n-api: add napi_get_version

Add napi_get_version function so that addons can
query the level of N-API supported.

PR-URL: https://github.com/nodejs/node/pull/13207
Fixes: https://github.com/nodejs/abi-stable-node/issues/231
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jason Ginchereau <jasongin@microsoft.com>

doc/api/n-api.md

@@ -2946,6 +2946,35 @@ the `complete` callback will be invoked with a status value of
 `napi_cancelled`. The work should not be deleted before the `complete`
 callback invocation, even if it has been successfully cancelled.
 
+## Version Management
+
+### napi_get_version
+<!-- YAML
+added: REPLACEME
+-->
+```C
+NAPI_EXTERN napi_status napi_get_version(napi_env env,
+                                         uint32_t* result);
+```
+
+- `[in] env`: The environment that the API is invoked under.
+- `[out] result`: The highest version of N-API supported.
+
+Returns `napi_ok` if the API succeeded.
+
+This API returns the highest N-API version supported by the
+Node.js runtime.  N-API is planned to be additive such that
+newer releases of Node.js may support additional API functions.
+In order to allow an addon to use a newer function when running with
+versions of Node.js that support it, while providing
+fallback behavior when running with Node.js versions that don't
+support it:
+
+* Call `napi_get_version()` to determine if the API is available.
+* If available, dynamically load a pointer to the function using `uv_dlsym()`.
+* Use the dynamically loaded pointer to invoke the function.
+* If the function is not available, provide an alternate implementation
+  that does not use the function.
 
 [Aynchronous Operations]: #n_api_asynchronous_operations
 [Basic N-API Data Types]: #n_api_basic_n_api_data_types

src/node_api.cc

@@ -18,6 +18,8 @@
 #include "uv.h"
 #include "node_api.h"
 
+#define NAPI_VERSION  1
+
 static
 napi_status napi_set_last_error(napi_env env, napi_status error_code,
                                 uint32_t engine_error_code = 0,
@@ -2713,6 +2715,13 @@ napi_status napi_get_typedarray_info(napi_env env,
   return napi_clear_last_error(env);
 }
 
+napi_status napi_get_version(napi_env env, uint32_t* result) {
+  CHECK_ENV(env);
+  CHECK_ARG(env, result);
+  *result = NAPI_VERSION;
+  return napi_clear_last_error(env);
+}
+
 namespace uvimpl {
 
 static napi_status ConvertUVErrorCode(int code) {

src/node_api.h

@@ -478,6 +478,10 @@ NAPI_EXTERN napi_status napi_queue_async_work(napi_env env,
 NAPI_EXTERN napi_status napi_cancel_async_work(napi_env env,
                                                napi_async_work work);
 
+
+// version management
+NAPI_EXTERN napi_status napi_get_version(napi_env env, uint32_t* result);
+
 EXTERN_C_END
 
 #endif  // SRC_NODE_API_H_

test/addons-napi/test_general/test.js

@@ -30,3 +30,7 @@ assert.strictEqual(test_general.testGetPrototype(extendedObject),
 assert.ok(test_general.testGetPrototype(baseObject) !==
           test_general.testGetPrototype(extendedObject),
           'Prototypes for base and extended should be different');
+
+// test version management funcitons
+// expected version is currently 1
+assert.strictEqual(test_general.testGetVersion(), 1);

test/addons-napi/test_general/test_general.c

@@ -25,10 +25,19 @@ napi_value testGetPrototype(napi_env env, napi_callback_info info) {
   return result;
 }
 
+napi_value testGetVersion(napi_env env, napi_callback_info info) {
+  uint32_t version;
+  napi_value result;
+  NAPI_CALL(env, napi_get_version(env, &version));
+  NAPI_CALL(env ,napi_create_number(env, version, &result));
+  return result;
+}
+
 void Init(napi_env env, napi_value exports, napi_value module, void* priv) {
   napi_property_descriptor descriptors[] = {
     DECLARE_NAPI_PROPERTY("testStrictEquals", testStrictEquals),
     DECLARE_NAPI_PROPERTY("testGetPrototype", testGetPrototype),
+    DECLARE_NAPI_PROPERTY("testGetVersion", testGetVersion),
   };
 
   NAPI_CALL_RETURN_VOID(env, napi_define_properties(