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.

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)