Add read support for all missing variants of the GNU sparse

extensions. Thus, in addition to GNUTYPE_SPARSE headers, sparse
information in pax headers created by GNU tar can now be decoded.
All three formats 0.0, 0.1 and 1.0 are supported.
On filesystems that support this, holes in files are now restored
whenever a sparse member is extracted.

Doc/library/tarfile.rst

@@ -20,7 +20,8 @@ Some facts and figures:
 * read/write support for the POSIX.1-1988 (ustar) format.
 
 * read/write support for the GNU tar format including *longname* and *longlink*
-  extensions, read-only support for the *sparse* extension.
+  extensions, read-only support for all variants of the *sparse* extension
+  including restoration of sparse files.
 
 * read/write support for the POSIX.1-2001 (pax) format.
 

Lib/test/test_tarfile.py

@@ -526,6 +526,22 @@ class MemberReadTest(ReadTest):
         tarinfo = self.tar.getmember("ustar/sparse")
         self._test_member(tarinfo, size=86016, chksum=md5_sparse)
 
+    def test_find_gnusparse(self):
+        tarinfo = self.tar.getmember("gnu/sparse")
+        self._test_member(tarinfo, size=86016, chksum=md5_sparse)
+
+    def test_find_gnusparse_00(self):
+        tarinfo = self.tar.getmember("gnu/sparse-0.0")
+        self._test_member(tarinfo, size=86016, chksum=md5_sparse)
+
+    def test_find_gnusparse_01(self):
+        tarinfo = self.tar.getmember("gnu/sparse-0.1")
+        self._test_member(tarinfo, size=86016, chksum=md5_sparse)
+
+    def test_find_gnusparse_10(self):
+        tarinfo = self.tar.getmember("gnu/sparse-1.0")
+        self._test_member(tarinfo, size=86016, chksum=md5_sparse)
+
     def test_find_umlauts(self):
         tarinfo = self.tar.getmember("ustar/umlauts-\xc4\xd6\xdc\xe4\xf6\xfc\xdf")
         self._test_member(tarinfo, size=7011, chksum=md5_regtype)
@@ -589,13 +605,53 @@ class GNUReadTest(LongnameTest):
     subdir = "gnu"
     longnametype = tarfile.GNUTYPE_LONGNAME
 
-    def test_sparse_file(self):
-        tarinfo1 = self.tar.getmember("ustar/sparse")
-        fobj1 = self.tar.extractfile(tarinfo1)
-        tarinfo2 = self.tar.getmember("gnu/sparse")
-        fobj2 = self.tar.extractfile(tarinfo2)
-        self.assertEqual(fobj1.read(), fobj2.read(),
-                "sparse file extraction failed")
+    # Since 3.2 tarfile is supposed to accurately restore sparse members and
+    # produce files with holes. This is what we actually want to test here.
+    # Unfortunately, not all platforms/filesystems support sparse files, and
+    # even on platforms that do it is non-trivial to make reliable assertions
+    # about holes in files. Therefore, we first do one basic test which works
+    # an all platforms, and after that a test that will work only on
+    # platforms/filesystems that prove to support sparse files.
+    def _test_sparse_file(self, name):
+        self.tar.extract(name, TEMPDIR)
+        filename = os.path.join(TEMPDIR, name)
+        with open(filename, "rb") as fobj:
+            data = fobj.read()
+        self.assertEqual(md5sum(data), md5_sparse,
+                "wrong md5sum for %s" % name)
+
+        if self._fs_supports_holes():
+            s = os.stat(filename)
+            self.assertTrue(s.st_blocks * 512 < s.st_size)
+
+    def test_sparse_file_old(self):
+        self._test_sparse_file("gnu/sparse")
+
+    def test_sparse_file_00(self):
+        self._test_sparse_file("gnu/sparse-0.0")
+
+    def test_sparse_file_01(self):
+        self._test_sparse_file("gnu/sparse-0.1")
+
+    def test_sparse_file_10(self):
+        self._test_sparse_file("gnu/sparse-1.0")
+
+    @staticmethod
+    def _fs_supports_holes():
+        # Return True if the platform knows the st_blocks stat attribute and
+        # uses st_blocks units of 512 bytes, and if the filesystem is able to
+        # store holes in files.
+        if sys.platform == "linux2":
+            # Linux evidentially has 512 byte st_blocks units.
+            name = os.path.join(TEMPDIR, "sparse-test")
+            with open(name, "wb") as fobj:
+                fobj.seek(4096)
+                fobj.truncate()
+            s = os.stat(name)
+            os.remove(name)
+            return s.st_blocks == 0
+        else:
+            return False
 
 
 class PaxReadTest(LongnameTest):

Misc/NEWS

@@ -54,6 +54,9 @@ Core and Builtins
 Library
 -------
 
+- tarfile.py: Add support for all missing variants of the GNU sparse
+  extensions and create files with holes when extracting sparse members.
+
 - Issue #10218: Return timeout status from ``Condition.wait`` in threading.
 
 - Issue #7351: Add ``zipfile.BadZipFile`` spelling of the exception name