5 files changed, 83 insertions, 8 deletions

diff --git a/Doc/c-api/init.rst b/Doc/c-api/init.rst
index 2f77bb3..a9927ab 100644
--- a/Doc/c-api/init.rst
+++ b/Doc/c-api/init.rst
@@ -58,8 +58,9 @@ The following functions can be safely called before Python is initialized:
 
    The following functions **should not be called** before
    :c:func:`Py_Initialize`: :c:func:`Py_EncodeLocale`, :c:func:`Py_GetPath`,
-   :c:func:`Py_GetPrefix`, :c:func:`Py_GetExecPrefix` and
-   :c:func:`Py_GetProgramFullPath` and :c:func:`Py_GetPythonHome`.
+   :c:func:`Py_GetPrefix`, :c:func:`Py_GetExecPrefix`,
+   :c:func:`Py_GetProgramFullPath`, :c:func:`Py_GetPythonHome` and
+   :c:func:`PyEval_InitThreads`.
 
 
 .. _global-conf-vars:
diff --git a/Lib/test/test_embed.py b/Lib/test/test_embed.py
index 8d44543..c7f45b5 100644
--- a/Lib/test/test_embed.py
+++ b/Lib/test/test_embed.py
@@ -198,6 +198,16 @@ class EmbeddingTests(unittest.TestCase):
         self.assertEqual(out, '')
         self.assertEqual(err, '')
 
+    def test_bpo20891(self):
+        """
+        bpo-20891: Calling PyGILState_Ensure in a non-Python thread before
+        calling PyEval_InitThreads() must not crash. PyGILState_Ensure() must
+        call PyEval_InitThreads() for us in this case.
+        """
+        out, err = self.run_embedded_interpreter("bpo20891")
+        self.assertEqual(out, '')
+        self.assertEqual(err, '')
+
 
 if __name__ == "__main__":
     unittest.main()
diff --git a/Misc/NEWS.d/next/C API/2017-11-30-18-13-45.bpo-20891.wBnMdF.rst b/Misc/NEWS.d/next/C API/2017-11-30-18-13-45.bpo-20891.wBnMdF.rst
new file mode 100644
index 0000000..e89cf12
--- /dev/null
+++ b/Misc/NEWS.d/next/C API/2017-11-30-18-13-45.bpo-20891.wBnMdF.rst
@@ -0,0 +1,3 @@
+Fix PyGILState_Ensure(). When PyGILState_Ensure() is called in a non-Python
+thread before PyEval_InitThreads(), only call PyEval_InitThreads() after
+calling PyThreadState_New() to fix a crash.
diff --git a/Programs/_testembed.c b/Programs/_testembed.c
index 21aa76e..a528f3e 100644
--- a/Programs/_testembed.c
+++ b/Programs/_testembed.c
@@ -1,4 +1,5 @@
 #include <Python.h>
+#include "pythread.h"
 #include <inttypes.h>
 #include <stdio.h>
 
@@ -147,6 +148,53 @@ static int test_pre_initialization_api(void)
     return 0;
 }
 
+static void bpo20891_thread(void *lockp)
+{
+    PyThread_type_lock lock = *((PyThread_type_lock*)lockp);
+
+    PyGILState_STATE state = PyGILState_Ensure();
+    if (!PyGILState_Check()) {
+        fprintf(stderr, "PyGILState_Check failed!");
+        abort();
+    }
+
+    PyGILState_Release(state);
+
+    PyThread_release_lock(lock);
+
+    PyThread_exit_thread();
+}
+
+static int test_bpo20891(void)
+{
+    /* bpo-20891: Calling PyGILState_Ensure in a non-Python thread before
+       calling PyEval_InitThreads() must not crash. PyGILState_Ensure() must
+       call PyEval_InitThreads() for us in this case. */
+    PyThread_type_lock lock = PyThread_allocate_lock();
+    if (!lock) {
+        fprintf(stderr, "PyThread_allocate_lock failed!");
+        return 1;
+    }
+
+    _testembed_Py_Initialize();
+
+    unsigned long thrd = PyThread_start_new_thread(bpo20891_thread, &lock);
+    if (thrd == PYTHREAD_INVALID_THREAD_ID) {
+        fprintf(stderr, "PyThread_start_new_thread failed!");
+        return 1;
+    }
+    PyThread_acquire_lock(lock, WAIT_LOCK);
+
+    Py_BEGIN_ALLOW_THREADS
+    /* wait until the thread exit */
+    PyThread_acquire_lock(lock, WAIT_LOCK);
+    Py_END_ALLOW_THREADS
+
+    PyThread_free_lock(lock);
+
+    return 0;
+}
+
 
 /* *********************************************************
  * List of test cases and the function that implements it.
@@ -170,6 +218,7 @@ static struct TestCase TestCases[] = {
     { "forced_io_encoding", test_forced_io_encoding },
     { "repeated_init_and_subinterpreters", test_repeated_init_and_subinterpreters },
     { "pre_initialization_api", test_pre_initialization_api },
+    { "bpo20891", test_bpo20891 },
     { NULL, NULL }
 };
 
diff --git a/Python/pystate.c b/Python/pystate.c
index 0fb8ed0..500f967 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -922,6 +922,8 @@ PyGILState_Ensure(void)
 {
     int current;
     PyThreadState *tcur;
+    int need_init_threads = 0;
+
     /* Note that we do not auto-init Python here - apart from
        potential races with 2 threads auto-initializing, pep-311
        spells out other issues.  Embedders are expected to have
@@ -929,12 +931,10 @@ PyGILState_Ensure(void)
     */
     /* Py_Initialize() hasn't been called! */
     assert(_PyRuntime.gilstate.autoInterpreterState);
+
     tcur = (PyThreadState *)PyThread_tss_get(&_PyRuntime.gilstate.autoTSSkey);
     if (tcur == NULL) {
-        /* At startup, Python has no concrete GIL. If PyGILState_Ensure() is
-           called from a new thread for the first time, we need the create the
-           GIL. */
-        PyEval_InitThreads();
+        need_init_threads = 1;
 
         /* Create a new thread state for this thread */
         tcur = PyThreadState_New(_PyRuntime.gilstate.autoInterpreterState);
@@ -945,16 +945,28 @@ PyGILState_Ensure(void)
         tcur->gilstate_counter = 0;
         current = 0; /* new thread state is never current */
     }
-    else
+    else {
         current = PyThreadState_IsCurrent(tcur);
-    if (current == 0)
+    }
+
+    if (current == 0) {
         PyEval_RestoreThread(tcur);
+    }
+
     /* Update our counter in the thread-state - no need for locks:
        - tcur will remain valid as we hold the GIL.
        - the counter is safe as we are the only thread "allowed"
          to modify this value
     */
     ++tcur->gilstate_counter;
+
+    if (need_init_threads) {
+        /* At startup, Python has no concrete GIL. If PyGILState_Ensure() is
+           called from a new thread for the first time, we need the create the
+           GIL. */
+        PyEval_InitThreads();
+    }
+
     return current ? PyGILState_LOCKED : PyGILState_UNLOCKED;
 }