fix(cuda.core): fall back to driver when nvJitLink < 12.3 is installed

cuda_core/cuda/core/_linker.pyx

@@ -684,10 +684,10 @@ def _decide_nvjitlink_or_driver() -> bool:
         " For best results, consider upgrading to a recent version of"
     )
 
-    nvjitlink_module = _optional_cuda_import(
-        "cuda.bindings.nvjitlink",
-        probe_function=lambda module: module.version(),  # probe triggers nvJitLink runtime load
-    )
+    # Do not probe via module.version(): nvJitLink 12.0-12.2 lacks the unversioned
+    # nvJitLinkVersion symbol, so calling version() raises FunctionNotFoundError.
+    # Use _nvjitlink_has_version_symbol() below instead (symbol pointer inspection).
+    nvjitlink_module = _optional_cuda_import("cuda.bindings.nvjitlink")
     if nvjitlink_module is None:
         warn_txt = f"cuda.bindings.nvjitlink is not available, therefore {warn_txt_common} cuda-bindings."
     else:

cuda_core/docs/source/release/1.2.0-notes.rst

@@ -18,6 +18,15 @@ Fixes and enhancements
   (`#2357 <https://github.com/NVIDIA/cuda-python/pull/2357>`__,
   `#2371 <https://github.com/NVIDIA/cuda-python/pull/2371>`__)
 
+- :meth:`Linker.which_backend` and constructing a :class:`Linker` no longer
+  raise ``FunctionNotFoundError`` when an nvJitLink older than 12.3
+  (12.0–12.2) is installed. These versions do not export the unversioned
+  ``nvJitLinkVersion`` symbol, so probing the version crashed instead of
+  falling back. ``cuda.core`` now warns and falls back to the driver
+  (``cuLink``) backend, restoring the pre-0.7.0 behavior.
+  (`#2409 <https://github.com/NVIDIA/cuda-python/pull/2409>`__,
+  closes `#2408 <https://github.com/NVIDIA/cuda-python/issues/2408>`__)
+
 Deprecation Notices
 -------------------
 

cuda_core/tests/test_linker.py

@@ -303,6 +303,24 @@ def fake_decide():
         assert result == "nvJitLink"
         assert called, "_decide_nvjitlink_or_driver was not called"
 
+    def test_which_backend_falls_back_when_nvjitlink_too_old(self, monkeypatch):
+        """Regression test for #2408: old nvJitLink must not crash which_backend()."""
+        monkeypatch.setattr(_linker, "_use_nvjitlink_backend", None)
+        monkeypatch.setattr(_linker, "_driver", None)
+
+        def fake__optional_cuda_import(modname, probe_function=None):
+            assert modname == "cuda.bindings.nvjitlink"
+            assert probe_function is None
+            return object()
+
+        monkeypatch.setattr(_linker, "_optional_cuda_import", fake__optional_cuda_import)
+        monkeypatch.setattr(_linker, "_nvjitlink_has_version_symbol", lambda _nvjitlink: False)
+
+        with pytest.warns(RuntimeWarning, match="too old \\(<12.3\\)"):
+            assert Linker.which_backend() == "driver"
+
+        assert _linker._use_nvjitlink_backend is False
+
     def test_which_backend_is_classmethod(self):
         attr = inspect.getattr_static(Linker, "which_backend")
         assert isinstance(attr, classmethod)

cuda_core/tests/test_optional_dependency_imports.py

@@ -78,7 +78,7 @@ def fake__optional_cuda_import(modname, probe_function=None):
 def test_decide_nvjitlink_or_driver_reraises_nested_module_not_found(monkeypatch):
     def fake__optional_cuda_import(modname, probe_function=None):
         assert modname == "cuda.bindings.nvjitlink"
-        assert probe_function is not None
+        assert probe_function is None
         err = ModuleNotFoundError("No module named 'not_a_real_dependency'")
         err.name = "not_a_real_dependency"
         raise err
@@ -93,7 +93,7 @@ def fake__optional_cuda_import(modname, probe_function=None):
 def test_decide_nvjitlink_or_driver_falls_back_when_module_missing(monkeypatch):
     def fake__optional_cuda_import(modname, probe_function=None):
         assert modname == "cuda.bindings.nvjitlink"
-        assert probe_function is not None
+        assert probe_function is None
         return None
 
     monkeypatch.setattr(_linker, "_optional_cuda_import", fake__optional_cuda_import)
@@ -103,3 +103,34 @@ def fake__optional_cuda_import(modname, probe_function=None):
 
     assert use_driver_backend is True
     assert _linker._use_nvjitlink_backend is False
+
+
+def test_decide_nvjitlink_or_driver_falls_back_when_nvjitlink_too_old(monkeypatch):
+    def fake__optional_cuda_import(modname, probe_function=None):
+        assert modname == "cuda.bindings.nvjitlink"
+        assert probe_function is None
+        return object()
+
+    monkeypatch.setattr(_linker, "_optional_cuda_import", fake__optional_cuda_import)
+    monkeypatch.setattr(_linker, "_nvjitlink_has_version_symbol", lambda _nvjitlink: False)
+
+    with pytest.warns(RuntimeWarning, match="too old \\(<12.3\\)"):
+        use_driver_backend = _linker._decide_nvjitlink_or_driver()
+
+    assert use_driver_backend is True
+    assert _linker._use_nvjitlink_backend is False
+
+
+def test_decide_nvjitlink_or_driver_selects_nvjitlink_when_version_symbol_present(monkeypatch):
+    def fake__optional_cuda_import(modname, probe_function=None):
+        assert modname == "cuda.bindings.nvjitlink"
+        assert probe_function is None
+        return object()
+
+    monkeypatch.setattr(_linker, "_optional_cuda_import", fake__optional_cuda_import)
+    monkeypatch.setattr(_linker, "_nvjitlink_has_version_symbol", lambda _nvjitlink: True)
+
+    use_driver_backend = _linker._decide_nvjitlink_or_driver()
+
+    assert use_driver_backend is False
+    assert _linker._use_nvjitlink_backend is True