From a89d22aff3fb1e67f8f08eac22b08a58dfa048a8 Mon Sep 17 00:00:00 2001
From: Serhiy Storchaka <storchaka@gmail.com>
Date: Sun, 30 Oct 2016 20:52:29 +0200
Subject: [PATCH] Issue #28449: tarfile.open() with mode "r" or "r:" now tries
 to open a tar file with compression before trying to open it without
 compression.  Otherwise it had 50% chance failed with ignore_zeros=True.

---
 Lib/tarfile.py           | 4 +++-
 Lib/test/test_tarfile.py | 8 +++++++-
 Misc/NEWS                | 4 ++++
 3 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/Lib/tarfile.py b/Lib/tarfile.py
index 721f9d7f918..999a99b978f 100755
--- a/Lib/tarfile.py
+++ b/Lib/tarfile.py
@@ -1549,7 +1549,9 @@ class TarFile(object):
 
         if mode in ("r", "r:*"):
             # Find out which *open() is appropriate for opening the file.
-            for comptype in cls.OPEN_METH:
+            def not_compressed(comptype):
+                return cls.OPEN_METH[comptype] == 'taropen'
+            for comptype in sorted(cls.OPEN_METH, key=not_compressed):
                 func = getattr(cls, cls.OPEN_METH[comptype])
                 if fileobj is not None:
                     saved_pos = fileobj.tell()
diff --git a/Lib/test/test_tarfile.py b/Lib/test/test_tarfile.py
index abfb34dfb81..1efb841bb22 100644
--- a/Lib/test/test_tarfile.py
+++ b/Lib/test/test_tarfile.py
@@ -3,6 +3,7 @@ import os
 import io
 from hashlib import md5
 from contextlib import contextmanager
+from random import Random
 
 import unittest
 import unittest.mock
@@ -349,12 +350,17 @@ class CommonReadTest(ReadTest):
 
     def test_ignore_zeros(self):
         # Test TarFile's ignore_zeros option.
+        # generate 512 pseudorandom bytes
+        data = Random(0).getrandbits(512*8).to_bytes(512, 'big')
         for char in (b'\0', b'a'):
             # Test if EOFHeaderError ('\0') and InvalidHeaderError ('a')
             # are ignored correctly.
             with self.open(tmpname, "w") as fobj:
                 fobj.write(char * 1024)
-                fobj.write(tarfile.TarInfo("foo").tobuf())
+                tarinfo = tarfile.TarInfo("foo")
+                tarinfo.size = len(data)
+                fobj.write(tarinfo.tobuf())
+                fobj.write(data)
 
             tar = tarfile.open(tmpname, mode="r", ignore_zeros=True)
             try:
diff --git a/Misc/NEWS b/Misc/NEWS
index f7226fddf07..298d3aa64e2 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -113,6 +113,10 @@ Core and Builtins
 Library
 -------
 
+- Issue #28449: tarfile.open() with mode "r" or "r:" now tries to open a tar
+  file with compression before trying to open it without compression.  Otherwise
+  it had 50% chance failed with ignore_zeros=True.
+
 - Issue #23262: The webbrowser module now supports Firefox 36+ and derived
   browsers.  Based on patch by Oleg Broytman.