Add the ability for EventLoopHandlers to participate in the event loop

Author: ksperling-apple

src/lib/support/IntrusiveList.h

@@ -84,7 +84,7 @@ enum class IntrusiveMode
 class IntrusiveListNodePrivateBase
 {
 public:
-    IntrusiveListNodePrivateBase() : mPrev(nullptr), mNext(nullptr) {}
+    constexpr IntrusiveListNodePrivateBase() : mPrev(nullptr), mNext(nullptr) {}
     ~IntrusiveListNodePrivateBase() { VerifyOrDie(!IsInList()); }
 
     // Note: The copy construct/assignment is not provided because the list node state is not copyable.
@@ -98,7 +98,7 @@ class IntrusiveListNodePrivateBase
 
 private:
     friend class IntrusiveListBase;
-    IntrusiveListNodePrivateBase(IntrusiveListNodePrivateBase * prev, IntrusiveListNodePrivateBase * next) :
+    constexpr IntrusiveListNodePrivateBase(IntrusiveListNodePrivateBase * prev, IntrusiveListNodePrivateBase * next) :
         mPrev(prev), mNext(next)
     {}
 
@@ -284,7 +284,7 @@ class IntrusiveListBase
     //   ^                                           |
     //    \------------------------------------------/
     //
-    IntrusiveListBase() : mNode(&mNode, &mNode) {}
+    constexpr IntrusiveListBase() : mNode(&mNode, &mNode) {}
     ~IntrusiveListBase()
     {
         VerifyOrDie(Empty());
@@ -399,7 +399,7 @@ template <typename T, IntrusiveMode Mode = IntrusiveMode::Strict, typename Hook
 class IntrusiveList : public IntrusiveListBase
 {
 public:
-    IntrusiveList() : IntrusiveListBase() {}
+    constexpr IntrusiveList() : IntrusiveListBase() {}
 
     IntrusiveList(IntrusiveList &&)             = default;
     IntrusiveList & operator=(IntrusiveList &&) = default;

src/system/SystemLayer.h

@@ -38,6 +38,7 @@
 #include <system/SystemEvent.h>
 
 #if CHIP_SYSTEM_CONFIG_USE_SOCKETS
+#include <lib/support/IntrusiveList.h>
 #include <system/SocketEvents.h>
 #endif // CHIP_SYSTEM_CONFIG_USE_SOCKETS
 
@@ -242,6 +243,7 @@ class LayerSockets : public Layer
      * Initialize watching for events on a file descriptor.
      *
      * Returns an opaque token through @a tokenOut that must be passed to subsequent operations for this file descriptor.
+     * Multiple calls to start watching the same file descriptor will return the same token.
      * StopWatchingSocket() must be called before closing the file descriptor.
      */
     virtual CHIP_ERROR StartWatchingSocket(int fd, SocketWatchToken * tokenOut) = 0;
@@ -287,6 +289,23 @@ class LayerSockets : public Layer
     virtual SocketWatchToken InvalidSocketWatchToken() = 0;
 };
 
+class LayerSocketsLoop;
+
+/**
+ * Enables the participation of subordinate event loops in the SystemLayer event loop.
+ */
+class EventLoopHandler : public chip::IntrusiveListNodeBase<>
+{
+public:
+    virtual ~EventLoopHandler() {}
+
+    /** Prepares events and returns the next requested wake time. */
+    virtual Clock::Timestamp PrepareEvents(Clock::Timestamp now) { return Clock::Timestamp::max(); }
+
+    /** Handles / dispatches pending events. */
+    virtual void HandleEvents() = 0;
+};
+
 class LayerSocketsLoop : public LayerSockets
 {
 public:
@@ -297,6 +316,9 @@ class LayerSocketsLoop : public LayerSockets
     virtual void HandleEvents()    = 0;
     virtual void EventLoopEnds()   = 0;
 
+    virtual void AddLoopHandler(EventLoopHandler & handler)    = 0;
+    virtual void RemoveLoopHandler(EventLoopHandler & handler) = 0;
+
 #if CHIP_SYSTEM_CONFIG_USE_DISPATCH
     virtual void SetDispatchQueue(dispatch_queue_t dispatchQueue) = 0;
     virtual dispatch_queue_t GetDispatchQueue()                   = 0;

src/system/SystemLayerImplSelect.cpp

@@ -370,8 +370,9 @@ CHIP_ERROR LayerImplSelect::StartWatchingSocket(int fd, SocketWatchToken * token
     {
         if (w.mFD == fd)
         {
-            // Duplicate registration is an error.
-            return CHIP_ERROR_INVALID_ARGUMENT;
+            // Already registered, return the existing token
+            *tokenOut = reinterpret_cast<SocketWatchToken>(&w);
+            return CHIP_NO_ERROR;
         }
         if ((w.mFD == kInvalidFd) && (watch == nullptr))
         {
@@ -621,6 +622,15 @@ void LayerImplSelect::PrepareEvents()
         awakenTime = timer->AwakenTime();
     }
 
+    for (auto & loop : mLoopHandlers)
+    {
+        Clock::Timestamp requestedWakeTime = loop.PrepareEvents(currentTime);
+        if (requestedWakeTime < awakenTime)
+        {
+            awakenTime = requestedWakeTime;
+        }
+    }
+
     const Clock::Timestamp sleepTime = (awakenTime > currentTime) ? (awakenTime - currentTime) : Clock::kZero;
     Clock::ToTimeval(sleepTime, mNextTimeout);
 
@@ -685,16 +695,21 @@ void LayerImplSelect::HandleEvents()
 
     for (auto & w : mSocketWatchPool)
     {
-        if (w.mFD != kInvalidFd)
+        if (w.mFD != kInvalidFd && w.mCallback != nullptr)
         {
             SocketEvents events = SocketEventsFromFDs(w.mFD, mSelected.mReadSet, mSelected.mWriteSet, mSelected.mErrorSet);
-            if (events.HasAny() && w.mCallback != nullptr)
+            if (events.HasAny())
             {
                 w.mCallback(events, w.mCallbackData);
             }
         }
     }
 
+    for (auto & loop : mLoopHandlers)
+    {
+        loop.HandleEvents();
+    }
+
 #if CHIP_SYSTEM_CONFIG_POSIX_LOCKING
     mHandleSelectThread = PTHREAD_NULL;
 #endif // CHIP_SYSTEM_CONFIG_POSIX_LOCKING

src/system/SystemLayerImplSelect.h

@@ -87,6 +87,9 @@ class LayerImplSelect : public LayerSocketsLoop
     void HandleEvents() override;
     void EventLoopEnds() override {}
 
+    void AddLoopHandler(EventLoopHandler & handler) override { mLoopHandlers.PushBack(&handler); }
+    void RemoveLoopHandler(EventLoopHandler & handler) override { mLoopHandlers.Remove(&handler); }
+
 #if CHIP_SYSTEM_CONFIG_USE_DISPATCH
     void SetDispatchQueue(dispatch_queue_t dispatchQueue) override { mDispatchQueue = dispatchQueue; };
     dispatch_queue_t GetDispatchQueue() override { return mDispatchQueue; };
@@ -135,6 +138,8 @@ class LayerImplSelect : public LayerSocketsLoop
     TimerList mExpiredTimers;
     timeval mNextTimeout;
 
+    IntrusiveList<EventLoopHandler> mLoopHandlers;
+
     // Members for select loop
     struct SelectSets
     {