prefer wheel-provided libucxx.so in load_library, use RTLD_LOCAL (#322)

Contributes to rapidsai/build-planning#118

Modifies `libucxx.load_library()` in the following ways:

* prefer wheel-provided `libucxx.so` to system installation
* expose environment variable `RAPIDS_LIBUCXX_PREFER_SYSTEM_LIBRARY` for switching that preference
* load `libucxx.so` with `RTLD_LOCAL`, to prevent adding symbols to the global namespace ([dlopen docs](https://linux.die.net/man/3/dlopen))

Authors:
  - James Lamb (https://github.com/jameslamb)

Approvers:
  - Vyas Ramasubramani (https://github.com/vyasr)
  - Peter Andreas Entschev (https://github.com/pentschev)

URL: #322

python/libucxx/libucxx/load.py

@@ -15,8 +15,35 @@
 import ctypes
 import os
 
+# Loading with RTLD_LOCAL adds the library itself to the loader's
+# loaded library cache without loading any symbols into the global
+# namespace. This allows libraries that express a dependency on
+# this library to be loaded later and successfully satisfy this dependency
+# without polluting the global symbol table with symbols from
+# libucxx that could conflict with symbols from other DSOs.
+PREFERRED_LOAD_FLAG = ctypes.RTLD_LOCAL
+
+
+def _load_system_installation(soname: str):
+    """Try to dlopen() the library indicated by ``soname``
+
+    Raises ``OSError`` if library cannot be loaded.
+    """
+    return ctypes.CDLL(soname, PREFERRED_LOAD_FLAG)
+
+
+def _load_wheel_installation(soname: str):
+    """Try to dlopen() the library indicated by ``soname``
+
+    Returns ``None`` if the library cannot be loaded.
+    """
+    if os.path.isfile(lib := os.path.join(os.path.dirname(__file__), "lib64", soname)):
+        return ctypes.CDLL(lib, PREFERRED_LOAD_FLAG)
+    return None
+
 
 def load_library():
+    """Dynamically load libucxx.so and its dependencies"""
     # If libucx was installed as a wheel, we must request it to load the library
     # symbols. Otherwise, we assume that the library was installed in a system path
     # that ld can find.
@@ -28,25 +55,33 @@ def load_library():
         libucx.load_library()
         del libucx
 
-    # Dynamically load libucxx.so. Prefer a system library if one is present to
-    # avoid clobbering symbols that other packages might expect, but if no
-    # other library is present use the one in the wheel.
+    prefer_system_installation = (
+        os.getenv("RAPIDS_LIBUCXX_PREFER_SYSTEM_LIBRARY", "false").lower() != "false"
+    )
+
+    soname = "libucxx.so"
     libucxx_lib = None
-    try:
-        libucxx_lib = ctypes.CDLL("libucxx.so", ctypes.RTLD_GLOBAL)
-    except OSError:
-        # If neither of these directories contain the library, we assume we are in an
-        # environment where the C++ library is already installed somewhere else and the
-        # CMake build of the libucxx Python package was a no-op. Note that this approach
-        # won't work for real editable installs of the libucxx package, but that's not a
-        # use case I think we need to support. scikit-build-core has limited support for
-        # importlib.resources so there isn't a clean way to support that case yet.
-        for lib_dir in ("lib", "lib64"):
-            if os.path.isfile(
-                lib := os.path.join(os.path.dirname(__file__), lib_dir, "libucxx.so")
-            ):
-                libucxx_lib = ctypes.CDLL(lib, ctypes.RTLD_GLOBAL)
-                break
+    if prefer_system_installation:
+        # Prefer a system library if one is present to
+        # avoid clobbering symbols that other packages might expect, but if no
+        # other library is present use the one in the wheel.
+        try:
+            libucxx_lib = _load_system_installation(soname)
+        except OSError:
+            libucxx_lib = _load_wheel_installation(soname)
+    else:
+        # Prefer the libraries bundled in this package. If they aren't found
+        # (which might be the case in builds where the library was prebuilt
+        # before packaging the wheel), look for a system installation.
+        try:
+            libucxx_lib = _load_wheel_installation(soname)
+            if libucxx_lib is None:
+                libucxx_lib = _load_system_installation(soname)
+        except OSError:
+            # If none of the searches above succeed, just silently return None
+            # and rely on other mechanisms (like RPATHs on other DSOs) to
+            # help the loader find the library.
+            pass
 
     # The caller almost never needs to do anything with this library, but no
     # harm in offering the option since this object at least provides a handle