fix: load gopy extensions with RTLD_LOCAL to prevent Go runtime interposition

On macOS, Python's default dlopen flags are RTLD_NOW|RTLD_GLOBAL
(Py_RTLD_DEFAULT in configure.ac for Darwin).  Every .so extension
imported via the normal Python import machinery is therefore loaded into
the process-wide flat namespace.  When two gopy extensions are loaded in
the same process, the second extension's Go runtime symbols (TLS keys,
mheap_, cgo init pointers) get interposed by the first extension's
definitions, causing the two independent Go runtimes to share GC state
and triggering 'fatal error: bad sweepgen in refill' (issue #385).

The generated Python wrapper now temporarily clears RTLD_GLOBAL before
importing the underlying _<pkg>.so, so each extension's Go runtime keeps
its own isolated copy of these globals.  The original flags are restored
immediately after import so the rest of the program is unaffected.

Author: b-long

bind/gen.go

@@ -317,7 +317,22 @@ except ImportError:
 cwd = os.getcwd()
 currentdir = os.path.dirname(os.path.abspath(inspect.getfile(inspect.currentframe())))
 os.chdir(currentdir)
+# Load the extension with RTLD_LOCAL so each gopy .so keeps its own copy of
+# the Go runtime globals (mheap_, TLS keys, etc.).  On macOS, Python's default
+# dlopen flags include RTLD_GLOBAL which causes symbol interposition across
+# independently-built Go runtimes loaded in the same process (issue #385).
+if hasattr(sys, 'getdlopenflags'):
+	try:
+		import ctypes as _gopy_ctypes
+		_gopy_saved_flags = sys.getdlopenflags()
+		sys.setdlopenflags(_gopy_saved_flags & ~getattr(_gopy_ctypes, 'RTLD_GLOBAL', 0))
+	except Exception:
+		_gopy_saved_flags = None
+else:
+	_gopy_saved_flags = None
 %[6]s
+if _gopy_saved_flags is not None:
+	sys.setdlopenflags(_gopy_saved_flags)
 os.chdir(cwd)
 
 # to use this code in your end-user python file, import it as follows: