@@ -645,6 +645,10 @@ loops that truncate the stream.
    used anywhere else; otherwise, the *iterable* could get advanced without
    the tee objects being informed.
 
+   ``tee`` iterators are not threadsafe. A :exc:`RuntimeError` may be
+   raised when using simultaneously iterators returned by the same :func:`tee`
+   call, even if the original *iterable* is threadsafe.
+
    This itertool may require significant auxiliary storage (depending on how
    much temporary data needs to be stored). In general, if one iterator uses
    most or all of the data before another iterator starts, it is faster to use
 
@@ -11,6 +11,7 @@
 from functools import reduce
 import sys
 import struct
+import threading
 maxsize = support.MAX_Py_ssize_t
 minsize = -maxsize-1
 
@@ -1494,6 +1495,42 @@ def test_tee_del_backward(self):
             del forward, backward
             raise
 
+    def test_tee_reenter(self):
+        class I:
+            first = True
+            def __iter__(self):
+                return self
+            def __next__(self):
+                first = self.first
+                self.first = False
+                if first:
+                    return next(b)
+
+        a, b = tee(I())
+        with self.assertRaisesRegex(RuntimeError, "tee"):
+            next(a)
+
+    def test_tee_concurrent(self):
+        start = threading.Event()
+        finish = threading.Event()
+        class I:
+            def __iter__(self):
+                return self
+            def __next__(self):
+                start.set()
+                finish.wait()
+
+        a, b = tee(I())
+        thread = threading.Thread(target=next, args=[a])
+        thread.start()
+        try:
+            start.wait()
+            with self.assertRaisesRegex(RuntimeError, "tee"):
+                next(b)
+        finally:
+            finish.set()
+            thread.join()
+
     def test_StopIteration(self):
         self.assertRaises(StopIteration, next, zip())
 
 
@@ -0,0 +1,2 @@
+Fixed a crash in the :func:`tee` iterator when re-enter it. RuntimeError is
+now raised in this case.
@@ -443,6 +443,7 @@ typedef struct {
     PyObject_HEAD
     PyObject *it;
     int numread;                /* 0 <= numread <= LINKCELLS */
+    int running;
     PyObject *nextlink;
     PyObject *(values[LINKCELLS]);
 } teedataobject;
@@ -465,6 +466,7 @@ teedataobject_newinternal(PyObject *it)
     if (tdo == NULL)
         return NULL;
 
+    tdo->running = 0;
     tdo->numread = 0;
     tdo->nextlink = NULL;
     Py_INCREF(it);
@@ -493,7 +495,14 @@ teedataobject_getitem(teedataobject *tdo, int i)
     else {
         /* this is the lead iterator, so fetch more data */
         assert(i == tdo->numread);
+        if (tdo->running) {
+            PyErr_SetString(PyExc_RuntimeError,
+                            "cannot re-enter the tee iterator");
+            return NULL;
+        }
+        tdo->running = 1;
         value = PyIter_Next(tdo->it);
+        tdo->running = 0;
         if (value == NULL)
             return NULL;
         tdo->numread++;
Original file line number	Diff line number	Diff line change
@@ -0,0 +1,2 @@
	1	+ Fixed a crash in the :func:`tee` iterator when re-enter it. RuntimeError is
	2	+ now raised in this case.