bpo-41435: Add sys._current_exceptions() function (GH-21689)

This adds a new function named sys._current_exceptions() which is equivalent ot
sys._current_frames() except that it returns the exceptions currently handled
by other threads. It is equivalent to calling sys.exc_info() for each running
thread.

7 files changed, 185 insertions, 1 deletions

diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
index 2f0840e2a74..f0acfcfe639 100644
--- a/Doc/library/sys.rst
+++ b/Doc/library/sys.rst
@@ -196,6 +196,18 @@ always available.
 
    .. audit-event:: sys._current_frames "" sys._current_frames
 
+.. function:: _current_exceptions()
+
+   Return a dictionary mapping each thread's identifier to the topmost exception
+   currently active in that thread at the time the function is called.
+   If a thread is not currently handling an exception, it is not included in
+   the result dictionary.
+
+   This is most useful for statistical profiling.
+
+   This function should be used for internal and specialized purposes only.
+
+   .. audit-event:: sys._current_exceptions "" sys._current_exceptions
 
 .. function:: breakpointhook()
 
diff --git a/Include/cpython/pystate.h b/Include/cpython/pystate.h
index 5d5e4e33197..25522b4dbec 100644
--- a/Include/cpython/pystate.h
+++ b/Include/cpython/pystate.h
@@ -167,6 +167,11 @@ PyAPI_FUNC(PyInterpreterState *) _PyGILState_GetInterpreterStateUnsafe(void);
 */
 PyAPI_FUNC(PyObject *) _PyThread_CurrentFrames(void);
 
+/* The implementation of sys._current_exceptions()  Returns a dict mapping
+   thread id to that thread's current exception.
+*/
+PyAPI_FUNC(PyObject *) _PyThread_CurrentExceptions(void);
+
 /* Routines for advanced debuggers, requested by David Beazley.
    Don't use unless you know what you are doing! */
 PyAPI_FUNC(PyInterpreterState *) PyInterpreterState_Main(void);
diff --git a/Lib/test/test_sys.py b/Lib/test/test_sys.py
index 30c29a26a99..332ed8f550c 100644
--- a/Lib/test/test_sys.py
+++ b/Lib/test/test_sys.py
@@ -432,6 +432,73 @@ class SysModuleTest(unittest.TestCase):
         leave_g.set()
         t.join()
 
+    @threading_helper.reap_threads
+    def test_current_exceptions(self):
+        import threading
+        import traceback
+
+        # Spawn a thread that blocks at a known place.  Then the main
+        # thread does sys._current_frames(), and verifies that the frames
+        # returned make sense.
+        entered_g = threading.Event()
+        leave_g = threading.Event()
+        thread_info = []  # the thread's id
+
+        def f123():
+            g456()
+
+        def g456():
+            thread_info.append(threading.get_ident())
+            entered_g.set()
+            while True:
+                try:
+                    raise ValueError("oops")
+                except ValueError:
+                    if leave_g.wait(timeout=support.LONG_TIMEOUT):
+                        break
+
+        t = threading.Thread(target=f123)
+        t.start()
+        entered_g.wait()
+
+        # At this point, t has finished its entered_g.set(), although it's
+        # impossible to guess whether it's still on that line or has moved on
+        # to its leave_g.wait().
+        self.assertEqual(len(thread_info), 1)
+        thread_id = thread_info[0]
+
+        d = sys._current_exceptions()
+        for tid in d:
+            self.assertIsInstance(tid, int)
+            self.assertGreater(tid, 0)
+
+        main_id = threading.get_ident()
+        self.assertIn(main_id, d)
+        self.assertIn(thread_id, d)
+        self.assertEqual((None, None, None), d.pop(main_id))
+
+        # Verify that the captured thread frame is blocked in g456, called
+        # from f123.  This is a litte tricky, since various bits of
+        # threading.py are also in the thread's call stack.
+        exc_type, exc_value, exc_tb = d.pop(thread_id)
+        stack = traceback.extract_stack(exc_tb.tb_frame)
+        for i, (filename, lineno, funcname, sourceline) in enumerate(stack):
+            if funcname == "f123":
+                break
+        else:
+            self.fail("didn't find f123() on thread's call stack")
+
+        self.assertEqual(sourceline, "g456()")
+
+        # And the next record must be for g456().
+        filename, lineno, funcname, sourceline = stack[i+1]
+        self.assertEqual(funcname, "g456")
+        self.assertTrue(sourceline.startswith("if leave_g.wait("))
+
+        # Reap the spawned thread.
+        leave_g.set()
+        t.join()
+
     def test_attributes(self):
         self.assertIsInstance(sys.api_version, int)
         self.assertIsInstance(sys.argv, list)
diff --git a/Misc/NEWS.d/next/Core and Builtins/2020-08-07-13-42-48.bpo-41435.qPWjJA.rst b/Misc/NEWS.d/next/Core and Builtins/2020-08-07-13-42-48.bpo-41435.qPWjJA.rst
new file mode 100644
index 00000000000..d2978f9b4ec
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2020-08-07-13-42-48.bpo-41435.qPWjJA.rst
@@ -0,0 +1 @@
+Add `sys._current_exceptions()` function to retrieve a dictionary mapping each thread's identifier to the topmost exception currently active in that thread at the time the function is called.
\ No newline at end of file
diff --git a/Python/clinic/sysmodule.c.h b/Python/clinic/sysmodule.c.h
index c1a9a2d69f0..addd58922e7 100644
--- a/Python/clinic/sysmodule.c.h
+++ b/Python/clinic/sysmodule.c.h
@@ -801,6 +801,26 @@ sys__current_frames(PyObject *module, PyObject *Py_UNUSED(ignored))
     return sys__current_frames_impl(module);
 }
 
+PyDoc_STRVAR(sys__current_exceptions__doc__,
+"_current_exceptions($module, /)\n"
+"--\n"
+"\n"
+"Return a dict mapping each thread\'s identifier to its current raised exception.\n"
+"\n"
+"This function should be used for specialized purposes only.");
+
+#define SYS__CURRENT_EXCEPTIONS_METHODDEF    \
+    {"_current_exceptions", (PyCFunction)sys__current_exceptions, METH_NOARGS, sys__current_exceptions__doc__},
+
+static PyObject *
+sys__current_exceptions_impl(PyObject *module);
+
+static PyObject *
+sys__current_exceptions(PyObject *module, PyObject *Py_UNUSED(ignored))
+{
+    return sys__current_exceptions_impl(module);
+}
+
 PyDoc_STRVAR(sys_call_tracing__doc__,
 "call_tracing($module, func, args, /)\n"
 "--\n"
@@ -945,4 +965,4 @@ sys_getandroidapilevel(PyObject *module, PyObject *Py_UNUSED(ignored))
 #ifndef SYS_GETANDROIDAPILEVEL_METHODDEF
     #define SYS_GETANDROIDAPILEVEL_METHODDEF
 #endif /* !defined(SYS_GETANDROIDAPILEVEL_METHODDEF) */
-/*[clinic end generated code: output=87baa3357293ea65 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=bbc4963fe86a29d9 input=a9049054013a1b77]*/
diff --git a/Python/pystate.c b/Python/pystate.c
index e88898670cd..e37cbd5a657 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -1222,6 +1222,69 @@ done:
     return result;
 }
 
+PyObject *
+_PyThread_CurrentExceptions(void)
+{
+    PyThreadState *tstate = _PyThreadState_GET();
+
+    _Py_EnsureTstateNotNULL(tstate);
+
+    if (_PySys_Audit(tstate, "sys._current_exceptions", NULL) < 0) {
+        return NULL;
+    }
+
+    PyObject *result = PyDict_New();
+    if (result == NULL) {
+        return NULL;
+    }
+
+    /* for i in all interpreters:
+     *     for t in all of i's thread states:
+     *          if t's frame isn't NULL, map t's id to its frame
+     * Because these lists can mutate even when the GIL is held, we
+     * need to grab head_mutex for the duration.
+     */
+    _PyRuntimeState *runtime = tstate->interp->runtime;
+    HEAD_LOCK(runtime);
+    PyInterpreterState *i;
+    for (i = runtime->interpreters.head; i != NULL; i = i->next) {
+        PyThreadState *t;
+        for (t = i->tstate_head; t != NULL; t = t->next) {
+            _PyErr_StackItem *err_info = _PyErr_GetTopmostException(t);
+            if (err_info == NULL) {
+                continue;
+            }
+            PyObject *id = PyLong_FromUnsignedLong(t->thread_id);
+            if (id == NULL) {
+                goto fail;
+            }
+            PyObject *exc_info = PyTuple_Pack(
+                3,
+                err_info->exc_type != NULL ? err_info->exc_type : Py_None,
+                err_info->exc_value != NULL ? err_info->exc_value : Py_None,
+                err_info->exc_traceback != NULL ? err_info->exc_traceback : Py_None);
+            if (exc_info == NULL) {
+                Py_DECREF(id);
+                goto fail;
+            }
+            int stat = PyDict_SetItem(result, id, exc_info);
+            Py_DECREF(id);
+            Py_DECREF(exc_info);
+            if (stat < 0) {
+                goto fail;
+            }
+        }
+    }
+    goto done;
+
+fail:
+    Py_CLEAR(result);
+
+done:
+    HEAD_UNLOCK(runtime);
+    return result;
+}
+
 /* Python "auto thread state" API. */
 
 /* Keep this as a static, as it is not reliable!  It can only
diff --git a/Python/sysmodule.c b/Python/sysmodule.c
index 749b96455d6..945e639ca57 100644
--- a/Python/sysmodule.c
+++ b/Python/sysmodule.c
@@ -1838,6 +1838,21 @@ sys__current_frames_impl(PyObject *module)
 }
 
 /*[clinic input]
+sys._current_exceptions
+
+Return a dict mapping each thread's identifier to its current raised exception.
+
+This function should be used for specialized purposes only.
+[clinic start generated code]*/
+
+static PyObject *
+sys__current_exceptions_impl(PyObject *module)
+/*[clinic end generated code: output=2ccfd838c746f0ba input=0e91818fbf2edc1f]*/
+{
+    return _PyThread_CurrentExceptions();
+}
+
+/*[clinic input]
 sys.call_tracing
 
     func: object
@@ -1953,6 +1968,7 @@ static PyMethodDef sys_methods[] = {
      METH_FASTCALL | METH_KEYWORDS, breakpointhook_doc},
     SYS__CLEAR_TYPE_CACHE_METHODDEF
     SYS__CURRENT_FRAMES_METHODDEF
+    SYS__CURRENT_EXCEPTIONS_METHODDEF
     SYS_DISPLAYHOOK_METHODDEF
     SYS_EXC_INFO_METHODDEF
     SYS_EXCEPTHOOK_METHODDEF