[core] guard against duplicate layer ids

src/mbgl/style/style.cpp

@@ -199,6 +199,15 @@ Layer* Style::getLayer(const std::string& id) const {
 Layer* Style::addLayer(std::unique_ptr<Layer> layer, optional<std::string> before) {
     // TODO: verify source
 
+    // Guard against duplicate layer ids
+    auto it = std::find_if(layers.begin(), layers.end(), [&](const auto& existing) {
+        return existing->getID() == layer->getID();
+    });
+
+    if (it != layers.end()) {
+        throw std::runtime_error(std::string{"Layer "} + layer->getID() + " already exists");
+    }
+
     if (SymbolLayer* symbolLayer = layer->as<SymbolLayer>()) {
         if (!symbolLayer->impl->spriteAtlas) {
             symbolLayer->impl->spriteAtlas = spriteAtlas.get();

test/style/style_layer.test.cpp

@@ -1,5 +1,7 @@
 #include <mbgl/test/util.hpp>
 #include <mbgl/test/stub_layer_observer.hpp>
+#include <mbgl/test/stub_file_source.hpp>
+#include <mbgl/style/style.hpp>
 #include <mbgl/style/layers/background_layer.hpp>
 #include <mbgl/style/layers/background_layer_impl.hpp>
 #include <mbgl/style/layers/circle_layer.hpp>
@@ -15,6 +17,10 @@
 #include <mbgl/style/layers/symbol_layer.hpp>
 #include <mbgl/style/layers/symbol_layer_impl.hpp>
 #include <mbgl/util/color.hpp>
+#include <mbgl/util/run_loop.hpp>
+#include <mbgl/util/io.hpp>
+
+#include <memory>
 
 using namespace mbgl;
 using namespace mbgl::style;
@@ -268,3 +274,25 @@ TEST(Layer, Observer) {
     layer->setLineCap(lineCap);
     EXPECT_FALSE(layoutPropertyChanged);
 }
+
+TEST(Layer, DuplicateLayer) {
+    util::RunLoop loop;
+
+    //Setup style
+    StubFileSource fileSource;
+    Style style { fileSource, 1.0 };
+    style.setJSON(util::read_file("test/fixtures/resources/style-unused-sources.json"));
+
+    //Add initial layer
+    style.addLayer(std::make_unique<LineLayer>("line", "unusedsource"));
+
+    //Try to add duplicate
+    try {
+        style.addLayer(std::make_unique<LineLayer>("line", "unusedsource"));
+        FAIL() << "Should not have been allowed to add a duplicate layer id";
+    } catch (const std::runtime_error e) {
+        //Expected
+        ASSERT_STREQ("Layer line already exists", e.what());
+    }
+}
+