Add support to re-activate watcher from wait_callback in watcher async wait.

Returns wait result when cancelling watcher.

etcd/Watcher.hpp

@@ -70,13 +70,17 @@ namespace etcd
      * An async wait, the callback will be called when the task has been stopped.
      *
      * The callback parameter would be true if the watch is been normally cancalled.
+     * If callback returns true, will reactivate the watcher and continue receiving 
+     * watch events, otherwise the watcher ends (default behaviour).
      */
-    void Wait(std::function<void(bool)> callback);
+    void Wait(std::function<bool(bool)> callback);
 
     /**
      * Stop the watching action.
+     * 
+     * Returns true if the watcher has been cancelled, otherwise false.
      */
-    void Cancel();
+    bool Cancel();
 
     ~Watcher();
 
@@ -88,7 +92,7 @@ namespace etcd
 
     int index;
     std::function<void(Response)> callback;
-    std::function<void(bool)> wait_callback;
+    std::function<bool(bool)> wait_callback;
 
     // Don't use `pplx::task` to avoid sharing thread pool with other actions on the client
     // to avoid any potential blocking, which may block the keepalive loop and evict the lease.

src/Watcher.cpp

@@ -109,7 +109,7 @@ bool etcd::Watcher::Wait()
   return stubs->call->Cancelled();
 }
 
-void etcd::Watcher::Wait(std::function<void(bool)> callback)
+void etcd::Watcher::Wait(std::function<bool(bool)> callback)
 {
   if (wait_callback == nullptr) {
     wait_callback = callback;
@@ -118,10 +118,10 @@ void etcd::Watcher::Wait(std::function<void(bool)> callback)
   }
 }
 
-void etcd::Watcher::Cancel()
+bool etcd::Watcher::Cancel()
 {
   stubs->call->CancelWatch();
-  this->Wait();
+  return this->Wait();
 }
 
 void etcd::Watcher::doWatch(std::string const & key,
@@ -141,11 +141,19 @@ void etcd::Watcher::doWatch(std::string const & key,
 
   stubs->call.reset(new etcdv3::AsyncWatchAction(params));
 
-  task_ = std::thread([this, callback]() {
-    stubs->call->waitForResponse(callback);
-    if (wait_callback != nullptr) {
-      wait_callback(stubs->call->Cancelled());
-    }
+  task_ = std::thread([this, params, callback]() {
+    bool continue_watch = false;  // Loop should iterate only once when no 'wait_callback' (to preserve previous behaviour).
+    do
+    {
+      stubs->call->waitForResponse(callback);
+      if (wait_callback != nullptr) {
+        continue_watch = wait_callback(stubs->call->Cancelled());
+        if (continue_watch)
+            stubs->call.reset(new etcdv3::AsyncWatchAction(params));
+      }
+    } while (continue_watch);
+
+    stubs->call->CancelWatch();
   });
   cancelled.store(false);
 }