allow callbacks to be set and called on updates

Summary:
Allow callbacks to be attached to a SettingCore (via SettingWrapper),
with lifetime managed by a guard object which automatically unregisters
the callback at destruction. This provides an API for clients to react
to changes in settings without polling.

Reviewed By: yfeldblum

Differential Revision: D25592427

fbshipit-source-id: a11a98ed651ef56a0fc86f5f5e4ed235f2289746

folly/experimental/settings/Settings.h

@@ -36,6 +36,8 @@ namespace detail {
 template <class T, std::atomic<uint64_t>* TrivialPtr>
 class SettingWrapper {
  public:
+  using CallbackHandle = typename SettingCore<T>::CallbackHandle;
+
   /**
    * Returns the setting's current value.
    *
@@ -78,6 +80,19 @@ class SettingWrapper {
    */
   void set(const T& t, StringPiece reason = "api") { core_.set(t, reason); }
 
+  /**
+   * Adds a callback to be invoked any time the setting is updated. Callback
+   * is not invoked for snapshot updates unless published.
+   *
+   * @param callback  void function that accepts a SettingsContents with value
+   *        and reason, to be invoked on updates
+   * @returns  a handle object which automatically removes the callback from
+   *           processing once destroyd
+   */
+  CallbackHandle addCallback(typename SettingCore<T>::UpdateCallback callback) {
+    return core_.addCallback(std::move(callback));
+  }
+
   /**
    * Returns the default value this setting was constructed with.
    * NOTE: SettingsMetadata is type-agnostic, so it only stores the string

folly/experimental/settings/detail/SettingsImpl.h

@@ -22,10 +22,12 @@
 #include <typeindex>
 
 #include <folly/Conv.h>
+#include <folly/Function.h>
 #include <folly/Range.h>
 #include <folly/SharedMutex.h>
 #include <folly/ThreadLocal.h>
 #include <folly/Utility.h>
+#include <folly/container/F14Set.h>
 #include <folly/experimental/settings/SettingsMetadata.h>
 #include <folly/lang/Aligned.h>
 
@@ -308,6 +310,7 @@ class SettingCore : public SettingCoreBase {
       return;
     }
 
+    {
       SharedMutex::WriteHolder lg(globalLock_);
 
       if (globalValue_) {
@@ -322,9 +325,44 @@ class SettingCore : public SettingCoreBase {
       }
       *settingVersion_ = nextGlobalVersion();
     }
+    invokeCallbacks(Contents(reason.str(), t));
+  }
 
   const T& defaultValue() const { return defaultValue_; }
 
+  using UpdateCallback = folly::Function<void(const Contents&)>;
+  class CallbackHandle {
+   public:
+    CallbackHandle(
+        std::shared_ptr<UpdateCallback> callback,
+        SettingCore<T>& setting)
+        : callback_(std::move(callback)), setting_(setting) {}
+    ~CallbackHandle() {
+      if (callback_) {
+        SharedMutex::WriteHolder lg(setting_.globalLock_);
+        setting_.callbacks_.erase(callback_);
+      }
+    }
+    CallbackHandle(const CallbackHandle&) = delete;
+    CallbackHandle& operator=(const CallbackHandle&) = delete;
+    CallbackHandle(CallbackHandle&&) = default;
+    CallbackHandle& operator=(CallbackHandle&&) = default;
+
+   private:
+    std::shared_ptr<UpdateCallback> callback_;
+    SettingCore<T>& setting_;
+  };
+  CallbackHandle addCallback(UpdateCallback callback) {
+    auto callbackPtr = copy_to_shared_ptr(std::move(callback));
+
+    auto copiedPtr = callbackPtr;
+    {
+      SharedMutex::WriteHolder lg(globalLock_);
+      callbacks_.emplace(std::move(copiedPtr));
+    }
+    return CallbackHandle(std::move(callbackPtr), *this);
+  }
+
   SettingCore(
       SettingMetadata meta,
       T defaultValue,
@@ -350,6 +388,8 @@ class SettingCore : public SettingCoreBase {
 
   std::atomic<uint64_t>& trivialStorage_;
 
+  folly::F14FastSet<std::shared_ptr<UpdateCallback>> callbacks_;
+
   /* Thread local versions start at 0, this will force a read on first access.
    */
   cacheline_aligned<std::atomic<Version>> settingVersion_{in_place, 1};
@@ -375,6 +415,20 @@ class SettingCore : public SettingCoreBase {
     }
     return value.second;
   }
+
+  void invokeCallbacks(const Contents& contents) {
+    auto callbacksSnapshot = invoke([&] {
+      SharedMutex::ReadHolder lg(globalLock_);
+      // invoking arbitrary user code under the lock is dangerous
+      return std::vector<std::shared_ptr<UpdateCallback>>(
+          callbacks_.begin(), callbacks_.end());
+    });
+
+    for (auto& callbackPtr : callbacksSnapshot) {
+      auto& callback = *callbackPtr;
+      callback(contents);
+    }
+  }
 };
 
 } // namespace detail

folly/experimental/settings/test/SettingsTest.cpp

@@ -447,3 +447,49 @@ TEST(Settings, snapshot) {
         123);
   }
 }
+
+TEST(SettingsTest, callback) {
+  size_t callbackInvocations = 0;
+  std::string lastCallbackValue;
+
+  EXPECT_EQ(*some_ns::FOLLY_SETTING(follytest, some_flag), "default");
+  {
+    auto handle = some_ns::FOLLY_SETTING(follytest, some_flag)
+                      .addCallback([&](const auto& contents) {
+                        ++callbackInvocations;
+                        lastCallbackValue = contents.value;
+                      });
+
+    some_ns::FOLLY_SETTING(follytest, some_flag).set("a");
+    EXPECT_EQ(callbackInvocations, 1);
+    EXPECT_EQ(lastCallbackValue, "a");
+
+    size_t secondCallbackInvocations = 0;
+    // Test adding multiple callbacks and letting the handle go out of scope
+    {
+      auto secondHandle = some_ns::FOLLY_SETTING(follytest, some_flag)
+                              .addCallback([&](const auto& /* contents */) {
+                                ++secondCallbackInvocations;
+                              });
+      some_ns::FOLLY_SETTING(follytest, some_flag).set("b");
+      EXPECT_EQ(callbackInvocations, 2);
+      EXPECT_EQ(lastCallbackValue, "b");
+      EXPECT_EQ(secondCallbackInvocations, 1);
+    }
+
+    some_ns::FOLLY_SETTING(follytest, some_flag).set("c");
+    EXPECT_EQ(callbackInvocations, 3);
+    EXPECT_EQ(lastCallbackValue, "c");
+    // Second callback no longer invoked
+    EXPECT_EQ(secondCallbackInvocations, 1);
+
+    auto movedHandle = std::move(handle);
+    some_ns::FOLLY_SETTING(follytest, some_flag).set("d");
+    EXPECT_EQ(callbackInvocations, 4);
+    EXPECT_EQ(lastCallbackValue, "d");
+  }
+  // Main callback no longer invoked
+  some_ns::FOLLY_SETTING(follytest, some_flag).set("e");
+  EXPECT_EQ(callbackInvocations, 4);
+  EXPECT_EQ(lastCallbackValue, "d");
+}