[LLDB] Fix Python GIL-not-held issues

The GIL must be held when calling any Python C API functions. In multithreaded applications that use callbacks this requirement can easily be violated by accident. A general tool to ensure GIL health is not available, but patching Python Py_INCREF to add an assert provides a basic health check:

```
+int PyGILState_Check(void); /* Include/internal/pystate.h */
+
 #define Py_INCREF(op) (                         \
+    assert(PyGILState_Check()),                 \
     _Py_INC_REFTOTAL  _Py_REF_DEBUG_COMMA       \
     ((PyObject *)(op))->ob_refcnt++)

 #define Py_DECREF(op)                                   \
     do {                                                \
+        assert(PyGILState_Check());                     \
         PyObject *_py_decref_tmp = (PyObject *)(op);    \
         if (_Py_DEC_REFTOTAL  _Py_REF_DEBUG_COMMA       \
         --(_py_decref_tmp)->ob_refcnt != 0)             \
```

Adding this assertion causes around 50 test failures in LLDB. Adjusting the scope of things guarded by `py_lock` fixes them.

More background: https://docs.python.org/3/glossary.html#term-global-interpreter-lock

Patch by Ralf Grosse-Kunstleve

Differential Revision: https://reviews.llvm.org/D114722

1 files changed, 21 insertions, 4 deletions

diff --git a/lldb/source/Plugins/ScriptInterpreter/Python/PythonDataObjects.h b/lldb/source/Plugins/ScriptInterpreter/Python/PythonDataObjects.h
index 56bc55d..9d2cdca 100644
--- a/lldb/source/Plugins/ScriptInterpreter/Python/PythonDataObjects.h
+++ b/lldb/source/Plugins/ScriptInterpreter/Python/PythonDataObjects.h
@@ -88,12 +88,21 @@ public:
   StructuredPythonObject() : StructuredData::Generic() {}
 
   StructuredPythonObject(void *obj) : StructuredData::Generic(obj) {
+    assert(PyGILState_Check());
     Py_XINCREF(GetValue());
   }
 
   ~StructuredPythonObject() override {
-    if (Py_IsInitialized())
-      Py_XDECREF(GetValue());
+    if (Py_IsInitialized()) {
+      if (_Py_IsFinalizing()) {
+        // Leak GetValue() rather than crashing the process.
+        // https://docs.python.org/3/c-api/init.html#c.PyGILState_Ensure
+      } else {
+        PyGILState_STATE state = PyGILState_Ensure();
+        Py_XDECREF(GetValue());
+        PyGILState_Release(state);
+      }
+    }
     SetValue(nullptr);
   }
 
@@ -264,8 +273,16 @@ public:
   ~PythonObject() { Reset(); }
 
   void Reset() {
-    if (m_py_obj && Py_IsInitialized())
-      Py_DECREF(m_py_obj);
+    if (m_py_obj && Py_IsInitialized()) {
+      if (_Py_IsFinalizing()) {
+        // Leak m_py_obj rather than crashing the process.
+        // https://docs.python.org/3/c-api/init.html#c.PyGILState_Ensure
+      } else {
+        PyGILState_STATE state = PyGILState_Ensure();
+        Py_DECREF(m_py_obj);
+        PyGILState_Release(state);
+      }
+    }
     m_py_obj = nullptr;
   }