Fix logsparse on multiprocess spawn (#480)

src/viztracer/patch.py

@@ -157,7 +157,8 @@ def run(self) -> None:
         tracer = viztracer.VizTracer(**self._viztracer_kwargs)
         install_all_hooks(tracer, self._cmdline_args)
         tracer.register_exit()
-        tracer.start()
+        if not self._viztracer_kwargs.get("log_sparse"):
+            tracer.start()
         self._run()
 
 

tests/test_logsparse.py

@@ -76,6 +76,27 @@ def f(x):
         time.sleep(0.1)
 """
 
+file_multiprocess_spawn = """
+import multiprocessing
+from viztracer import get_tracer
+
+def bar():
+    pass
+
+def child(it):
+    for idx in it:
+        with get_tracer().log_event(f"custom_event"):
+            bar()
+
+def main():
+    p = multiprocessing.get_context("spawn").Process(target=child, args=(range(3), ))
+    p.start()
+    p.join()
+
+if __name__ == "__main__":
+    main()
+"""
+
 file_context_manager = """
 from viztracer import VizTracer, log_sparse
 
@@ -199,6 +220,14 @@ def test_multiprocess(self):
                 if not os.getenv("COVERAGE_RUN"):
                     raise e
 
+    def test_multiprocess_spawn(self):
+        self.template(["viztracer", "-o", "result.json", "--log_sparse", "cmdline_test.py"],
+                      script=file_multiprocess_spawn,
+                      expected_output_file="result.json",
+                      expected_entries=3,
+                      check_func=functools.partial(self.check_func, target=['custom_event'] * 3),
+                      concurrency="multiprocessing")
+
     def test_context_manager(self):
         self.template(["python", "cmdline_test.py"], script=file_context_manager,
                       expected_output_file="result.json", expected_entries=4,