Fix misuse of offset param in seek (#140633)

Fixes #115630.

The size of BufferAdapter has been calculated wrongly due to misuse of python method seek. Causes miniz reader initialized with wrong size.

Pull Request resolved: https://github.com/pytorch/pytorch/pull/140633
Approved by: https://github.com/ezyang

Co-authored-by: Edward Z. Yang <ezyang@fb.com>

test/test_serialization.py

@@ -4394,6 +4394,15 @@ class TestSerialization(TestCase, SerializationMixin):
                 with zipfile.ZipFile(f) as zip_file:
                     zip_file.extractall(path=temp_dir)
 
+    def test_serialization_with_header(self):
+        orig = torch.randn(3, 3)
+        with BytesIOContext() as f:
+            f.write(b'header')
+            torch.save(orig, f)
+            f.seek(6)
+            loaded = torch.load(f)
+            self.assertEqual(orig, loaded)
+
     def test_get_unsafe_globals_in_checkpoint(self):
         t = torch.randn(2, 3)
         tt = TwoTensor(t, t)

torch/csrc/jit/python/init.cpp

@@ -1514,7 +1514,7 @@ void initJITBindings(PyObject* module) {
       // Jump to the end of the buffer to get its size
       auto current = buffer.attr("tell")();
       start_offset_ = py::cast<size_t>(current);
-      buffer.attr("seek")(current, py::module::import("os").attr("SEEK_END"));
+      buffer.attr("seek")(0, py::module::import("os").attr("SEEK_END"));
       size_ = py::cast<size_t>(buffer.attr("tell")()) - start_offset_;
       buffer.attr("seek")(current);
       // If we can read directly into a buffer, do that instead of an extra copy