add test on thread safety

numexpr/tests/test_numexpr.py

@@ -1201,6 +1201,7 @@ def run(self):
         test.join()
 
     def test_multithread(self):
+
         import threading
 
         # Running evaluate() from multiple threads shouldn't crash
@@ -1218,6 +1219,77 @@ def work(n):
         for t in threads:
             t.join()
 
+    def test_thread_safety(self):
+        """
+        Expected output
+
+        When not safe (before the pr this test is commited)
+        AssertionError: Thread-0 failed: result does not match expected
+
+        When safe (after the pr this test is commited)
+        Should pass without failure
+        """
+        import threading
+        import time
+
+        barrier = threading.Barrier(4)
+
+        # Function that each thread will run with different expressions
+        def thread_function(a_value, b_value, expression, expected_result, results, index):
+            validate(expression, local_dict={"a": a_value, "b": b_value})
+            # Wait for all threads to reach this point
+            # such that they all set _numexpr_last
+            barrier.wait()
+
+            # Simulate some work or a context switch delay
+            time.sleep(0.1)
+
+            result = re_evaluate(local_dict={"a": a_value, "b": b_value})
+            results[index] = np.array_equal(result, expected_result)
+
+        def test_thread_safety_with_numexpr():
+            num_threads = 4
+            array_size = 1000000
+
+            expressions = [
+                "a + b",
+                "a - b",
+                "a * b",
+                "a / b"
+            ]
+
+            a_value = [np.full(array_size, i + 1) for i in range(num_threads)]
+            b_value = [np.full(array_size, (i + 1) * 2) for i in range(num_threads)]
+
+            expected_results = [
+                a_value[i] + b_value[i] if expr == "a + b" else
+                a_value[i] - b_value[i] if expr == "a - b" else
+                a_value[i] * b_value[i] if expr == "a * b" else
+                a_value[i] / b_value[i] if expr == "a / b" else None
+                for i, expr in enumerate(expressions)
+            ]
+
+            results = [None] * num_threads
+            threads = []
+
+            # Create and start threads with different expressions
+            for i in range(num_threads):
+                thread = threading.Thread(
+                    target=thread_function,
+                    args=(a_value[i], b_value[i], expressions[i], expected_results[i], results, i)
+                )
+                threads.append(thread)
+                thread.start()
+
+            for thread in threads:
+                thread.join()
+
+            for i in range(num_threads):
+                if not results[i]:
+                    self.fail(f"Thread-{i} failed: result does not match expected")
+
+        test_thread_safety_with_numexpr()
+
 
 # The worker function for the subprocess (needs to be here because Windows
 # has problems pickling nested functions with the multiprocess module :-/)