Set back to the default log function after using an override

src/core/lib/support/log.c

@@ -96,4 +96,6 @@ void gpr_log_verbosity_init() {
   }
 }
 
-void gpr_set_log_function(gpr_log_func f) { g_log_func = f; }
+void gpr_set_log_function(gpr_log_func f) {
+  g_log_func = f ? f : gpr_default_log;
+}

test/core/support/log_test.c

@@ -78,11 +78,13 @@ int main(int argc, char **argv) {
   gpr_set_log_function(test_callback);
   gpr_log_message(GPR_INFO, "hello 1 2 3");
   gpr_log(GPR_INFO, "hello %d %d %d", 1, 2, 3);
+  gpr_set_log_function(NULL);
 
   /* gpr_log_verbosity_init() will be effective only once, and only before
    * gpr_set_log_verbosity() is called */
   gpr_setenv("GRPC_VERBOSITY", "ERROR");
   gpr_log_verbosity_init();
+
   test_log_function_reached(GPR_ERROR);
   test_log_function_unreached(GPR_INFO);
   test_log_function_unreached(GPR_DEBUG);