Track if a setting has had callbacks

Summary: * Add an API to tell if a setting has ever had a callback (RFC on naming here, seems like we could do better)

Reviewed By: Gownta

Differential Revision: D67534645

fbshipit-source-id: 45223bb408528224ab319c0d25a0f1b9473c13bf

folly/settings/detail/SettingsImpl.h

@@ -74,6 +74,7 @@ class SettingCoreBase {
   virtual void forceResetToDefault(SnapshotBase* snapshot) = 0;
   virtual const SettingMetadata& meta() const = 0;
   virtual uint64_t accessCount() const = 0;
+  virtual bool hasHadCallbacks() const = 0;
   virtual ~SettingCoreBase() {}
 
   /**
@@ -192,6 +193,7 @@ class SnapshotBase {
     std::pair<std::string, std::string> valueAndReason() const;
     const std::string& fullName() const { return fullName_; }
     uint64_t accessCount() const { return core_.accessCount(); }
+    bool hasHadCallbacks() const { return core_.hasHadCallbacks(); }
 
    private:
     const std::string& fullName_;
@@ -335,6 +337,8 @@ class SettingCore : public SettingCoreBase {
 
   uint64_t accessCount() const override { return AccessCounter::count(); }
 
+  bool hasHadCallbacks() const override { return hasHadCallbacks_.load(); }
+
   /**
    * @param trivialStorage must refer to the same location
    *   as the internal trivialStorage_.  This hint will
@@ -387,6 +391,7 @@ class SettingCore : public SettingCoreBase {
     SettingCore<T, Tag>& setting_;
   };
   CallbackHandle addCallback(UpdateCallback callback) {
+    hasHadCallbacks_.store(true);
     auto callbackPtr = copy_to_shared_ptr(std::move(callback));
 
     auto copiedPtr = callbackPtr;
@@ -420,6 +425,7 @@ class SettingCore : public SettingCoreBase {
   std::atomic<uint64_t>& trivialStorage_;
 
   F14FastSet<std::shared_ptr<UpdateCallback>> callbacks_;
+  std::atomic<bool> hasHadCallbacks_{false};
 
   /* Thread local versions start at 0, this will force a read on first access.
    */

folly/settings/test/SettingsTest.cpp

@@ -645,6 +645,11 @@ TEST(SettingsTest, callback) {
   some_ns::FOLLY_SETTING(follytest, some_flag).set("e");
   EXPECT_EQ(callbackInvocations, 4);
   EXPECT_EQ(lastCallbackValue, "d");
+
+  folly::settings::Snapshot snapshot;
+  snapshot.forEachSetting([](const auto& s) {
+    EXPECT_EQ(s.hasHadCallbacks(), s.fullName() == "follytest_some_flag");
+  });
 }
 
 TEST(SettingsTest, observers) {
@@ -661,6 +666,11 @@ TEST(SettingsTest, observers) {
   folly::observer_detail::ObserverManager::waitForAllUpdates();
   EXPECT_EQ(**observer, "new value");
   EXPECT_EQ(updatedFromCallback, "new value");
+
+  folly::settings::Snapshot snapshot;
+  snapshot.forEachSetting([](const auto& s) {
+    EXPECT_EQ(s.hasHadCallbacks(), s.fullName() == "follytest_some_flag");
+  });
 }
 
 TEST(Settings, immutables) {