// GetResponse looks up a response matching the specified cmdID and
// returns true if found. The response is deserialized into the
// supplied reply parameter. If no response is found, returns
// false. If a command is pending already for the cmdID, then this
// method will block until the the command is completed or the
// response cache is cleared.
func (rc *ResponseCache) GetResponse(e engine.Engine, cmdID proto.ClientCmdID, reply proto.Response) (bool, error) {
	// Do nothing if command ID is empty.
	if cmdID.IsEmpty() {
		return false, nil
	}

	// Pull response from the cache and read into reply if available.
	rwResp := proto.ReadWriteCmdResponse{}
	key := keys.ResponseCacheKey(rc.raftID, &cmdID)
	ok, err := engine.MVCCGetProto(e, key, proto.ZeroTimestamp, true, nil, &rwResp)
	if ok && err == nil {
		gogoproto.Merge(reply, rwResp.GetValue().(gogoproto.Message))
	}
	return ok, err
}

// GetResponse looks up a response matching the specified cmdID and
// returns true if found. The response is deserialized into the
// supplied reply parameter. If no response is found, returns
// false. If a command is pending already for the cmdID, then this
// method will block until the the command is completed or the
// response cache is cleared.
func (rc *ResponseCache) GetResponse(cmdID proto.ClientCmdID, reply proto.Response) (bool, error) {
	// Do nothing if command ID is empty.
	if cmdID.IsEmpty() {
		return false, nil
	}

	// If the response is in the cache or we experienced an error, return.
	rwResp := proto.ReadWriteCmdResponse{}
	key := keys.ResponseCacheKey(rc.raftID, &cmdID)
	ok, err := engine.MVCCGetProto(rc.engine, key, proto.ZeroTimestamp, true, nil, &rwResp)
	if ok && err == nil {
		gogoproto.Merge(reply, rwResp.GetValue().(gogoproto.Message))
	}
	return ok, err
}

// GetResponse looks up a response matching the specified cmdID. If the
// response is found, it is returned along with its associated error.
// If the response is not found, nil is returned for both the response
// and its error. In all cases, the third return value is the error
// returned from the engine when reading the on-disk cache.
func (rc *ResponseCache) GetResponse(e engine.Engine, cmdID proto.ClientCmdID) (proto.ResponseWithError, error) {
	// Do nothing if command ID is empty.
	if cmdID.IsEmpty() {
		return proto.ResponseWithError{}, nil
	}

	// Pull response from the cache and read into reply if available.
	var rwResp proto.ReadWriteCmdResponse
	key := keys.ResponseCacheKey(rc.raftID, &cmdID)
	ok, err := engine.MVCCGetProto(e, key, proto.ZeroTimestamp, true, nil, &rwResp)
	if err != nil {
		return proto.ResponseWithError{}, err
	}
	if ok {
		resp := rwResp.GetValue().(proto.Response)
		header := resp.Header()
		defer func() { header.Error = nil }()
		return proto.ResponseWithError{resp, header.GoError()}, nil
	}
	return proto.ResponseWithError{}, nil
}

// GetResponse looks up a response matching the specified cmdID and
// returns true if found. The response is deserialized into the
// supplied reply parameter. If no response is found, returns
// false. If a command is pending already for the cmdID, then this
// method will block until the the command is completed or the
// response cache is cleared.
func (rc *ResponseCache) GetResponse(cmdID proto.ClientCmdID, reply proto.Response) (bool, error) {
	// Do nothing if command ID is empty.
	if cmdID.IsEmpty() {
		return false, nil
	}
	// If the command is inflight, wait for it to complete.
	rc.Lock()
	for {
		if cond, ok := rc.inflight[makeCmdIDKey(cmdID)]; ok {
			log.Infof("waiting on cmdID: %s", &cmdID)
			cond.Wait()
		} else {
			break
		}
	}
	// Adding inflight here is preemptive; we don't want to hold lock
	// while fetching from the on-disk cache. The vast, vast majority of
	// calls to GetResponse will be cache misses, so this saves us
	// from acquiring the lock twice: once here and once below in the
	// event we experience a cache miss.
	rc.addInflightLocked(cmdID)
	rc.Unlock()

	// If the response is in the cache or we experienced an error, return.
	rwResp := proto.ReadWriteCmdResponse{}
	key := engine.ResponseCacheKey(rc.raftID, &cmdID)
	if ok, err := engine.MVCCGetProto(rc.engine, key, proto.ZeroTimestamp, nil, &rwResp); ok || err != nil {
		rc.Lock() // Take lock after fetching response from cache.
		defer rc.Unlock()
		rc.removeInflightLocked(cmdID)
		if err == nil && rwResp.GetValue() != nil {
			gogoproto.Merge(reply.(gogoproto.Message), rwResp.GetValue().(gogoproto.Message))
		}
		return ok, err
	}
	// There's no command result cached for this ID; but inflight was added above.
	return false, nil
}

// PutResponse writes a response and an error associated with it to the
// cache for the specified cmdID.
func (rc *ResponseCache) PutResponse(e engine.Engine, cmdID proto.ClientCmdID, replyWithErr proto.ResponseWithError) error {
	// Do nothing if command ID is empty.
	if cmdID.IsEmpty() {
		return nil
	}

	// Write the response value to the engine.
	if rc.shouldCacheResponse(replyWithErr) {
		// Write the error into the reply before caching.
		header := replyWithErr.Reply.Header()
		header.SetGoError(replyWithErr.Err)
		// Be sure to clear it when you're done!
		defer func() { header.Error = nil }()

		key := keys.ResponseCacheKey(rc.raftID, &cmdID)
		var rwResp proto.ReadWriteCmdResponse
		if !rwResp.SetValue(replyWithErr.Reply) {
			panic(fmt.Sprintf("response %T not supported by response cache", replyWithErr.Reply))
		}
		return engine.MVCCPutProto(e, nil, key, proto.ZeroTimestamp, nil, &rwResp)
	}

	return nil
}