bpo-39688: tarfile: compute next header offset using pax size for sparse file

In case of a sparse file, the tarinfo.size attribute is set to the sparse
file expanded size (pax attribute GNU.sparse.size or GNU.sparse.size) and
do not correspond to the actual size of the data block. The size of the
data block is specified by the size pax header if present or by the ustar
size header.
Moreover, for GNU sparse 1.0 files, the data block start at the beginning of
the sparse mapping and not after the sparse mapping and so the offset should
be computed from here.

Author: nitmir

Lib/tarfile.py

@@ -1292,17 +1292,22 @@ def _proc_pax(self, tarfile):
         if self.type in (XHDTYPE, SOLARIS_XHDTYPE):
             # Patch the TarInfo object with the extended header info.
             next._apply_pax_info(pax_headers, tarfile.encoding, tarfile.errors)
-            next.offset = self.offset
 
             if "size" in pax_headers:
                 # If the extended header replaces the size field,
                 # we need to recalculate the offset where the next
                 # header starts.
-                offset = next.offset_data
+                offset = next.offset + BLOCKSIZE
                 if next.isreg() or next.type not in SUPPORTED_TYPES:
-                    offset += next._block(next.size)
+                    try:
+                        size = PAX_NUMBER_FIELDS["size"](pax_headers["size"])
+                    except ValueError:
+                        size = 0
+                    offset += next._block(size)
                 tarfile.offset = offset
 
+        next.offset = self.offset
+
         return next
 
     def _proc_gnusparse_00(self, next, pax_headers, buf):