Expose wasm_runtime_get_exec_env_singleton to the API users (#1388)

Sometimes it can be useful to access the singleton.
E.g. use wasm_runtime_set_user_data on it.

core/iwasm/common/wasm_runtime_common.h

@@ -592,7 +592,8 @@ wasm_runtime_call_indirect(WASMExecEnv *exec_env, uint32 element_indices,
 bool
 wasm_runtime_create_exec_env_singleton(WASMModuleInstanceCommon *module_inst);
 
-WASMExecEnv *
+/* See wasm_export.h for description */
+WASM_RUNTIME_API_EXTERN WASMExecEnv *
 wasm_runtime_get_exec_env_singleton(WASMModuleInstanceCommon *module_inst);
 
 /* See wasm_export.h for description */

core/iwasm/include/wasm_export.h

@@ -485,6 +485,23 @@ wasm_runtime_create_exec_env(wasm_module_inst_t module_inst,
 WASM_RUNTIME_API_EXTERN void
 wasm_runtime_destroy_exec_env(wasm_exec_env_t exec_env);
 
+/**
+ * Get the singleton execution environment for the instance.
+ *
+ * Note: The singleton execution environment is the execution
+ * environment used internally by the runtime for the API functions
+ * like wasm_application_execute_main, which don't take explicit
+ * execution environment. It's associated to the corresponding
+ * module instance and managed by the runtime. The API user should
+ * not destroy it with wasm_runtime_destroy_exec_env.
+ *
+ * @param module_inst the module instance
+ *
+ * @return exec_env the execution environment to destroy
+ */
+WASM_RUNTIME_API_EXTERN wasm_exec_env_t
+wasm_runtime_get_exec_env_singleton(wasm_module_inst_t module_inst);
+
 /**
  * Start debug instance based on given execution environment.
  * Note: