Merge branch 'bugfixes/1849-gzip-compression-header-cpython-bug39389'maint/17.x

This change implements a fix to compression in gzip tool that CPython
applied per https://bugs.python.org/issue39389. It uses proper XFL
value in gzip header per https://tools.ietf.org/html/rfc1952, section
2.3.1.

Refs:
* https://github.com/python/cpython/pull/18077 (CPython 3.9.0+)
* https://github.com/python/cpython/pull/18100 (CPython 3.8.2+)
* https://github.com/python/cpython/pull/18101 (CPython 3.7.7+)

Fixes #1849

(cherry picked from commit 3925ae9dffb3d0b8d7792ccd1fd8fefae6f20089)

3 files changed, 44 insertions, 2 deletions

diff --git a/CHANGES.rst b/CHANGES.rst
index c72ab792..898f1f74 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -1,6 +1,10 @@
 v17.4.3
 -------
 
+* :issue:`1849` via :pr:`1879`: Fixed XLF flag in gzip header
+  emitted by gzip compression tool per
+  :rfc:`1952#section-2.3.1`.
+
 * :issue:`1874`: Restricted depending on pywin32 only under
   CPython so that it won't get pulled-in under PyPy
   -- by :user:`webknjaz`.
diff --git a/cherrypy/lib/encoding.py b/cherrypy/lib/encoding.py
index 3d001ca6..4f9495e4 100644
--- a/cherrypy/lib/encoding.py
+++ b/cherrypy/lib/encoding.py
@@ -11,6 +11,10 @@ from cherrypy.lib import is_closable_iterator
 from cherrypy.lib import set_vary_header
 
 
+_COMPRESSION_LEVEL_FAST = 1
+_COMPRESSION_LEVEL_BEST = 9
+
+
 def decode(encoding=None, default_encoding='utf-8'):
     """Replace or extend the list of charsets used to decode a request entity.
 
@@ -287,13 +291,29 @@ def compress(body, compress_level):
     """Compress 'body' at the given compress_level."""
     import zlib
 
-    # See http://www.gzip.org/zlib/rfc-gzip.html
+    # See https://tools.ietf.org/html/rfc1952
     yield b'\x1f\x8b'       # ID1 and ID2: gzip marker
     yield b'\x08'           # CM: compression method
     yield b'\x00'           # FLG: none set
     # MTIME: 4 bytes
     yield struct.pack('<L', int(time.time()) & int('FFFFFFFF', 16))
-    yield b'\x02'           # XFL: max compression, slowest algo
+
+    # RFC 1952, section 2.3.1:
+    #
+    # XFL (eXtra FLags)
+    #    These flags are available for use by specific compression
+    #    methods.  The "deflate" method (CM = 8) sets these flags as
+    #    follows:
+    #
+    #       XFL = 2 - compressor used maximum compression,
+    #                 slowest algorithm
+    #       XFL = 4 - compressor used fastest algorithm
+    if compress_level == _COMPRESSION_LEVEL_BEST:
+        yield b'\x02'       # XFL: max compression, slowest algo
+    elif compress_level == _COMPRESSION_LEVEL_FAST:
+        yield b'\x04'       # XFL: min compression, fastest algo
+    else:
+        yield b'\x00'       # XFL: compression unset/tradeoff
     yield b'\xff'           # OS: unknown
 
     crc = zlib.crc32(b'')
diff --git a/cherrypy/test/test_tools.py b/cherrypy/test/test_tools.py
index a73a3898..dbed9faf 100644
--- a/cherrypy/test/test_tools.py
+++ b/cherrypy/test/test_tools.py
@@ -18,6 +18,17 @@ from cherrypy._cpcompat import ntou
 from cherrypy.test import helper, _test_decorators
 
 
+PY_VER_PATCH = sys.version_info[:3]
+PY_VER_MINOR = PY_VER_PATCH[:2]
+# Refs:
+# bugs.python.org/issue39389
+# docs.python.org/3.7/whatsnew/changelog.html#python-3-7-7-release-candidate-1
+# docs.python.org/3.8/whatsnew/changelog.html#python-3-8-2-release-candidate-1
+HAS_GZIP_COMPRESSION_HEADER_FIXED = PY_VER_PATCH >= (3, 8, 2) or (
+    PY_VER_MINOR == (3, 7) and PY_VER_PATCH >= (3, 7, 7)
+)
+
+
 timeout = 0.2
 europoundUnicode = ntou('\x80\xa3')
 
@@ -363,6 +374,13 @@ class ToolTests(helper.CPWebCase):
                          ('Accept-Charset', 'ISO-8859-1,utf-8;q=0.7,*;q=0.7')])
         self.assertInBody(zbuf.getvalue()[:3])
 
+        if not HAS_GZIP_COMPRESSION_HEADER_FIXED:
+            # NOTE: CherryPy adopts a fix from the CPython bug 39389
+            # NOTE: introducing a variable compression XFL flag that
+            # NOTE: was hardcoded to "best compression" before. And so
+            # NOTE: we can only test it on CPython versions that also
+            # NOTE: implement this fix.
+            return
         zbuf = io.BytesIO()
         zfile = gzip.GzipFile(mode='wb', fileobj=zbuf, compresslevel=6)
         zfile.write(expectedResult)