[node] Reset Style::Impl::lastError when loading a new style

platform/node/test/js/request.test.js

@@ -114,3 +114,54 @@ test(`render ignores request functions calling the callback a second time`, func
     });
 });
 
+test(`render reports an error from loading the current style`, function(t) {
+    var map = new mbgl.Map({
+        request: function(req, callback) {
+            var data = mockfs.dataForRequest(req);
+            if (mockfs.source_vector === data) {
+                callback(new Error('message'));
+            } else {
+                callback(null, { data: data });
+            }
+        }
+    });
+    map.load(mockfs.style_vector);
+    map.render({ zoom: 16 }, function(err, pixels) {
+        t.assert(err);
+        t.assert(/message/.test(err.message));
+        t.assert(!pixels);
+
+        map.render({ zoom: 16 }, function(err, pixels) {
+            t.assert(err);
+            t.assert(/message/.test(err.message));
+            t.assert(!pixels);
+            t.end();
+        });
+    });
+});
+
+test(`render does not report an error from rendering a previous style`, function(t) {
+    var map = new mbgl.Map({
+        request: function(req, callback) {
+            var data = mockfs.dataForRequest(req);
+            if (mockfs.source_vector === data) {
+                callback(new Error('message'));
+            } else {
+                callback(null, { data: data });
+            }
+        }
+    });
+    map.load(mockfs.style_vector);
+    map.render({ zoom: 16 }, function(err, pixels) {
+        t.assert(err);
+        t.assert(/message/.test(err.message));
+        t.assert(!pixels);
+
+        map.load(mockfs.style_raster);
+        map.render({ zoom: 16 }, function(err, pixels) {
+            t.error(err);
+            t.assert(pixels);
+            t.end();
+        });
+    });
+});

src/mbgl/style/style_impl.cpp

@@ -38,13 +38,15 @@ Style::Impl::Impl(Scheduler& scheduler_, FileSource& fileSource_, float pixelRat
 Style::Impl::~Impl() = default;
 
 void Style::Impl::loadJSON(const std::string& json_) {
+    lastError = nullptr;
     observer->onStyleLoading();
 
     url.clear();
     parse(json_);
 }
 
 void Style::Impl::loadURL(const std::string& url_) {
+    lastError = nullptr;
     observer->onStyleLoading();
 
     loaded = false;