Issue #1531: Read fileobj from the current offset, do not seek to

the start.

(backport from r59260)

Lib/tarfile.py

@@ -1065,7 +1065,8 @@ class TarFile(object):
         self.closed = False
         self.members = []       # list of members as TarInfo objects
         self._loaded = False    # flag if all members have been read
-        self.offset = 0L        # current position in the archive file
+        self.offset = self.fileobj.tell()
+                                # current position in the archive file
         self.inodes = {}        # dictionary caching the inodes of
                                 # archive members already added
 

Lib/test/test_tarfile.py

@@ -260,6 +260,38 @@ class ReadStreamAsteriskTest(ReadStreamTest):
         mode = self.mode + self.sep + "*"
         self.tar = tarfile.open(tarname(self.comp), mode)
 
+class ReadFileobjTest(BaseTest):
+
+    def test_fileobj_with_offset(self):
+        # Skip the first member and store values from the second member
+        # of the testtar.
+        self.tar.next()
+        t = self.tar.next()
+        name = t.name
+        offset = t.offset
+        data = self.tar.extractfile(t).read()
+        self.tar.close()
+
+        # Open the testtar and seek to the offset of the second member.
+        if self.comp == "gz":
+            _open = gzip.GzipFile
+        elif self.comp == "bz2":
+            _open = bz2.BZ2File
+        else:
+            _open = open
+        fobj = _open(tarname(self.comp), "rb")
+        fobj.seek(offset)
+
+        # Test if the tarfile starts with the second member.
+        self.tar = tarfile.open(tarname(self.comp), "r:", fileobj=fobj)
+        t = self.tar.next()
+        self.assertEqual(t.name, name)
+        # Read to the end of fileobj and test if seeking back to the
+        # beginning works.
+        self.tar.getmembers()
+        self.assertEqual(self.tar.extractfile(t).read(), data,
+                "seek back did not work")
+
 class WriteTest(BaseTest):
     mode = 'w'
 
@@ -624,6 +656,8 @@ class ReadAsteriskTestGzip(ReadAsteriskTest):
     comp = "gz"
 class ReadStreamAsteriskTestGzip(ReadStreamAsteriskTest):
     comp = "gz"
+class ReadFileobjTestGzip(ReadFileobjTest):
+    comp = "gz"
 
 # Filemode test cases
 
@@ -680,6 +714,8 @@ if bz2:
         comp = "bz2"
     class ReadStreamAsteriskTestBzip2(ReadStreamAsteriskTest):
         comp = "bz2"
+    class ReadFileobjTestBzip2(ReadFileobjTest):
+        comp = "bz2"
 
 # If importing gzip failed, discard the Gzip TestCases.
 if not gzip:
@@ -713,6 +749,7 @@ def test_main():
         ReadDetectFileobjTest,
         ReadAsteriskTest,
         ReadStreamAsteriskTest,
+        ReadFileobjTest,
         WriteTest,
         Write100Test,
         WriteSize0Test,
@@ -730,7 +767,8 @@ def test_main():
             ReadTestGzip, ReadStreamTestGzip,
             WriteTestGzip, WriteStreamTestGzip,
             ReadDetectTestGzip, ReadDetectFileobjTestGzip,
-            ReadAsteriskTestGzip, ReadStreamAsteriskTestGzip
+            ReadAsteriskTestGzip, ReadStreamAsteriskTestGzip,
+            ReadFileobjTestGzip
         ])
 
     if bz2:
@@ -738,7 +776,8 @@ def test_main():
             ReadTestBzip2, ReadStreamTestBzip2,
             WriteTestBzip2, WriteStreamTestBzip2,
             ReadDetectTestBzip2, ReadDetectFileobjTestBzip2,
-            ReadAsteriskTestBzip2, ReadStreamAsteriskTestBzip2
+            ReadAsteriskTestBzip2, ReadStreamAsteriskTestBzip2,
+            ReadFileobjTestBzip2
         ])
     try:
         test_support.run_unittest(*tests)

Misc/NEWS

@@ -46,6 +46,9 @@ Core and builtins
 Library
 -------
 
+- Issue #1531: tarfile.py: Read fileobj from the current offset, do not
+  seek to the start.
+
 - Issue 1429818: patch for trace and doctest modules so they play nicely
   together.