bpo-39389: gzip: fix compression level metadata (GH-18077)
As described in RFC 1952, section 2.3.1, the XFL (eXtra FLags) byte of a
gzip member header should indicate whether the DEFLATE algorithm was
tuned for speed or compression ratio. Prior to this patch, archives
emitted by the `gzip` module always indicated maximum compression.
(cherry picked from commit eab3b3f1c60afecfb4db3c3619109684cb04bd60)
Co-authored-by: William Chargin <wchargin@gmail.com>
diff --git a/Lib/gzip.py b/Lib/gzip.py
index 2968f47..87b553d 100644
--- a/Lib/gzip.py
+++ b/Lib/gzip.py
@@ -201,7 +201,7 @@
self.fileobj = fileobj
if self.mode == WRITE:
- self._write_gzip_header()
+ self._write_gzip_header(compresslevel)
@property
def filename(self):
@@ -228,7 +228,7 @@
self.bufsize = 0
self.offset = 0 # Current file offset for seek(), tell(), etc
- def _write_gzip_header(self):
+ def _write_gzip_header(self, compresslevel):
self.fileobj.write(b'\037\213') # magic header
self.fileobj.write(b'\010') # compression method
try:
@@ -249,7 +249,13 @@
if mtime is None:
mtime = time.time()
write32u(self.fileobj, int(mtime))
- self.fileobj.write(b'\002')
+ if compresslevel == _COMPRESS_LEVEL_BEST:
+ xfl = b'\002'
+ elif compresslevel == _COMPRESS_LEVEL_FAST:
+ xfl = b'\004'
+ else:
+ xfl = b'\000'
+ self.fileobj.write(xfl)
self.fileobj.write(b'\377')
if fname:
self.fileobj.write(fname + b'\000')