Make sure header has reasonable values

Author: rhpvorderman

src/isal/igzip.py

@@ -123,6 +123,16 @@ def __repr__(self):
     def flush(self, zlib_mode=isal_zlib.Z_SYNC_FLUSH):
         super().flush(zlib_mode)
 
+    def _write_gzip_header(self, compresslevel):
+        # Determine what xfl flag is written for the compression level.
+        # Equate the fast level to gzip level 1. All the other levels are
+        # medium.
+        if compresslevel == _COMPRESS_LEVEL_FAST:
+            compresslevel = gzip._COMPRESS_LEVEL_FAST
+        else:
+            compresslevel = gzip._COMPRESS_LEVEL_TRADEOFF
+        super()._write_gzip_header(compresslevel)
+
     def write(self, data):
         self._check_not_closed()
         if self.mode != gzip.WRITE:

tests/test_gzip_compliance.py

@@ -397,8 +397,8 @@ def test_compresslevel_metadata(self):
         # specifically, discussion of XFL in section 2.3.1
         cases = [
             ('fast', 0, b'\x04'),
-            ('best', 3, b'\x02'),
-            ('tradeoff', 2, b'\x00'),
+            ('best', 3, b'\x00'),  # Comparable to medium gzip level.
+            ('tradeoff', 2, b'\x00'),  # Dito
         ]
         xflOffset = 8