convert: prevent ndarray conversion in LazyChunkedTensor (#27869)

gguf-py/gguf/gguf_writer.py

@@ -467,10 +467,15 @@ class GGUFWriter:
                     shard_bar.reset(total=(total if total > 0 else None))
 
                 # relying on the fact that Python dicts preserve insertion order (since 3.7)
-                for ti in tensors.values():
+                for name, ti in tensors.items():
                     assert ti.tensor is not None  # can only iterate once over the tensors
                     assert ti.tensor.nbytes == ti.nbytes
+                    start = fout.tell()
                     ti.tensor.tofile(fout)
+                    # a short write here would only surface as a corrupt file at load time
+                    if fout.tell() - start != ti.nbytes:
+                        raise ValueError(
+                            f"tensor {name!r} wrote {fout.tell() - start} bytes, expected {ti.nbytes}")
                     if shard_bar is not None:
                         shard_bar.update(ti.nbytes)
                     if bar is not None: