mgmt/lang/core/example/flipflop.go

// Mgmt
// Copyright (C) James Shubin and the project contributors
// Written by James Shubin <james@shubin.ca> and the project contributors
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program.  If not, see <https://www.gnu.org/licenses/>.
//
// Additional permission under GNU GPL version 3 section 7
//
// If you modify this program, or any covered work, by linking or combining it
// with embedded mcl code and modules (and that the embedded mcl code and
// modules which link with this program, contain a copy of their source code in
// the authoritative form) containing parts covered by the terms of any other
// license, the licensors of this program grant you additional permission to
// convey the resulting work. Furthermore, the licensors of this program grant
// the original author, James Shubin, additional permission to update this
// additional permission if he deems it necessary to achieve the goals of this
// additional permission.

package coreexample

import (
	"context"
	"sync"
	"time"

	"github.com/purpleidea/mgmt/lang/funcs"
	"github.com/purpleidea/mgmt/lang/interfaces"
	"github.com/purpleidea/mgmt/lang/types"
)

const (
	// FlipFlopFuncName is the name this fact is registered as. It's still a
	// Func Name because this is the name space the fact is actually using.
	FlipFlopFuncName = "flipflop"
)

func init() {
	funcs.ModuleRegister(ModuleName, FlipFlopFuncName, func() interfaces.Func { return &FlipFlop{} }) // must register the fact and name
}

// FlipFlop is a fact which flips a bool repeatedly. This is an example fact and
// is not meant for serious computing. This would be better served by a flip
// function which you could specify an interval for.
type FlipFlop struct {
	init  *interfaces.Init
	mutex *sync.Mutex
	value bool
}

// String returns a simple name for this fact. This is needed so this struct can
// satisfy the pgraph.Vertex interface.
func (obj *FlipFlop) String() string {
	return FlipFlopFuncName
}

// Validate makes sure we've built our struct properly.
func (obj *FlipFlop) Validate() error {
	return nil
}

// Info returns some static info about itself.
func (obj *FlipFlop) Info() *interfaces.Info {
	return &interfaces.Info{
		Pure: false, // non-constant facts can't be pure!
		Memo: false,
		Fast: false,
		Spec: false,
		Sig:  types.NewType("func() bool"),
	}
}

// Init runs some startup code for this fact.
func (obj *FlipFlop) Init(init *interfaces.Init) error {
	obj.init = init
	obj.mutex = &sync.Mutex{}
	return nil
}

// Stream starts a mainloop and runs Event when it's time to Call() again.
func (obj *FlipFlop) Stream(ctx context.Context) error {
	// TODO: don't hard code 5 sec interval
	ticker := time.NewTicker(time.Duration(5) * time.Second)
	defer ticker.Stop()

	// streams must generate an initial event on startup
	// even though ticker will send one, we want to be faster to first event
	startChan := make(chan struct{}) // start signal
	close(startChan)                 // kick it off!

	for {
		select {
		case <-startChan:
			startChan = nil // disable

		case <-ticker.C: // received the timer event
			// pass

		case <-ctx.Done():
			return nil
		}

		obj.mutex.Lock()
		obj.value = !obj.value // flip it
		obj.mutex.Unlock()

		if err := obj.init.Event(ctx); err != nil {
			return err
		}
	}
}

// Call this fact and return the value if it is possible to do so at this time.
func (obj *FlipFlop) Call(ctx context.Context, args []types.Value) (types.Value, error) {
	if obj.mutex == nil {
		return nil, funcs.ErrCantSpeculate
	}
	obj.mutex.Lock() // TODO: could be a read lock
	value := obj.value
	obj.mutex.Unlock()
	return &types.BoolValue{
		V: value,
	}, nil
}