Add a copy of webrtc's TaskRunner abstraction, which is going to be deleted in webrtc.

third_party/libjingle_xmpp/task_runner/task.h

Index: third_party/libjingle_xmpp/task_runner/task.h
diff --git a/third_party/libjingle_xmpp/task_runner/task.h b/third_party/libjingle_xmpp/task_runner/task.h
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+/*
+ *  Copyright 2004 The WebRTC Project Authors. All rights reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef WEBRTC_BASE_TASK_H__
+#define WEBRTC_BASE_TASK_H__
+
+#include <stdint.h>
+
+#include <string>
+
+#include "third_party/libjingle_xmpp/task_runner/taskparent.h"
+#include "third_party/webrtc/base/sigslot.h"
+
+/////////////////////////////////////////////////////////////////////
+//
+// TASK
+//
+/////////////////////////////////////////////////////////////////////
+//
+// Task is a state machine infrastructure.  States are pushed forward by
+// pushing forwards a TaskRunner that holds on to all Tasks.  The purpose
+// of Task is threefold:
+//
+// (1) It manages ongoing work on the UI thread.  Multitasking without
+// threads, keeping it easy, keeping it real. :-)  It does this by
+// organizing a set of states for each task.  When you return from your
+// Process*() function, you return an integer for the next state.  You do
+// not go onto the next state yourself.  Every time you enter a state,
+// you check to see if you can do anything yet.  If not, you return
+// STATE_BLOCKED.  If you _could_ do anything, do not return
+// STATE_BLOCKED - even if you end up in the same state, return
+// STATE_mysamestate.  When you are done, return STATE_DONE and then the
+// task will self-delete sometime afterwards.
+//
+// (2) It helps you avoid all those reentrancy problems when you chain
+// too many triggers on one thread.  Basically if you want to tell a task
+// to process something for you, you feed your task some information and
+// then you Wake() it.  Don't tell it to process it right away.  If it
+// might be working on something as you send it information, you may want
+// to have a queue in the task.
+//
+// (3) Finally it helps manage parent tasks and children.  If a parent
+// task gets aborted, all the children tasks are too.  The nice thing
+// about this, for example, is if you have one parent task that
+// represents, say, and Xmpp connection, then you can spawn a whole bunch
+// of infinite lifetime child tasks and now worry about cleaning them up.
+//  When the parent task goes to STATE_DONE, the task engine will make
+// sure all those children are aborted and get deleted.
+//
+// Notice that Task has a few built-in states, e.g.,
+//
+// STATE_INIT - the task isn't running yet
+// STATE_START - the task is in its first state
+// STATE_RESPONSE - the task is in its second state
+// STATE_DONE - the task is done
+//
+// STATE_ERROR - indicates an error - we should audit the error code in
+// light of any usage of it to see if it should be improved.  When I
+// first put down the task stuff I didn't have a good sense of what was
+// needed for Abort and Error, and now the subclasses of Task will ground
+// the design in a stronger way.
+//
+// STATE_NEXT - the first undefined state number.  (like WM_USER) - you
+// can start defining more task states there.
+//
+// When you define more task states, just override Process(int state) and
+// add your own switch statement.  If you want to delegate to
+// Task::Process, you can effectively delegate to its switch statement.
+// No fancy method pointers or such - this is all just pretty low tech,
+// easy to debug, and fast.
+//
+// Also notice that Task has some primitive built-in timeout functionality.
+//
+// A timeout is defined as "the task stays in STATE_BLOCKED longer than
+// timeout_seconds_."
+//
+// Descendant classes can override this behavior by calling the
+// various protected methods to change the timeout behavior.  For
+// instance, a descendand might call SuspendTimeout() when it knows
+// that it isn't waiting for anything that might timeout, but isn't
+// yet in the STATE_DONE state.
+//
+
+namespace rtc {
+
+// Executes a sequence of steps
+class Task : public TaskParent {
+ public:
+  Task(TaskParent *parent);
+  ~Task() override;
+
+  int32_t unique_id() { return unique_id_; }
+
+  void Start();
+  void Step();
+  int GetState() const { return state_; }
+  bool HasError() const { return (GetState() == STATE_ERROR); }
+  bool Blocked() const { return blocked_; }
+  bool IsDone() const { return done_; }
+  int64_t ElapsedTime();
+
+  // Called from outside to stop task without any more callbacks
+  void Abort(bool nowake = false);
+
+  bool TimedOut();
+
+  int64_t timeout_time() const { return timeout_time_; }
+  int timeout_seconds() const { return timeout_seconds_; }
+  void set_timeout_seconds(int timeout_seconds);
+
+  sigslot::signal0<> SignalTimeout;
+
+  // Called inside the task to signal that the task may be unblocked
+  void Wake();
+
+ protected:
+
+  enum {
+    STATE_BLOCKED = -1,
+    STATE_INIT = 0,
+    STATE_START = 1,
+    STATE_DONE = 2,
+    STATE_ERROR = 3,
+    STATE_RESPONSE = 4,
+    STATE_NEXT = 5,  // Subclasses which need more states start here and higher
+  };
+
+  // Called inside to advise that the task should wake and signal an error
+  void Error();
+
+  int64_t CurrentTime();
+
+  virtual std::string GetStateName(int state) const;
+  virtual int Process(int state);
+  virtual void Stop();
+  virtual int ProcessStart() = 0;
+  virtual int ProcessResponse();
+
+  void ResetTimeout();
+  void ClearTimeout();
+
+  void SuspendTimeout();
+  void ResumeTimeout();
+
+ protected:
+  virtual int OnTimeout();
+
+ private:
+  void Done();
+
+  int state_;
+  bool blocked_;
+  bool done_;
+  bool aborted_;
+  bool busy_;
+  bool error_;
+  int64_t start_time_;
+  int64_t timeout_time_;
+  int timeout_seconds_;
+  bool timeout_suspended_;
+  int32_t unique_id_;
+
+  static int32_t unique_id_seed_;
+};
+
+}  // namespace rtc
+
+#endif  // WEBRTC_BASE_TASK_H__