Acquire lock before releasing GIL

Author: Jgunde

apriltag_pywrap.c

@@ -68,6 +68,7 @@ typedef struct {
 
     apriltag_family_t*   tf;
     apriltag_detector_t* td;
+    PyThread_type_lock   det_lock;
     void (*destroy_func)(apriltag_family_t *tf);
 } apriltag_py_t;
 
@@ -85,6 +86,12 @@ apriltag_new(PyTypeObject *type, PyObject *args, PyObject *kwargs)
     self->tf = NULL;
     self->td = NULL;
 
+    self->det_lock = PyThread_allocate_lock();
+    if (self->det_lock == NULL) {
+        PyErr_SetString(PyExc_RuntimeError, "Unable to allocate detection lock");
+        goto done;
+    }
+
     const char* family          = NULL;
     int         Nthreads        = 1;
     int         maxhamming      = 1;
@@ -173,6 +180,11 @@ apriltag_new(PyTypeObject *type, PyObject *args, PyObject *kwargs)
                 self->destroy_func(self->tf);
                 self->tf = NULL;
             }
+            if(self->det_lock != NULL)
+            {
+                PyThread_free_lock(self->det_lock);
+                self->det_lock = NULL;
+            }
             Py_DECREF(self);
         }
         return NULL;
@@ -195,6 +207,11 @@ static void apriltag_dealloc(apriltag_py_t* self)
         self->destroy_func(self->tf);
         self->tf = NULL;
     }
+    if(self->det_lock != NULL)
+    {
+        PyThread_free_lock(self->det_lock);
+        self->det_lock = NULL;
+    }
 
     Py_TYPE(self)->tp_free((PyObject*)self);
 }
@@ -244,9 +261,14 @@ static PyObject* apriltag_detect(apriltag_py_t* self,
                      .buf    = PyArray_DATA(image)};
 
     zarray_t *detections;  // Declare detections variable outside of the GIL block
-    Py_BEGIN_ALLOW_THREADS  // Acquire the GIL before running detection
-        detections = apriltag_detector_detect(self->td, &im);
-    Py_END_ALLOW_THREADS  // Release the GIL after detection completes
+    Py_BEGIN_ALLOW_THREADS  // Acquire the GIL before acquiring the detection lock
+        if (PyThread_acquire_lock(self->det_lock, 1) == 0) {  // Acquire the detection lock before running the detector
+            PyErr_SetString(PyExc_RuntimeError, "Unable to acquire detection lock");
+            goto done;
+        }
+        detections = apriltag_detector_detect(self->td, &im);  // Run detection
+        PyThread_release_lock(self->det_lock);  // Release the detection lock
+    Py_END_ALLOW_THREADS  // Release the GIL after releasing the detection lock
 
     int N = zarray_size(detections);