Generators are not reentrant

src/runtime/generator.cpp

@@ -61,12 +61,17 @@ Box* generatorSend(Box* s, Box* v) {
     assert(s->cls == generator_cls);
     BoxedGenerator* self = static_cast<BoxedGenerator*>(s);
 
+    if (self->running)
+        raiseExcHelper(ValueError, "generator already executing");
+
     // check if the generator already exited
     if (self->entryExited)
         raiseExcHelper(StopIteration, "");
 
     self->returnValue = v;
+    self->running = true;
     swapcontext(&self->returnContext, &self->context);
+    self->running = false;
 
     // propagate exception to the caller
     if (self->exception)
@@ -130,7 +135,7 @@ extern "C" BoxedGenerator* createGenerator(BoxedFunction* function, Box* arg1, B
 
 extern "C" BoxedGenerator::BoxedGenerator(BoxedFunction* function, Box* arg1, Box* arg2, Box* arg3, Box** args)
     : Box(generator_cls), function(function), arg1(arg1), arg2(arg2), arg3(arg3), args(nullptr), entryExited(false),
-      returnValue(nullptr), exception(nullptr) {
+      running(false), returnValue(nullptr), exception(nullptr) {
 
     giveAttr("__name__", boxString(function->f->source->getName()));
 

src/runtime/types.h

@@ -524,6 +524,7 @@ public:
     GCdArray* args;
 
     bool entryExited;
+    bool running;
     Box* returnValue;
     Box* exception;
 

test/tests/generator_reentry.py

+# From PEP 255: "A generator cannot be resumed while it is actively running"
+
+def g():
+    i = me.next()
+    yield i
+me = g()
+try:
+    me.next()
+except ValueError, e:
+    # Should be: "generator already executing"
+    print e

test/tests/generator_thread_sharing.py

+# Start a generator on one thread, pass it to another, and have that execute it for a while
+
 def gen():
     while True:
         for i in xrange(100):

test/tests/generator_threads.py

+# Pass a started generator to two different threads, and make them both
+# try to run it at the same time.
+
+started = []
+def gen():
+    started.append(None)
+    while len(started) == 1:
+        pass
+
+    yield "done"
+
+done = []
+def run_through(g, i):
+    if i == 0:
+        print g.next()
+    else:
+        while len(started) < 1:
+            pass
+
+        try:
+            print g.next()
+        except ValueError, e:
+            print e
+
+        started.append(None)
+
+    done.append(None)
+
+g = gen()
+from thread import start_new_thread
+
+start_new_thread(run_through, (g, 0))
+start_new_thread(run_through, (g, 1))
+
+import time
+while len(done) < 2:
+    time.sleep(0.01)