fix: only check VirtualNDArray's shape versus count in from_buffers if both are known

src/awkward/operations/ak_from_buffers.py

@@ -189,7 +189,11 @@ def _from_buffer(
             raise ValueError(
                 f"Mismatch of dtypes. Got {dtype}, but VirtualNDArray has {buffer.dtype}."
             )
-        if count != buffer._shape[0]:
+        if (
+            count is not unknown_length
+            and buffer._shape[0] is not unknown_length
+            and count != buffer._shape[0]
+        ):
             raise ValueError(
                 f"Mismatch of lengths. Got {count}, but VirtualNDArray has {buffer._shape[0]}."
             )

tests/test_3667_virtual_array_shape_check_in_from_buffers.py

@@ -0,0 +1,63 @@
+# BSD 3-Clause License; see https://github.com/scikit-hep/awkward/blob/main/LICENSE
+
+from __future__ import annotations
+
+import numpy as np
+import pytest
+
+import awkward as ak
+from awkward._nplikes.numpy import Numpy
+from awkward._nplikes.shape import unknown_length
+from awkward._nplikes.virtual import VirtualNDArray
+
+
+@pytest.mark.parametrize("offsets_length", [5, unknown_length])
+@pytest.mark.parametrize("content_length", [9, unknown_length])
+def test(offsets_length, content_length):
+    offset_generator = lambda: np.array([0, 2, 4, 5, 6], dtype=np.int64)  # noqa: E731
+    data_generator = lambda: np.array([1, 2, 3, 4, 5, 6, 7, 8, 9], dtype=np.int64)  # noqa: E731
+
+    offsets = ak.index.Index(
+        VirtualNDArray(
+            Numpy.instance(),
+            shape=(offsets_length,),
+            dtype=np.int64,
+            generator=offset_generator,
+        )
+    )
+    data = ak.contents.NumpyArray(
+        VirtualNDArray(
+            Numpy.instance(),
+            shape=(content_length,),
+            dtype=np.int64,
+            generator=data_generator,
+        )
+    )
+    array = ak.Array(ak.contents.ListOffsetArray(offsets, data))
+    assert array.to_list() == [[1, 2], [3, 4], [5], [6]]
+
+    buffers = {"node0-offsets": offset_generator, "node1-data": data_generator}
+    form = ak.forms.ListOffsetForm(
+        "i64", ak.forms.NumpyForm("int64", form_key="node1"), form_key="node0"
+    )
+    array = ak.from_buffers(form, 4, buffers)
+    assert array.to_list() == [[1, 2], [3, 4], [5], [6]]
+
+    offsets = VirtualNDArray(
+        Numpy.instance(),
+        shape=(offsets_length,),
+        dtype=np.int64,
+        generator=offset_generator,
+    )
+    data = VirtualNDArray(
+        Numpy.instance(),
+        shape=(content_length,),
+        dtype=np.int64,
+        generator=data_generator,
+    )
+    buffers = {"node0-offsets": offsets, "node1-data": data}
+    form = ak.forms.ListOffsetForm(
+        "i64", ak.forms.NumpyForm("int64", form_key="node1"), form_key="node0"
+    )
+    array = ak.from_buffers(form, 4, buffers)
+    assert array.to_list() == [[1, 2], [3, 4], [5], [6]]