tarfile: Fix FileSection.skip to not rely on extended readinto args.

Commit 2ca1527 optimized
`FileSection.skip()` for memory use.  But that introduced a dependency on
the MicroPython-extension to stream read methods for an additional argument
specifying a maximum read size.  This optimization meant that all file-like
objects passed into TarFile must support the extended 2-argument `readinto`
form.

This is problematic for at least two use cases:
1. Nested tar files, because `FileSetion` itself doesn't support 2-argument
   `readinto`.
2. Using `mpremote mount` and reading a tar file from the remote mount,
   which also doesn't support 2-argument `readinto`.

Instead of requiring all file-like objects to implement this extended form
of `readinto`, this commit changes `FileSection.skip()` so that it doesn't
use this form.

A test is added for this case which fails without the fix here.

Signed-off-by: Damien George <damien@micropython.org>

Author: dpgeorge

python-stdlib/tarfile/manifest.py

@@ -1,4 +1,4 @@
-metadata(description="Read-only implementation of Python's tarfile.", version="0.4.1")
+metadata(description="Read-only implementation of Python's tarfile.", version="0.4.2")
 
 # Originally written by Paul Sokolovsky.
 

python-stdlib/tarfile/tarfile/__init__.py

@@ -55,9 +55,12 @@ def skip(self):
         if sz:
             buf = bytearray(16)
             while sz:
-                s = min(sz, 16)
-                self.f.readinto(buf, s)
-                sz -= s
+                if sz >= 16:
+                    self.f.readinto(buf)
+                    sz -= 16
+                else:
+                    self.f.read(sz)
+                    sz = 0
 
 
 class TarInfo:

python-stdlib/tarfile/test_tarfile.py

@@ -35,3 +35,11 @@ def test_iter(self):
     def test_contents(self):
         tf = tarfile.TarFile("test.tar")
         self.check_contents(test_tar_contents, tf)
+
+    def test_nested_tar(self):
+        tf = tarfile.TarFile("test.tar")
+        for file in tf:
+            if file.name == "tar.tar":
+                subf = tf.extractfile(file)
+                subtf = tarfile.TarFile(fileobj=subf)
+                self.check_contents(test_sub_tar_contents, subtf)