Optionally set __cause__.

Author: ericsnowcurrently

Python/crossinterp.c

@@ -199,19 +199,20 @@ _check_xidata(PyThreadState *tstate, _PyXIData_t *data)
 }
 
 static inline void
-_set_xid_lookup_failure(PyThreadState *tstate, PyObject *obj, const char *msg)
+_set_xid_lookup_failure(PyThreadState *tstate, PyObject *obj, const char *msg,
+                        PyObject *cause)
 {
     if (msg != NULL) {
         assert(obj == NULL);
-        set_notshareableerror(tstate, msg);
+        set_notshareableerror(tstate, cause, msg);
     }
     else if (obj == NULL) {
-        set_notshareableerror(
-                tstate, "object does not support cross-interpreter data");
+        msg = "object does not support cross-interpreter data";
+        set_notshareableerror(tstate, cause, msg);
     }
     else {
-        _PyXIData_FormatNotShareableError(
-                tstate, "%S does not support cross-interpreter data", obj);
+        format_notshareableerror(
+            tstate, cause, "%S does not support cross-interpreter data", obj);
     }
 }
 
@@ -225,7 +226,7 @@ _PyObject_CheckXIData(PyThreadState *tstate, PyObject *obj)
     xidatafunc getdata = lookup_getdata(&ctx, obj);
     if (getdata == NULL) {
         if (!PyErr_Occurred()) {
-            _set_xid_lookup_failure(tstate, obj, NULL);
+            _set_xid_lookup_failure(tstate, obj, NULL, NULL);
         }
         return -1;
     }
@@ -252,7 +253,7 @@ _PyObject_GetXIData(PyThreadState *tstate,
     if (getdata == NULL) {
         Py_DECREF(obj);
         if (!PyErr_Occurred()) {
-            _set_xid_lookup_failure(tstate, obj, NULL);
+            _set_xid_lookup_failure(tstate, obj, NULL, NULL);
         }
         return -1;
     }
@@ -1003,7 +1004,7 @@ _PyXI_ApplyErrorCode(_PyXI_errcode code, PyInterpreterState *interp)
                         "failed to apply namespace to __main__");
         break;
     case _PyXI_ERR_NOT_SHAREABLE:
-        _set_xid_lookup_failure(tstate, NULL, NULL);
+        _set_xid_lookup_failure(tstate, NULL, NULL, NULL);
         break;
     default:
 #ifdef Py_DEBUG
@@ -1066,7 +1067,7 @@ _PyXI_ApplyError(_PyXI_error *error)
     }
     else if (error->code == _PyXI_ERR_NOT_SHAREABLE) {
         // Propagate the exception directly.
-        _set_xid_lookup_failure(tstate, NULL, error->uncaught.msg);
+        _set_xid_lookup_failure(tstate, NULL, error->uncaught.msg, NULL);
     }
     else {
         // Raise an exception corresponding to the code.

Python/crossinterp_data_lookup.h

@@ -72,16 +72,18 @@ _PyXIData_GetNotShareableErrorType(PyThreadState *tstate)
 void
 _PyXIData_SetNotShareableError(PyThreadState *tstate, const char *msg)
 {
-    set_notshareableerror(tstate, msg);
+    PyObject *cause = NULL;
+    set_notshareableerror(tstate, cause, msg);
 }
 
 void
 _PyXIData_FormatNotShareableError(PyThreadState *tstate,
                                   const char *format, ...)
 {
+    PyObject *cause = NULL;
     va_list vargs;
     va_start(vargs, format);
-    format_notshareableerror_v(tstate, format, vargs);
+    format_notshareableerror_v(tstate, cause, format, vargs);
     va_end(vargs);
 }
 

Python/crossinterp_exceptions.h

@@ -1,4 +1,24 @@
 
+static void
+_ensure_current_cause(PyThreadState *tstate, PyObject *cause)
+{
+    if (cause == NULL) {
+        return;
+    }
+    PyObject *exc = _PyErr_GetRaisedException(tstate);
+    assert(exc != NULL);
+    PyObject *ctx = PyException_GetContext(exc);
+    if (ctx == NULL) {
+        PyException_SetContext(exc, cause);
+    }
+    else {
+        Py_DECREF(ctx);
+    }
+    assert(PyException_GetCause(exc) == NULL);
+    PyException_SetCause(exc, cause);
+}
+
+
 /* InterpreterError extends Exception */
 
 static PyTypeObject _PyExc_InterpreterError = {
@@ -60,7 +80,7 @@ get_notshareableerror_type(PyThreadState *tstate)
 }
 
 static void
-set_notshareableerror(PyThreadState *tstate, const char *msg)
+set_notshareableerror(PyThreadState *tstate, PyObject *cause, const char *msg)
 {
     PyObject *exctype = get_notshareableerror_type(tstate);
     if (exctype == NULL) {
@@ -70,13 +90,12 @@ set_notshareableerror(PyThreadState *tstate, const char *msg)
     PyObject *ctx = _PyErr_GetRaisedException(tstate);
     _PyErr_SetString(tstate, exctype, msg);
     _PyErr_ChainExceptions1Tstate(tstate, ctx);
-    // XXX Ideally we would also append the currently handling exception
-    // (_PyErr_GetTopmostException()->exc_value) to the end of ctx's
-    // context chain.  (See _PyErr_SetObject().)
+    _ensure_current_cause(tstate, cause);
 }
 
 static void
-format_notshareableerror_v(PyThreadState *tstate, const char *format, va_list vargs)
+format_notshareableerror_v(PyThreadState *tstate, PyObject *cause,
+                           const char *format, va_list vargs)
 {
     PyObject *exctype = get_notshareableerror_type(tstate);
     if (exctype == NULL) {
@@ -86,9 +105,17 @@ format_notshareableerror_v(PyThreadState *tstate, const char *format, va_list va
     PyObject *ctx = _PyErr_GetRaisedException(tstate);
     _PyErr_FormatV(tstate, exctype, format, vargs);
     _PyErr_ChainExceptions1Tstate(tstate, ctx);
-    // XXX Ideally we would also append the currently handling exception
-    // (_PyErr_GetTopmostException()->exc_value) to the end of ctx's
-    // context chain.  (See _PyErr_SetObject().)
+    _ensure_current_cause(tstate, cause);
+}
+
+static void
+format_notshareableerror(PyThreadState *tstate, PyObject *cause,
+                         const char *format, ...)
+{
+    va_list vargs;
+    va_start(vargs, format);
+    format_notshareableerror_v(tstate, cause, format, vargs);
+    va_end(vargs);
 }