py/objgenerator: Protect against reentering a generator.

Generators that are already executing cannot be reexecuted.  This patch
puts in a check for such a case.

Thanks to @jepler for finding the bug.

1 files changed, 15 insertions, 0 deletions

diff --git a/tests/basics/gen_yield_from_executing.py b/tests/basics/gen_yield_from_executing.py
new file mode 100644
index 000000000..cad0c7695
--- /dev/null
+++ b/tests/basics/gen_yield_from_executing.py
@@ -0,0 +1,15 @@
+# yielding from an already executing generator is not allowed
+
+def f():
+    yield 1
+    # g here is already executing so this will raise an exception
+    yield from g
+
+g = f()
+
+print(next(g))
+
+try:
+    next(g)
+except ValueError:
+    print('ValueError')