mgmt/lang/core/world/getval.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 coreworld

import (
	"context"
	"fmt"

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

const (
	// GetValFuncName is the name this function is registered as.
	GetValFuncName = "getval"

	// arg names...
	getValArgNameKey = "key"

	// struct field names...
	getValFieldNameValue  = "value"
	getValFieldNameExists = "exists"
)

func init() {
	funcs.ModuleRegister(ModuleName, GetValFuncName, func() interfaces.Func { return &GetValFunc{} })
}

var _ interfaces.StreamableFunc = &GetValFunc{}

// GetValFunc is special function which returns the value of a given key in the
// exposed world.
type GetValFunc struct {
	init *interfaces.Init

	input chan string // stream of inputs
	key   *string     // the active key
}

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

// ArgGen returns the Nth arg name for this function.
func (obj *GetValFunc) ArgGen(index int) (string, error) {
	seq := []string{getValArgNameKey}
	if l := len(seq); index >= l {
		return "", fmt.Errorf("index %d exceeds arg length of %d", index, l)
	}
	return seq[index], nil
}

// Validate makes sure we've built our struct properly. It is usually unused for
// normal functions that users can use directly.
func (obj *GetValFunc) Validate() error {
	return nil
}

// Info returns some static info about itself.
func (obj *GetValFunc) Info() *interfaces.Info {
	return &interfaces.Info{
		Pure: false, // definitely false
		Memo: false,
		Fast: false,
		Spec: false,
		// output is a struct with two fields:
		// value is the zero value if not exists. A bool for that in other field.
		Sig: types.NewType(fmt.Sprintf("func(%s str) struct{%s str; %s bool}", getValArgNameKey, getValFieldNameValue, getValFieldNameExists)),
		Err: obj.Validate(),
	}
}

// Init runs some startup code for this function.
func (obj *GetValFunc) Init(init *interfaces.Init) error {
	obj.init = init
	obj.input = make(chan string)
	return nil
}

// Stream returns the changing values that this func has over time.
func (obj *GetValFunc) Stream(ctx context.Context) error {
	ctx, cancel := context.WithCancel(ctx)
	defer cancel() // important so that we cleanup the watch when exiting

	watchChan := make(chan error) // XXX: sender should close this, but did I implement that part yet???

	for {
		select {
		// TODO: should this first chan be run as a priority channel to
		// avoid some sort of glitch? is that even possible? can our
		// hostname check with reality (below) fix that?
		case key, ok := <-obj.input:
			if !ok {
				obj.input = nil // don't infinite loop back
				return fmt.Errorf("unexpected close")
			}

			if obj.key != nil && *obj.key == key {
				continue // nothing changed
			}

			// TODO: support changing the key over time...
			if obj.key == nil {
				obj.key = &key // store
				var err error
				//  Don't send a value right away, wait for the
				// first ValueWatch startup event to get one!
				watchChan, err = obj.init.World.StrWatch(ctx, key) // watch for var changes
				if err != nil {
					return err
				}
				continue // we get values on the watch chan, not here!
			}

			if *obj.key == key {
				continue // skip duplicates
			}

			// *obj.key != key
			return fmt.Errorf("can't change key, previously: `%s`", *obj.key)

		case err, ok := <-watchChan:
			if !ok { // closed
				// XXX: if we close, perhaps the engine is
				// switching etcd hosts and we should retry?
				// maybe instead we should get an "etcd
				// reconnect" signal, and the lang will restart?
				return nil
			}
			if err != nil {
				return errwrap.Wrapf(err, "channel watch failed on `%s`", *obj.key)
			}

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

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

// Call this function with the input args and return the value if it is possible
// to do so at this time. This was previously getValue which gets the value
// we're looking for.
func (obj *GetValFunc) Call(ctx context.Context, args []types.Value) (types.Value, error) {
	if len(args) < 1 {
		return nil, fmt.Errorf("not enough args")
	}
	key := args[0].Str()
	exists := true // assume true
	if key == "" {
		return nil, fmt.Errorf("can't use an empty key")
	}

	// Check before we send to a chan where we'd need Stream to be running.
	if obj.init == nil {
		return nil, funcs.ErrCantSpeculate
	}

	if obj.init.Debug {
		obj.init.Logf("key: %s", key)
	}

	select {
	case obj.input <- key:
	case <-ctx.Done():
		return nil, ctx.Err()
	}

	val, err := obj.init.World.StrGet(ctx, key)
	if err != nil && obj.init.World.StrIsNotExist(err) {
		exists = false // val doesn't exist
	} else if err != nil {
		return nil, errwrap.Wrapf(err, "channel read failed on `%s`", key)
	}

	s := &types.StrValue{V: val}
	b := &types.BoolValue{V: exists}
	st := types.NewStruct(obj.Info().Sig.Out)
	if err := st.Set(getValFieldNameValue, s); err != nil {
		return nil, errwrap.Wrapf(err, "struct could not add field `%s`, val: `%s`", getValFieldNameValue, s)
	}
	if err := st.Set(getValFieldNameExists, b); err != nil {
		return nil, errwrap.Wrapf(err, "struct could not add field `%s`, val: `%s`", getValFieldNameExists, b)
	}

	return st, nil // put struct into interface type
}