Close #17666: Fix reading gzip files with an extra field.

Lib/gzip.py

@@ -202,7 +202,8 @@ class GzipFile(io.BufferedIOBase):
 
         if flag & FEXTRA:
             # Read & discard the extra field, if present
-            self._read_exact(struct.unpack("<H", self._read_exact(2)))
+            extra_len, = struct.unpack("<H", self._read_exact(2))
+            self._read_exact(extra_len)
         if flag & FNAME:
             # Read and discard a null-terminated string containing the filename
             while True:

Lib/test/test_gzip.py

@@ -306,6 +306,13 @@ class TestGzip(unittest.TestCase):
             with gzip.GzipFile(fileobj=io.BytesIO(truncated[:i])) as f:
                 self.assertRaises(EOFError, f.read, 1)
 
+    def test_read_with_extra(self):
+        # Gzip data with an extra field
+        gzdata = (b'\x1f\x8b\x08\x04\xb2\x17cQ\x02\xff'
+                  b'\x05\x00Extra'
+                  b'\x0bI-.\x01\x002\xd1Mx\x04\x00\x00\x00')
+        with gzip.GzipFile(fileobj=io.BytesIO(gzdata)) as f:
+            self.assertEqual(f.read(), b'Test')
 
 def test_main(verbose=None):
     test_support.run_unittest(TestGzip)

Misc/NEWS

@@ -14,6 +14,8 @@ Core and Builtins
 Library
 -------
 
+- Issue #17666: Fix reading gzip files with an extra field.
+
 - Issue #13150, #17512: sysconfig no longer parses the Makefile and config.h
   files when imported, instead doing it at build time.  This makes importing
   sysconfig faster and reduces Python startup time by 20%.