Merged revisions 88528 via svnmerge from

svn+ssh://pythondev@svn.python.org/python/branches/py3k

........
  r88528 | lars.gustaebel | 2011-02-23 12:42:22 +0100 (Wed, 23 Feb 2011) | 16 lines

  Issue #11224: Improved sparse file read support (r85916) introduced a
  regression in _FileInFile which is used in file-like objects returned
  by TarFile.extractfile(). The inefficient design of the
  _FileInFile.read() method causes various dramatic side-effects and
  errors:

    - The data segment of a file member is read completely into memory
      every(!) time a small block is accessed. This is not only slow
      but may cause unexpected MemoryErrors with very large files.
    - Reading members from compressed tar archives is even slower
      because of the excessive backwards seeking which is done when the
      same data segment is read over and over again.
    - As a backwards seek on a TarFile opened in stream mode is not
      possible, using extractfile() fails with a StreamError.
........

1 files changed, 16 insertions, 0 deletions

diff --git a/Lib/test/test_tarfile.py b/Lib/test/test_tarfile.py
index 94ef61c0ce..68e094d5db 100644
--- a/Lib/test/test_tarfile.py
+++ b/Lib/test/test_tarfile.py
@@ -419,6 +419,22 @@ class StreamReadTest(CommonReadTest):
 
     mode="r|"
 
+    def test_read_through(self):
+        # Issue #11224: A poorly designed _FileInFile.read() method
+        # caused seeking errors with stream tar files.
+        for tarinfo in self.tar:
+            if not tarinfo.isreg():
+                continue
+            fobj = self.tar.extractfile(tarinfo)
+            while True:
+                try:
+                    buf = fobj.read(512)
+                except tarfile.StreamError:
+                    self.fail("simple read-through using TarFile.extractfile() failed")
+                if not buf:
+                    break
+            fobj.close()
+
     def test_fileobj_regular_file(self):
         tarinfo = self.tar.next() # get "regtype" (can't use getmember)
         fobj = self.tar.extractfile(tarinfo)