Avoid segfaults on decrefs from c++ destructors during exit

Author: msimacek

graalpython/com.oracle.graal.python.jni/src/capi_native.c

@@ -422,6 +422,16 @@ void Py_IncRef(PyObject *a) {
 }
 */
 
+/*
+This is a workaround for C++ modules, namely PyTorch, that declare global/static variables with destructors that call
+_Py_DECREF. The destructors get called by libc during exit during which we cannot make upcalls as that would segfault.
+So we rebind them to no-ops when exitting.
+*/
+static void nop_Py_DecRef(PyObject* obj) {}
+void finalizeCAPI() {
+    __target___Py_DecRef = nop_Py_DecRef;
+    __target__Py_DecRef = nop_Py_DecRef;
+}
 
 PyObject* PyTuple_Pack(Py_ssize_t n, ...) {
     va_list vargs;

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/PythonLanguage.java

@@ -932,4 +932,11 @@ public RootCallTarget getDescriptorCallTarget(BuiltinMethodDescriptor descriptor
         assert callTarget != null : "Missing call target for builtin slot descriptor " + descriptor;
         return callTarget;
     }
+
+    @Override
+    protected void exitContext(PythonContext context, ExitMode exitMode, int exitCode) {
+        if (context.getCApiContext() != null) {
+            context.getCApiContext().finalizeCapi();
+        }
+    }
 }

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/objects/cext/capi/CApiContext.java

@@ -118,6 +118,7 @@
 import com.oracle.truffle.api.TruffleLogger;
 import com.oracle.truffle.api.frame.VirtualFrame;
 import com.oracle.truffle.api.interop.ArityException;
+import com.oracle.truffle.api.interop.InteropException;
 import com.oracle.truffle.api.interop.InteropLibrary;
 import com.oracle.truffle.api.interop.TruffleObject;
 import com.oracle.truffle.api.interop.UnknownIdentifierException;
@@ -693,6 +694,19 @@ public static CApiContext ensureCapiWasLoaded(Node node, PythonContext context,
         return context.getCApiContext();
     }
 
+    @TruffleBoundary
+    public void finalizeCapi() {
+        if (nativeLibrary != null) {
+            try {
+                Object initFunction = InteropLibrary.getUncached().readMember(nativeLibrary, "finalizeCAPI");
+                Object signature = PythonContext.get(null).getEnv().parseInternal(Source.newBuilder("nfi", "():VOID", "exec").build()).call();
+                SignatureLibrary.getUncached().call(signature, initFunction);
+            } catch (InteropException e) {
+                throw CompilerDirectives.shouldNotReachHere(e);
+            }
+        }
+    }
+
     @TruffleBoundary
     public Object initCApiModule(Node location, Object sharedLibrary, TruffleString initFuncName, ModuleSpec spec, InteropLibrary llvmInteropLib, CheckFunctionResultNode checkFunctionResultNode)
                     throws UnsupportedMessageException, ArityException, UnsupportedTypeException, ImportException {