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.
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 @@
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