 24, 2025
diff --git a/CHANGELOG.rst b/CHANGELOG.rst
@@ -11,6 +11,10 @@ Changelog
 version 1.8.0-dev
 -----------------
 + Python 3.8 is 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.
 + Change build backend to setuptools-scm which is more commonly used and
   supported.
 + Include test packages in the source distribution, so source distribution

diff --git a/src/isal/igzip_threaded.py b/src/isal/igzip_threaded.py
@@ -316,30 +316,26 @@ 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) -> None:
+        if self._closed:
+            return
+        self.flush()
         self.raw.write(isal_zlib.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()

diff --git a/tests/test_igzip_threaded.py b/tests/test_igzip_threaded.py
@@ -12,6 +12,7 @@
 import subprocess
 import sys
 import tempfile
+import zlib
 from pathlib import Path
 
 from isal import igzip_threaded
@@ -243,15 +244,29 @@ def test_threaded_program_can_exit_on_error(tmp_path, mode, threads):
 
 @pytest.mark.parametrize("threads", [1, 2])
 def test_flush(tmp_path, threads):
+    empty_block_end = b"\x00\x00\xff\xff"
+    compressobj = zlib.compressobj(wbits=-15)
+    deflate_last_block = compressobj.compress(b"") + compressobj.flush()
     test_file = tmp_path / "output.gz"
     with igzip_threaded.open(test_file, "wb", threads=threads) as f:
         f.write(b"1")
         f.flush()
-        assert gzip.decompress(test_file.read_bytes()) == b"1"
+        data = test_file.read_bytes()
+        assert data[-4:] == empty_block_end
+        # Cut off gzip header and end data with an explicit last block to
+        # test if the data was compressed correctly.
+        deflate_block = data[10:] + deflate_last_block
+        assert zlib.decompress(deflate_block, wbits=-15) == b"1"
         f.write(b"2")
         f.flush()
-        assert gzip.decompress(test_file.read_bytes()) == b"12"
+        data = test_file.read_bytes()
+        assert data[-4:] == empty_block_end
+        deflate_block = data[10:] + deflate_last_block
+        assert zlib.decompress(deflate_block, wbits=-15) == b"12"
         f.write(b"3")
         f.flush()
-        assert gzip.decompress(test_file.read_bytes()) == b"123"
+        data = test_file.read_bytes()
+        assert data[-4:] == empty_block_end
+        deflate_block = data[10:] + deflate_last_block
+        assert zlib.decompress(deflate_block, wbits=-15) == b"123"
     assert gzip.decompress(test_file.read_bytes()) == b"123"