Add some explanatory comments.
git-svn-id: https://llvm.org/svn/llvm-project/llvdb/trunk@127438 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/source/Interpreter/ScriptInterpreterPython.cpp b/source/Interpreter/ScriptInterpreterPython.cpp
index 68c120d..07eeafa 100644
--- a/source/Interpreter/ScriptInterpreterPython.cpp
+++ b/source/Interpreter/ScriptInterpreterPython.cpp
@@ -1384,6 +1384,17 @@
// The following call drops into the embedded interpreter loop and stays there until the
// user chooses to exit from the Python interpreter.
+ // When in the embedded interpreter, the user can call arbitrary system and Python stuff, which may require
+ // the ability to run multi-threaded stuff, so we need to surround the call the the embedded interpreter with
+ // calls to Py_BEGIN_ALLOW_THREADS and Py_END_ALLOW_THREADS.
+
+ // We ALSO need to surround the call to the embedded interpreter with calls to PyGILState_Ensure and
+ // PyGILState_Release. This is because this embedded interpreter is being run on a DIFFERENT THREAD than
+ // the thread on which the call to Py_Initialize (and PyEval_InitThreads) was called. Those initializations
+ // called PyGILState_Ensure on *that* thread, but it also needs to be called on *this* thread. Otherwise,
+ // if the user calls Python code that does threading stuff, the interpreter state will be off, and things could
+ // hang (it's happened before).
+
Py_BEGIN_ALLOW_THREADS
PyGILState_STATE gstate = PyGILState_Ensure();