Keep the offset of a member which follows a global header

_proc_pax() handles the global extended header too, so setting the offset
of the next member unconditionally made it the offset of the global
header instead of its own.  It only has to be set after the offset of the
next header is computed.

Author: serhiy-storchaka

Lib/tarfile.py

@@ -1625,7 +1625,7 @@ def _proc_pax(self, tarfile):
                     offset += next._block(size)
                 tarfile.offset = offset
 
-        next.offset = self.offset
+            next.offset = self.offset
 
         return next
 

Lib/test/test_tarfile.py

@@ -1503,6 +1503,31 @@ def test_pax_global_headers(self):
         finally:
             tar.close()
 
+    def test_offset_after_global_header(self):
+        # gh-83869: a global header is a member of its own, the member which
+        # follows it keeps the offset of its own header.
+        rec = b"30 comment=global header here\n"
+        glob = tarfile.TarInfo("././@PaxHeader")
+        glob.type = tarfile.XGLTYPE
+        glob.size = len(rec)
+        buf = glob.tobuf(tarfile.USTAR_FORMAT)
+        buf += rec + b"\0" * (-len(rec) % tarfile.BLOCKSIZE)
+
+        member = tarfile.TarInfo("member")
+        data = b"hello\n"
+        member.size = len(data)
+        offset = len(buf)
+        buf += member.tobuf(tarfile.USTAR_FORMAT)
+        buf += data + b"\0" * (-len(data) % tarfile.BLOCKSIZE)
+        buf += b"\0" * (tarfile.BLOCKSIZE * 2)
+
+        with tarfile.open(fileobj=io.BytesIO(buf)) as tar:
+            tarinfo = tar.getmember("member")
+            self.assertEqual(tarinfo.offset, offset)
+            self.assertEqual(tarinfo.pax_headers.get("comment"),
+                             "global header here")
+            self.assertEqual(tar.extractfile(tarinfo).read(), data)
+
     def test_pax_number_fields(self):
         # All following number fields are read from the pax header.
         tar = tarfile.open(tarname, encoding="iso8859-1")