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, 2 insertions, 3 deletions

diff --git a/Lib/tarfile.py b/Lib/tarfile.py
index e3747e9..0f9d1da 100644
--- a/Lib/tarfile.py
+++ b/Lib/tarfile.py
@@ -760,9 +760,8 @@ class _FileInFile(object):
                         self.map_index = 0
             length = min(size, stop - self.position)
             if data:
-                self.fileobj.seek(offset)
-                block = self.fileobj.read(stop - start)
-                buf += block[self.position - start:self.position + length]
+                self.fileobj.seek(offset + (self.position - start))
+                buf += self.fileobj.read(length)
             else:
                 buf += NUL * length
             size -= length