[FFI][BUGFIX] Grab GIL when check env signals

This PR updates the CheckSignals function to grab GIL.
This is needed because we now explicitly release gil when calling
any C functions. GIL will need to be obtained otherwise we will
run into segfault when checking the signal.

The update now enables us to run ctrl + C in long running C functions.

python/tvm/_ffi/_cython/base.pxi

@@ -201,6 +201,10 @@ cdef inline void* c_handle(object handle):
 # python env API
 cdef extern from "Python.h":
     int PyErr_CheckSignals()
+    void* PyGILState_Ensure()
+    void PyGILState_Release(void*)
+    void Py_IncRef(void*)
+    void Py_DecRef(void*)
 
 cdef extern from "tvm/runtime/c_backend_api.h":
     int TVMBackendRegisterEnvCAPI(const char* name, void* ptr)
@@ -210,11 +214,13 @@ cdef _init_env_api():
     # so backend can call tvm::runtime::EnvCheckSignals to check
     # signal when executing a long running function.
     #
-    # This feature is only enabled in cython for now due to problems of calling
-    # these functions in ctypes.
-    #
-    # When the functions are not registered, the signals will be handled
-    # only when the FFI function returns.
+    # Also registers the gil state release and ensure as PyErr_CheckSignals
+    # function is called with gil released and we need to regrab the gil
     CHECK_CALL(TVMBackendRegisterEnvCAPI(c_str("PyErr_CheckSignals"), <void*>PyErr_CheckSignals))
+    CHECK_CALL(TVMBackendRegisterEnvCAPI(c_str("PyGILState_Ensure"), <void*>PyGILState_Ensure))
+    CHECK_CALL(TVMBackendRegisterEnvCAPI(c_str("PyGILState_Release"), <void*>PyGILState_Release))
+    CHECK_CALL(TVMBackendRegisterEnvCAPI(c_str("PyGILState_Release"), <void*>PyGILState_Release))
+    CHECK_CALL(TVMBackendRegisterEnvCAPI(c_str("Py_IncRef"), <void*>Py_IncRef))
+    CHECK_CALL(TVMBackendRegisterEnvCAPI(c_str("Py_DecRef"), <void*>Py_DecRef))
 
 _init_env_api()

src/runtime/registry.cc

@@ -183,10 +183,14 @@ class EnvCAPIRegistry {
   // implementation of tvm::runtime::EnvCheckSignals
   void CheckSignals() {
     // check python signal to see if there are exception raised
-    if (pyerr_check_signals != nullptr && (*pyerr_check_signals)() != 0) {
-      // The error will let FFI know that the frontend environment
-      // already set an error.
-      throw EnvErrorAlreadySet("");
+    if (pyerr_check_signals != nullptr) {
+      // The C++ env comes without gil, so we need to grab gil here
+      WithGIL context(this);
+      if ((*pyerr_check_signals)() != 0) {
+        // The error will let FFI know that the frontend environment
+        // already set an error.
+        throw EnvErrorAlreadySet("");
+      }
     }
   }
 

src/support/ffi_testing.cc

@@ -178,6 +178,14 @@ TVM_REGISTER_GLOBAL("testing.sleep_in_ffi").set_body_typed([](double timeout) {
   std::this_thread::sleep_for(duration);
 });
 
+TVM_REGISTER_GLOBAL("testing.check_signals").set_body_typed([](double sleep_period) {
+  while (true) {
+    std::chrono::duration<int64_t, std::nano> duration(static_cast<int64_t>(sleep_period * 1e9));
+    std::this_thread::sleep_for(duration);
+    runtime::EnvCheckSignals();
+  }
+});
+
 TVM_REGISTER_GLOBAL("testing.ReturnsVariant").set_body_typed([](int x) -> Variant<String, IntImm> {
   if (x % 2 == 0) {
     return IntImm(DataType::Int(64), x / 2);