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// -*- mode:C++; tab-width:8; c-basic-offset:2; indent-tabs-mode:t -*-
// vim: ts=8 sw=2 smarttab
/*
* Ceph - scalable distributed file system
*
* Copyright (C) 2014 Red Hat
*
* This is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License version 2.1, as published by the Free Software
* Foundation. See file COPYING.
*
*/
#include "include/Context.h"
/**
* The Continuation interface is designed to help easily create multi-step
* operations that share data without having to pass it around or create
* custom Context classes for each step. To write a Continuation:
* 1) create a child class with a function for each stage.
* 2) Put all your shared data members into the class.
* 3) In the constructor, register each function stage with set_callback().
* 4) Whenever you need to provide a Context callback that activates the next
* stage, call get_callback(stage_number). If you need to proceed to another
* stage immediately, call immediate(stage, retcode) and return its result.
*
* To use a class:
* 1) Construct the child class on the heap.
* 2) Call begin().
* 3) The destructor will be called once one of your functions returns true to
* indicate it is done.
*
* Please note that while you can skip stages and get multiple Callback
* objects at once, you *cannot* have any stage report that the Continuation
* is completed while any other stage Callbacks are outstanding. It's best to
* be serial unless you want to maintain your own metadata about which stages
* are still pending.
*
* In fact, there are only two situations in which a stage should return
* true while others are running:
* 1) A Callback was issued and completed in the same thread,
* 2) you called immediate(stage) and it is returning true.
*/
class Continuation {
std::set<int> stages_in_flight;
std::set<int> stages_processing;
int rval;
Context *on_finish;
bool reported_done;
class Callback : public Context {
Continuation *continuation;
int stage_to_activate;
public:
Callback(Continuation *c, int stage) :
continuation(c),
stage_to_activate(stage) {}
void finish(int r) override {
continuation->continue_function(r, stage_to_activate);
}
};
protected:
typedef bool (Continuation::*stagePtr)(int r);
/**
* Continue immediately to the given stage. It will be executed
* immediately, in the given thread.
* @pre You are in a callback function.
* @param stage The stage to execute
* @param r The return code that will be provided to the next stage
*/
bool immediate(int stage, int r) {
ceph_assert(!stages_in_flight.count(stage));
ceph_assert(!stages_processing.count(stage));
stages_in_flight.insert(stage);
stages_processing.insert(stage);
return _continue_function(r, stage);
}
/**
* Obtain a Context * that when complete()ed calls back into the given stage.
* @pre You are in a callback function.
* @param stage The stage this Context should activate
*/
Context *get_callback(int stage) {
stages_in_flight.insert(stage);
return new Callback(this, stage);
}
/**
* Set the return code that is passed to the finally-activated Context.
* @param new_rval The return code to use.
*/
void set_rval(int new_rval) { rval = new_rval; }
int get_rval() { return rval; }
/**
* Register member functions as associated with a given stage. Start
* your stage IDs at 0 and make that one the setup phase.
* @pre There are no other functions associated with the stage.
* @param stage The stage to associate this function with
* @param func The function to use
*/
void set_callback(int stage, stagePtr func) {
ceph_assert(callbacks.find(stage) == callbacks.end());
callbacks[stage] = func;
}
/**
* Called when the Continuation is done, as determined by a stage returning
* true and us having finished all the currently-processing ones.
*/
virtual void _done() {
on_finish->complete(rval);
on_finish = NULL;
return;
}
private:
std::map<int, Continuation::stagePtr> callbacks;
bool _continue_function(int r, int n) {
set<int>::iterator stage_iter = stages_in_flight.find(n);
ceph_assert(stage_iter != stages_in_flight.end());
ceph_assert(callbacks.count(n));
stagePtr p = callbacks[n];
pair<set<int>::iterator,bool> insert_r = stages_processing.insert(n);
bool done = (this->*p)(r);
if (done)
reported_done = true;
stages_processing.erase(insert_r.first);
stages_in_flight.erase(stage_iter);
return done;
}
void continue_function(int r, int stage) {
bool done = _continue_function(r, stage);
assert (!done ||
stages_in_flight.size() == stages_processing.size());
if ((done || reported_done) && stages_processing.empty()) {
_done();
delete this;
}
}
public:
/**
* Construct a new Continuation object. Call this from your child class,
* obviously.
*
* @Param c The Context which should be complete()ed when this Continuation
* is done.
*/
Continuation(Context *c) :
rval(0), on_finish(c), reported_done(false) {}
/**
* Clean up.
*/
virtual ~Continuation() { ceph_assert(on_finish == NULL); }
/**
* Begin running the Continuation.
*/
void begin() { stages_in_flight.insert(0); continue_function(0, 0); }
};
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