runtime: changing snapshot access to be const

include/envoy/runtime/runtime.h

@@ -220,11 +220,11 @@ class Loader {
   virtual void initialize(Upstream::ClusterManager& cm) PURE;
 
   /**
-   * @return Snapshot& the current snapshot. This reference is safe to use for the duration of
+   * @return const Snapshot& the current snapshot. This reference is safe to use for the duration of
    *         the calling routine, but may be overwritten on a future event loop cycle so should be
    *         fetched again when needed.
    */
-  virtual Snapshot& snapshot() PURE;
+  virtual const Snapshot& snapshot() PURE;
 
   /**
    * Merge the given map of key-value pairs into the runtime's state. To remove a previous merge for

source/common/runtime/runtime_impl.cc

@@ -557,7 +557,7 @@ void LoaderImpl::loadNewSnapshot() {
   });
 }
 
-Snapshot& LoaderImpl::snapshot() { return tls_->getTyped<Snapshot>(); }
+const Snapshot& LoaderImpl::snapshot() { return tls_->getTyped<Snapshot>(); }
 
 void LoaderImpl::mergeValues(const std::unordered_map<std::string, std::string>& values) {
   if (admin_layer_ == nullptr) {

source/common/runtime/runtime_impl.h

@@ -243,7 +243,7 @@ class LoaderImpl : public Loader, Logger::Loggable<Logger::Id::runtime> {
 
   // Runtime::Loader
   void initialize(Upstream::ClusterManager& cm) override;
-  Snapshot& snapshot() override;
+  const Snapshot& snapshot() override;
   void mergeValues(const std::unordered_map<std::string, std::string>& values) override;
 
 private:

test/common/runtime/runtime_impl_test.cc

@@ -155,7 +155,7 @@ TEST_F(DiskLoaderImplTest, All) {
   EXPECT_EQ(123UL, loader_->snapshot().getInteger("file4", 1));
 
   bool value;
-  SnapshotImpl* snapshot = reinterpret_cast<SnapshotImpl*>(&loader_->snapshot());
+  const SnapshotImpl* snapshot = reinterpret_cast<const SnapshotImpl*>(&loader_->snapshot());
 
   // Validate that the layer name is set properly for static layers.
   EXPECT_EQ("base", snapshot->getLayers()[0]->name());
@@ -538,7 +538,7 @@ TEST_F(StaticLoaderImplTest, ProtoParsing) {
 
   // Boolean getting.
   bool value;
-  SnapshotImpl* snapshot = reinterpret_cast<SnapshotImpl*>(&loader_->snapshot());
+  const SnapshotImpl* snapshot = reinterpret_cast<const SnapshotImpl*>(&loader_->snapshot());
 
   EXPECT_EQ(true, snapshot->getBoolean("file11", value));
   EXPECT_EQ(true, value);

test/mocks/runtime/mocks.h

@@ -63,7 +63,7 @@ class MockLoader : public Loader {
   ~MockLoader() override;
 
   MOCK_METHOD1(initialize, void(Upstream::ClusterManager& cm));
-  MOCK_METHOD0(snapshot, Snapshot&());
+  MOCK_METHOD0(snapshot, const Snapshot&());
   MOCK_METHOD1(mergeValues, void(const std::unordered_map<std::string, std::string>&));
 
   testing::NiceMock<MockSnapshot> snapshot_;