python · ericsnowcurrently · Sep 14, 2018 · Sep 13, 2018 · Sep 13, 2018 · Sep 13, 2018
diff --git a/Include/internal/pystate.h b/Include/internal/pystate.h
@@ -218,6 +218,7 @@ PyAPI_FUNC(_PyInitError) _PyRuntime_Initialize(void);
 /* Other */
 
 PyAPI_FUNC(_PyInitError) _PyInterpreterState_Enable(_PyRuntimeState *);
+PyAPI_FUNC(void) _PyInterpreterState_DeleteExceptMain(void);
 
 #ifdef __cplusplus
 }

diff --git a/Lib/test/test__xxsubinterpreters.py b/Lib/test/test__xxsubinterpreters.py
@@ -824,23 +824,12 @@ def test_fork(self):
 
             expected = 'spam spam spam spam spam'
             script = dedent(f"""
-                # (inspired by Lib/test/test_fork.py)
                 import os
-                pid = os.fork()
-                if pid == 0:  # child
+                try:
+                    os.fork()
+                except RuntimeError:
                     with open('{file.name}', 'w') as out:
                         out.write('{expected}')
-                    # Kill the unittest runner in the child process.
-                    os._exit(1)
-                else:
-                    SHORT_SLEEP = 0.1
-                    import time
-                    for _ in range(10):
-                        spid, status = os.waitpid(pid, os.WNOHANG)
-                        if spid == pid:
-                            break
-                        time.sleep(SHORT_SLEEP)
-                    assert(spid == pid)
                 """)
             interpreters.run_string(self.id, script)
 

diff --git a/Misc/NEWS.d/next/Core and Builtins/2018-09-13-12-21-08.bpo-34651.v-bUeV.rst b/Misc/NEWS.d/next/Core and Builtins/2018-09-13-12-21-08.bpo-34651.v-bUeV.rst
@@ -0,0 +1,3 @@
+Only allow the main interpreter to fork.  The avoids the possibility of
+affecting the main interprerter, which is critical to operation of the
+runtime.
diff --git a/Modules/_posixsubprocess.c b/Modules/_posixsubprocess.c
@@ -576,6 +576,11 @@ subprocess_fork_exec(PyObject* self, PyObject *args)
             &restore_signals, &call_setsid, &preexec_fn))
         return NULL;
 
+    if (_PyInterpreterState_Get() != PyInterpreterState_Main()) {
+        PyErr_SetString(PyExc_RuntimeError, "fork not supported for subinterpreters");
+        return NULL;
+    }
+
     if (close_fds && errpipe_write < 3) {  /* precondition */
         PyErr_SetString(PyExc_ValueError, "errpipe_write must be >= 3");
         return NULL;

diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
@@ -32,6 +32,7 @@
 #else
 #include "winreparse.h"
 #endif
+#include "internal/pystate.h"
 
 /* On android API level 21, 'AT_EACCESS' is not declared although
  * HAVE_FACCESSAT is defined. */
@@ -420,6 +421,7 @@ void
 PyOS_AfterFork_Child(void)
 {
     _PyGILState_Reinit();
+    _PyInterpreterState_DeleteExceptMain();
     PyEval_ReInitThreads();
     _PyImport_ReInitLock();
     _PySignal_AfterFork();
@@ -5790,6 +5792,10 @@ os_fork1_impl(PyObject *module)
 {
     pid_t pid;
 
+    if (_PyInterpreterState_Get() != PyInterpreterState_Main()) {
+        PyErr_SetString(PyExc_RuntimeError, "fork not supported for subinterpreters");
+        return NULL;
+    }
     PyOS_BeforeFork();
     pid = fork1();
     if (pid == 0) {
@@ -5821,6 +5827,10 @@ os_fork_impl(PyObject *module)
 {
     pid_t pid;
 
+    if (_PyInterpreterState_Get() != PyInterpreterState_Main()) {
+        PyErr_SetString(PyExc_RuntimeError, "fork not supported for subinterpreters");
+        return NULL;
+    }
     PyOS_BeforeFork();
     pid = fork();
     if (pid == 0) {
@@ -6416,6 +6426,10 @@ os_forkpty_impl(PyObject *module)
     int master_fd = -1;
     pid_t pid;
 
+    if (_PyInterpreterState_Get() != PyInterpreterState_Main()) {
+        PyErr_SetString(PyExc_RuntimeError, "fork not supported for subinterpreters");
+        return NULL;
+    }
     PyOS_BeforeFork();
     pid = forkpty(&master_fd, NULL, NULL, NULL);
     if (pid == 0) {

diff --git a/Python/pystate.c b/Python/pystate.c
@@ -268,6 +268,44 @@ PyInterpreterState_Delete(PyInterpreterState *interp)
 }
 
 
+/*
+ * Delete all interpreter states except the main interpreter.  If there
+ * is a current interpreter state, it *must* be the main interpreter.
+ */
+void
+_PyInterpreterState_DeleteExceptMain()
+{
+    PyThreadState *tstate = PyThreadState_Swap(NULL);
+    if (tstate != NULL && tstate->interp != _PyRuntime.interpreters.main) {
+        Py_FatalError("PyInterpreterState_DeleteExceptMain: not main interpreter");
+    }
+
+    HEAD_LOCK();
+    PyInterpreterState *interp = _PyRuntime.interpreters.head;
+    _PyRuntime.interpreters.head = NULL;
+    for (; interp != NULL; interp = interp->next) {
+        if (interp == _PyRuntime.interpreters.main) {
+            _PyRuntime.interpreters.main->next = NULL;
+            _PyRuntime.interpreters.head = interp;
+            continue;
+        }
+
+        PyInterpreterState_Clear(interp);  // XXX must activate?
+        zapthreads(interp);
+        if (interp->id_mutex != NULL) {
+            PyThread_free_lock(interp->id_mutex);
+        }
+        PyMem_RawFree(interp);
+    }
+    HEAD_UNLOCK();
+
+    if (_PyRuntime.interpreters.head == NULL) {
+        Py_FatalError("PyInterpreterState_DeleteExceptMain: missing main");
+    }
+    PyThreadState_Swap(tstate);
+}
+
+
 PyInterpreterState *
 _PyInterpreterState_Get(void)
 {