fix(toolkit): populate Python3_SOABI from sysconfig when empty

When NO_STABLE_ABI=1, the build sets SOABI=".${Python3_SOABI}". If
CMake's FindPython3 fails to populate Python3_SOABI, the suffix
collapses to "." and extension modules are linked as `_solver..so` /
`_output..so`, which Python cannot import.

Fall back to `sysconfig.get_config_var('SOABI')` via the interpreter
when the CMake variable is empty.

Also add a ctest guardrail (ExtensionSuffix_output, ExtensionSuffix_solver)
that asserts each built module has no double-dot and ends with a valid
`importlib.machinery.EXTENSION_SUFFIXES` entry, and wire a regression
step into unit_test.yml so the NO_STABLE_ABI path is exercised on
Linux, macOS, and Windows.

Refs #155

Author: cheginit

.github/workflows/unit_test.yml

@@ -58,3 +58,17 @@ jobs:
           pip install -r test-requirements.txt
           pip install --no-index --find-links=./dist swmm_toolkit
           pytest
+
+      # Regression test for the Python3_SOABI fallback. Builds with
+      # NO_STABLE_ABI=1 and runs the ctest suffix guardrail to catch the
+      # double-dot filename bug (e.g. _solver..so) that occurs when
+      # CMake's FindPython3 fails to populate Python3_SOABI.
+      - name: Build + ctest (NO_STABLE_ABI path)
+        shell: bash
+        env:
+          NO_STABLE_ABI: "1"
+        run: |
+          pip install cmake swig
+          cmake -S . -B build_cmake
+          cmake --build build_cmake --config Release
+          ctest --test-dir build_cmake --output-on-failure -C Release

swmm-toolkit/CMakeLists.txt

@@ -36,6 +36,8 @@ cmake_policy(SET CMP0078 NEW)
 cmake_policy(SET CMP0086 NEW)
 include(${SWIG_USE_FILE})
 
+enable_testing()
+
 # Add project subdirectories
 add_subdirectory(swmm-solver)
 

swmm-toolkit/src/swmm/toolkit/CMakeLists.txt

@@ -28,6 +28,14 @@ set(PY_LIMITED_API_DEF "Py_LIMITED_API=0x03090000")
 string(TOLOWER "$ENV{NO_STABLE_ABI}" _abi_val)
 if(_abi_val STREQUAL "1" OR _abi_val STREQUAL "true")
     set(PY_LIMITED_API_DEF "")
+    if(NOT Python3_SOABI)
+        execute_process(
+            COMMAND ${Python3_EXECUTABLE} -c
+                "import sysconfig; print(sysconfig.get_config_var('SOABI') or '')"
+            OUTPUT_VARIABLE Python3_SOABI
+            OUTPUT_STRIP_TRAILING_WHITESPACE
+        )
+    endif()
     set(SOABI ".${Python3_SOABI}")
 endif()
 
@@ -188,6 +196,31 @@ install(
     COMPONENT python
 )
 
+#############################################################
+####################    SUFFIX TESTS     ####################
+#############################################################
+
+# Verify that each extension module was linked with a valid Python ABI suffix.
+# Guards against the double-dot bug (_solver..so) that occurs when
+# CMake's FindPython3 fails to populate Python3_SOABI.
+# Run with: cmake --build <build_dir> && ctest --test-dir <build_dir>
+
+foreach(_ext_target output solver)
+    add_test(
+        NAME ExtensionSuffix_${_ext_target}
+        COMMAND ${Python3_EXECUTABLE} -c
+            "import importlib.machinery, os; \
+p = r'$<TARGET_FILE:${_ext_target}>'; \
+valid = importlib.machinery.EXTENSION_SUFFIXES; \
+assert os.path.exists(p), f'Extension not found: {p!r}'; \
+assert '..' not in os.path.basename(p), \
+    f'Double-dot ABI bug in {p!r} -- Python3_SOABI was empty at configure time'; \
+assert any(p.endswith(s) for s in valid), \
+    f'Invalid suffix in {p!r}, expected one of {valid}'; \
+print('PASS:', p)"
+    )
+endforeach()
+
 # Copy libomp.dylib on macOS if using scikit-build-core
 if(APPLE AND DEFINED SKBUILD_PLATLIB_DIR)
     install(