gh-100228: Warn from os.fork() if other threads exist. (#100229)

Not comprehensive, best effort warning. There are cases when threads exist on some platforms that this code cannot detect. macOS when API permissions allow and Linux with a readable /proc procfs present are the currently supported cases where a warning should show up reliably.

Starting with a DeprecationWarning for now, it is less disruptive than something like RuntimeWarning and most likely to only be seen in people's CI tests - a good place to start with this messaging.

1 files changed, 47 insertions, 0 deletions

diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index c32fdb5..c777c3e 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -25,6 +25,9 @@
 #include "structmember.h"         // for offsetof(), T_OBJECT
 #include <float.h>                // FLT_MAX
 #include <signal.h>
+#ifndef MS_WINDOWS
+#include <unistd.h>
+#endif
 
 #ifdef HAVE_SYS_WAIT_H
 #include <sys/wait.h>             // W_STOPCODE
@@ -871,6 +874,46 @@ test_thread_state(PyObject *self, PyObject *args)
     Py_RETURN_NONE;
 }
 
+#ifndef MS_WINDOWS
+static PyThread_type_lock wait_done = NULL;
+
+static void wait_for_lock(void *unused) {
+    PyThread_acquire_lock(wait_done, 1);
+    PyThread_release_lock(wait_done);
+    PyThread_free_lock(wait_done);
+    wait_done = NULL;
+}
+
+// These can be used to test things that care about the existence of another
+// thread that the threading module doesn't know about.
+
+static PyObject *
+spawn_pthread_waiter(PyObject *self, PyObject *Py_UNUSED(ignored))
+{
+    if (wait_done) {
+        PyErr_SetString(PyExc_RuntimeError, "thread already running");
+        return NULL;
+    }
+    wait_done = PyThread_allocate_lock();
+    if (wait_done == NULL)
+        return PyErr_NoMemory();
+    PyThread_acquire_lock(wait_done, 1);
+    PyThread_start_new_thread(wait_for_lock, NULL);
+    Py_RETURN_NONE;
+}
+
+static PyObject *
+end_spawned_pthread(PyObject *self, PyObject *Py_UNUSED(ignored))
+{
+    if (!wait_done) {
+        PyErr_SetString(PyExc_RuntimeError, "call _spawn_pthread_waiter 1st");
+        return NULL;
+    }
+    PyThread_release_lock(wait_done);
+    Py_RETURN_NONE;
+}
+#endif  // not MS_WINDOWS
+
 /* test Py_AddPendingCalls using threads */
 static int _pending_callback(void *arg)
 {
@@ -3207,6 +3250,10 @@ static PyMethodDef TestMethods[] = {
     {"test_get_type_name",        test_get_type_name,            METH_NOARGS},
     {"test_get_type_qualname",    test_get_type_qualname,        METH_NOARGS},
     {"_test_thread_state",      test_thread_state,               METH_VARARGS},
+#ifndef MS_WINDOWS
+    {"_spawn_pthread_waiter",   spawn_pthread_waiter,            METH_NOARGS},
+    {"_end_spawned_pthread",    end_spawned_pthread,             METH_NOARGS},
+#endif
     {"_pending_threadfunc",     pending_threadfunc,              METH_VARARGS},
 #ifdef HAVE_GETTIMEOFDAY
     {"profile_int",             profile_int,                     METH_NOARGS},