Fix swapped XFL and OS bytes in igzip_threaded gzip headers

CHANGELOG.rst

@@ -9,6 +9,9 @@ Changelog
 
 version 1.10.0-dev
 ------------------
++ Fix the gzip header that ``igzip_threaded.open`` writes when ``threads`` is
+  not 0. It had the XFL and OS bytes swapped, so the ``file`` command reported
+  the output as coming from VM/CMS or a FAT file system.
 + Fix pickling of ``isal_zlib.error`` (``igzip_lib.IsalError``). An error
   raised in a ``multiprocessing`` worker process did not reach the parent
   process. Tracebacks now show ``isal.igzip_lib.IsalError`` instead of

src/isal/igzip_threaded.py

@@ -299,7 +299,7 @@ def _write_gzip_header(self):
         os = 0xff
         xfl = 4 if self.level == 0 else 0
         self.raw.write(struct.pack(
-            "BBBBIBB", magic1, magic2, method, flags, mtime, os, xfl))
+            "BBBBIBB", magic1, magic2, method, flags, mtime, xfl, os))
 
     def start(self):
         self.running = True

tests/test_igzip_threaded.py

@@ -59,6 +59,19 @@ def test_threaded_write(tmp_path, mode, threads, block_size):
     assert test_data == out_data
 
 
+@pytest.mark.parametrize(["level", "threads"],
+                         list(itertools.product([0, 1, 2, 3], [1, 2])))
+def test_threaded_write_header_xfl_os(level, threads):
+    buffer = io.BytesIO()
+    with igzip_threaded.open(buffer, "wb", compresslevel=level,
+                             threads=threads) as f:
+        f.write(b"thisisatest")
+    # RFC 1952 puts XFL before OS. IGzipFile writes the same two bytes:
+    # XFL 4 (fastest) at level 0, else 0, and OS 255 (unknown).
+    xfl = 4 if level == 0 else 0
+    assert buffer.getvalue()[8:10] == bytes([xfl, 255])
+
+
 def test_threaded_open_no_threads():
     with tempfile.TemporaryFile("rb") as tmp:
         klass = igzip_threaded.open(tmp, "rb", threads=0)