Use CMake FindPython3 for reliable library discovery

- Use CMake 3.18+ built-in FindPython3 module
- Pass Python library path as PYTHON_LIBRARY_PATH compile definition
- Use CMake-discovered path for dlopen with fallback to pattern-based

This ensures the correct Python library is loaded with RTLD_GLOBAL
on all platforms including FreeBSD and free-threaded Python.

Author: benoitc

c_src/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.14 FATAL_ERROR)
+cmake_minimum_required(VERSION 3.18 FATAL_ERROR)
 
 project(ErlangPythonNIF C)
 
@@ -51,50 +51,41 @@ endif()
 include(FindErlang)
 include_directories(${ERLANG_ERTS_INCLUDE_PATH})
 
-# Find Python - use the PYTHON_CONFIG env variable if set
+# Find Python using CMake's built-in FindPython3
+# Use PYTHON_CONFIG env variable to hint which Python to use
 if(DEFINED ENV{PYTHON_CONFIG})
-    set(Python3_CONFIG "$ENV{PYTHON_CONFIG}")
+    # Extract prefix from python-config for hinting
     execute_process(
-        COMMAND ${Python3_CONFIG} --prefix
-        OUTPUT_VARIABLE Python3_PREFIX
+        COMMAND $ENV{PYTHON_CONFIG} --prefix
+        OUTPUT_VARIABLE Python3_ROOT_DIR
         OUTPUT_STRIP_TRAILING_WHITESPACE
     )
-    execute_process(
-        COMMAND ${Python3_CONFIG} --includes
-        OUTPUT_VARIABLE Python3_INCLUDE_FLAGS
-        OUTPUT_STRIP_TRAILING_WHITESPACE
-    )
-    execute_process(
-        COMMAND ${Python3_CONFIG} --ldflags --embed
-        OUTPUT_VARIABLE Python3_LINK_FLAGS
-        OUTPUT_STRIP_TRAILING_WHITESPACE
-        RESULT_VARIABLE Python3_EMBED_RESULT
-    )
-    if(NOT Python3_EMBED_RESULT EQUAL 0)
-        execute_process(
-            COMMAND ${Python3_CONFIG} --ldflags
-            OUTPUT_VARIABLE Python3_LINK_FLAGS
-            OUTPUT_STRIP_TRAILING_WHITESPACE
-        )
-    endif()
-
-    # Extract include directories from flags
-    string(REGEX MATCHALL "-I[^ ]+" Python3_INCLUDE_DIRS_LIST "${Python3_INCLUDE_FLAGS}")
-    string(REGEX REPLACE "-I" "" Python3_INCLUDE_DIRS_LIST "${Python3_INCLUDE_DIRS_LIST}")
-
-    message(STATUS "Using Python config: ${Python3_CONFIG}")
-    message(STATUS "Python includes: ${Python3_INCLUDE_FLAGS}")
-    message(STATUS "Python link flags: ${Python3_LINK_FLAGS}")
-else()
-    # Use CMake's FindPython3
-    find_package(Python3 REQUIRED COMPONENTS Development)
-    set(Python3_INCLUDE_DIRS_LIST ${Python3_INCLUDE_DIRS})
-    set(Python3_LINK_FLAGS "${Python3_LIBRARIES}")
+    set(Python3_FIND_STRATEGY LOCATION)
 endif()
 
+find_package(Python3 REQUIRED COMPONENTS Interpreter Development)
+
+message(STATUS "Python3 executable: ${Python3_EXECUTABLE}")
+message(STATUS "Python3 version: ${Python3_VERSION}")
+message(STATUS "Python3 include dirs: ${Python3_INCLUDE_DIRS}")
+message(STATUS "Python3 libraries: ${Python3_LIBRARIES}")
+message(STATUS "Python3 library: ${Python3_LIBRARY}")
+
 # Create the NIF shared library
 add_library(py_nif MODULE py_nif.c)
 
+# Pass Python library path to NIF for dlopen
+# Python3_LIBRARY is the full path to the Python shared library
+if(Python3_LIBRARY)
+    target_compile_definitions(py_nif PRIVATE PYTHON_LIBRARY_PATH="${Python3_LIBRARY}")
+    message(STATUS "Using Python library path for dlopen: ${Python3_LIBRARY}")
+elseif(Python3_LIBRARIES)
+    # Fallback to first library in the list
+    list(GET Python3_LIBRARIES 0 Python3_FIRST_LIB)
+    target_compile_definitions(py_nif PRIVATE PYTHON_LIBRARY_PATH="${Python3_FIRST_LIB}")
+    message(STATUS "Using Python library path for dlopen: ${Python3_FIRST_LIB}")
+endif()
+
 # Set output name
 set_target_properties(py_nif PROPERTIES
     PREFIX ""
@@ -104,7 +95,7 @@ set_target_properties(py_nif PROPERTIES
 # Include directories
 target_include_directories(py_nif PRIVATE
     ${ERLANG_ERTS_INCLUDE_PATH}
-    ${Python3_INCLUDE_DIRS_LIST}
+    ${Python3_INCLUDE_DIRS}
 )
 
 # Compiler flags
@@ -140,13 +131,7 @@ elseif(UNIX)
 endif()
 
 # Link Python
-if(DEFINED ENV{PYTHON_CONFIG})
-    # Parse the link flags and add them properly
-    separate_arguments(Python3_LINK_FLAGS_LIST UNIX_COMMAND "${Python3_LINK_FLAGS}")
-    target_link_libraries(py_nif PRIVATE ${Python3_LINK_FLAGS_LIST})
-else()
-    target_link_libraries(py_nif PRIVATE Python3::Python)
-endif()
+target_link_libraries(py_nif PRIVATE Python3::Python)
 
 # Threads
 find_package(Threads REQUIRED)

c_src/py_nif.c

@@ -471,34 +471,42 @@ static ERL_NIF_TERM nif_py_init(ErlNifEnv *env, int argc, const ERL_NIF_TERM arg
      * extension modules can find Python symbols when dynamically loaded.
      * Without this, modules like _socket.so fail with "undefined symbol: PyByteArray_Type" */
     {
-        char libpython[256];
         void *handle = NULL;
 
+#ifdef PYTHON_LIBRARY_PATH
+        /* Use CMake-discovered library path (most reliable) */
+        handle = dlopen(PYTHON_LIBRARY_PATH, RTLD_NOW | RTLD_GLOBAL);
+#endif
+
+        /* Fallback: try pattern-based discovery if CMake path didn't work */
+        if (!handle) {
+            char libpython[256];
 #ifdef Py_GIL_DISABLED
-        /* Free-threaded Python has 't' suffix in library name (e.g., libpython3.13t.so) */
-        const char *patterns[] = {
-            "libpython%d.%dt.so.1.0",  /* Linux free-threaded with full version */
-            "libpython%d.%dt.so",      /* Linux/FreeBSD free-threaded */
-            "libpython%d.%dt.so.1",    /* Some systems free-threaded */
-            "libpython%d.%d.so.1.0",   /* Fallback: Linux with full version */
-            "libpython%d.%d.so",       /* Fallback: Linux/FreeBSD */
-            "libpython%d.%d.so.1",     /* Fallback: Some systems */
-            NULL
-        };
+            /* Free-threaded Python has 't' suffix in library name (e.g., libpython3.13t.so) */
+            const char *patterns[] = {
+                "libpython%d.%dt.so.1.0",  /* Linux free-threaded with full version */
+                "libpython%d.%dt.so",      /* Linux/FreeBSD free-threaded */
+                "libpython%d.%dt.so.1",    /* Some systems free-threaded */
+                "libpython%d.%d.so.1.0",   /* Fallback: Linux with full version */
+                "libpython%d.%d.so",       /* Fallback: Linux/FreeBSD */
+                "libpython%d.%d.so.1",     /* Fallback: Some systems */
+                NULL
+            };
 #else
-        /* Standard Python library names */
-        const char *patterns[] = {
-            "libpython%d.%d.so.1.0",  /* Linux with full version */
-            "libpython%d.%d.so",      /* Linux/FreeBSD */
-            "libpython%d.%d.so.1",    /* Some systems */
-            NULL
-        };
+            /* Standard Python library names */
+            const char *patterns[] = {
+                "libpython%d.%d.so.1.0",  /* Linux with full version */
+                "libpython%d.%d.so",      /* Linux/FreeBSD */
+                "libpython%d.%d.so.1",    /* Some systems */
+                NULL
+            };
 #endif
 
-        for (int i = 0; patterns[i] && !handle; i++) {
-            snprintf(libpython, sizeof(libpython), patterns[i],
-                     PY_MAJOR_VERSION, PY_MINOR_VERSION);
-            handle = dlopen(libpython, RTLD_NOW | RTLD_GLOBAL);
+            for (int i = 0; patterns[i] && !handle; i++) {
+                snprintf(libpython, sizeof(libpython), patterns[i],
+                         PY_MAJOR_VERSION, PY_MINOR_VERSION);
+                handle = dlopen(libpython, RTLD_NOW | RTLD_GLOBAL);
+            }
         }
         /* It's OK if this fails - the symbols might already be global */
     }