threadlocal: avoiding a dynamic cast in opt builds (envoyproxy#11900)

requested follow-up PR from envoyproxy#11796 as it's more consistent with our style guide.

Risk Level: low
Testing: n/a
Docs Changes: n/a
Release Notes: n/a

Signed-off-by: Alyssa Wilk <alyssar@chromium.org>

include/envoy/runtime/BUILD

@@ -14,6 +14,7 @@ envoy_cc_library(
     external_deps = ["abseil_optional"],
     deps = [
         "//include/envoy/stats:stats_interface",
+        "//include/envoy/thread_local:thread_local_interface",
         "//source/common/common:assert_lib",
         "//source/common/singleton:threadsafe_singleton",
         "@envoy_api//envoy/type/v3:pkg_cc_proto",

include/envoy/runtime/runtime.h

@@ -10,6 +10,7 @@
 
 #include "envoy/common/pure.h"
 #include "envoy/stats/store.h"
+#include "envoy/thread_local/thread_local.h"
 #include "envoy/type/v3/percent.pb.h"
 
 #include "common/common/assert.h"
@@ -70,10 +71,8 @@ using RandomGeneratorPtr = std::unique_ptr<RandomGenerator>;
 /**
  * A snapshot of runtime data.
  */
-class Snapshot {
+class Snapshot : public ThreadLocal::ThreadLocalObject {
 public:
-  virtual ~Snapshot() = default;
-
   struct Entry {
     std::string raw_string_value_;
     absl::optional<uint64_t> uint_value_;

include/envoy/thread_local/thread_local.h

@@ -45,10 +45,14 @@ class Slot {
 
   /**
    * This is a helper on top of get() that casts the object stored in the slot to the specified
-   * type. Since the slot only stores pointers to the base interface, dynamic_cast provides some
-   * level of protection via RTTI.
+   * type. Since the slot only stores pointers to the base interface, the static_cast operates
+   * in production for performance, and the dynamic_cast validates correctness in tests and debug
+   * builds.
    */
-  template <class T> T& getTyped() { return *std::dynamic_pointer_cast<T>(get()); }
+  template <class T> T& getTyped() {
+    ASSERT(std::dynamic_pointer_cast<T>(get()) != nullptr);
+    return *static_cast<T*>(get().get());
+  }
 
   /**
    * Run a callback on all registered threads.

source/common/runtime/runtime_impl.h

@@ -73,9 +73,7 @@ struct RuntimeStats {
 /**
  * Implementation of Snapshot whose source is the vector of layers passed to the constructor.
  */
-class SnapshotImpl : public Snapshot,
-                     public ThreadLocal::ThreadLocalObject,
-                     Logger::Loggable<Logger::Id::runtime> {
+class SnapshotImpl : public Snapshot, Logger::Loggable<Logger::Id::runtime> {
 public:
   SnapshotImpl(RandomGenerator& generator, RuntimeStats& stats,
                std::vector<OverrideLayerConstPtr>&& layers);