Fix cufinufft fallback (#536)

* py: fix bug in cufinufft library fallback

* py: add tests for fallback library loading

* py: skip fallback tests in CI

These tests checks that the fallback mechanism is working properly. In
other words, if there is no bundled library, it should look in the
system path (e.g. `LD_LIBRARY_PATH` on Unix systems) and try to load it
from there.

For some reason, these tests work locally, but in CI, the library is
imported as usual, even with the patched function. Perhaps this has
something to do with the fact that the module is already loaded, so
`import finufft` is a no-op (this is evidenced by the fact that any
`print` statement in `finufft._finufft` is not detected in the output).

Jenkinsfile

@@ -59,7 +59,7 @@ pipeline {
       source $HOME/bin/activate
       python3 -m pip install --no-cache-dir --upgrade pycuda cupy-cuda112 numba
       python3 -m pip install --no-cache-dir torch==1.12.1+cu113 -f https://download.pytorch.org/whl/torch_stable.html
-      python3 -m pip install --no-cache-dir pytest
+      python3 -m pip install --no-cache-dir pytest pytest-mock
       python -c "from numba import cuda; cuda.cudadrv.libs.test()"
       python3 -m pytest --framework=pycuda python/cufinufft
       python3 -m pytest --framework=numba python/cufinufft

python/cufinufft/cufinufft/_cufinufft.py

@@ -11,6 +11,7 @@
 import importlib.util
 import pathlib
 import numpy as np
+from ctypes.util import find_library
 
 from ctypes import c_double
 from ctypes import c_int

python/cufinufft/tests/test_fallback.py

@@ -0,0 +1,25 @@
+import pytest
+
+import numpy as np
+from ctypes.util import find_library
+
+
+# Check to make sure the fallback mechanism works if there is no bundled
+# dynamic library.
+@pytest.mark.skip(reason="Patching seems to fail in CI")
+def test_fallback(mocker):
+    def fake_load_library(lib_name, path):
+        if lib_name in ["libcufinufft", "cufinufft"]:
+            raise OSError()
+        else:
+            return np.ctypeslib.load_library(lib_name, path)
+
+    # Block out the bundled library.
+    mocker.patch("numpy.ctypeslib.load_library", fake_load_library)
+
+    # Make sure an error is raised if no system library is found.
+    if find_library("cufinufft") is None:
+        with pytest.raises(ImportError, match="suitable cufinufft"):
+            import cufinufft
+    else:
+        import cufinufft

python/finufft/pyproject.toml

@@ -59,7 +59,7 @@ input = "finufft/__init__.py"
 build-verbosity = 1
 # Not building for PyPy and musllinux for now.
 skip = "pp* *musllinux*"
-test-requires = "pytest"
+test-requires = ["pytest", "pytest-mock"]
 test-command = "pytest {project}/python/finufft/test"
 
 [tool.cibuildwheel.linux]

python/finufft/test/test_fallback.py

@@ -0,0 +1,20 @@
+import pytest
+
+import numpy as np
+from ctypes.util import find_library
+
+@pytest.mark.skip(reason="Patching seems to fail in CI")
+def test_fallback(mocker):
+    def fake_load_library(lib_name, path):
+        if lib_name in ["libfinufft", "finufft"]:
+            raise OSError()
+        else:
+            return np.ctypeslib.load_library(lib_name, path)
+
+    mocker.patch("numpy.ctypeslib.load_library", fake_load_library)
+
+    if find_library("finufft") is None:
+        with pytest.raises(ImportError, match="suitable finufft"):
+            import finufft
+    else:
+        import finufft