From 5396257be241a21a04f1db61e5dd99a6ddc8d823 Mon Sep 17 00:00:00 2001
From: Serhiy Storchaka <storchaka@gmail.com>
Date: Sun, 30 Oct 2016 20:52:55 +0200
Subject: 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 | 9 ++++++++-
 Misc/NEWS                | 4 ++++
 3 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/Lib/tarfile.py b/Lib/tarfile.py
index be68800..adf91d5 100644
--- a/Lib/tarfile.py
+++ b/Lib/tarfile.py
@@ -1665,7 +1665,9 @@ 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 3f9e996..89bd738 100644
--- a/Lib/test/test_tarfile.py
+++ b/Lib/test/test_tarfile.py
@@ -2,7 +2,9 @@ import sys
 import os
 import shutil
 import StringIO
+from binascii import unhexlify
 from hashlib import md5
+from random import Random
 import errno
 
 import unittest
@@ -276,12 +278,17 @@ class CommonReadTest(ReadTest):
         else:
             _open = open
 
+        # generate 512 pseudorandom bytes
+        data = unhexlify('%1024x' % Random(0).getrandbits(512*8))
         for char in ('\0', 'a'):
             # Test if EOFHeaderError ('\0') and InvalidHeaderError ('a')
             # are ignored correctly.
             with _open(tmpname, "wb") 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 12c3da2..760a6ea 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -60,6 +60,10 @@ 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 #25464: Fixed HList.header_exists() in Tix module by adding
   a workaround to Tix library bug.
 
-- 
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