mgmt/lang/core/os/readfilewait.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 coreos

import (
	"context"
	"fmt"
	"os"
	"sync"

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

const (
	// ReadFileWaitFuncName is the name this function is registered as.
	ReadFileWaitFuncName = "readfilewait"

	// arg names...
	readFileWaitArgNameFilename = "filename"
)

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

// ReadFileWaitFunc is a function that reads the full contents from a local
// file. If the file contents change or the file path changes, a new string will
// be sent. This is different from the normal readfile in that if will not error
// if the file doesn't exist. If the file is not found, it returns the empty
// string. If the file re-appears, it returns those new contents. This function
// will eventually be deprecated when the function graph error system is stable.
type ReadFileWaitFunc struct {
	init *interfaces.Init

	recWatcher *recwatch.RecWatcher
	events     chan error // internal events

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

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

// ArgGen returns the Nth arg name for this function.
func (obj *ReadFileWaitFunc) ArgGen(index int) (string, error) {
	seq := []string{readFileWaitArgNameFilename}
	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 *ReadFileWaitFunc) Validate() error {
	return nil
}

// Info returns some static info about itself.
func (obj *ReadFileWaitFunc) Info() *interfaces.Info {
	return &interfaces.Info{
		Pure: false, // maybe false because the file contents can change
		Memo: false,
		Fast: false,
		Spec: false,
		Sig:  types.NewType(fmt.Sprintf("func(%s str) str", readFileWaitArgNameFilename)),
	}
}

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

// Stream returns the changing values that this func has over time.
func (obj *ReadFileWaitFunc) Stream(ctx context.Context) error {
	//defer close(obj.input)  // if we close, this is a race with the sender
	defer close(obj.events) // clean up for fun
	wg := &sync.WaitGroup{}
	defer wg.Wait()
	defer func() {
		if obj.recWatcher != nil {
			obj.recWatcher.Close() // close previous watcher
			wg.Wait()
		}
	}()
	for {
		select {
		case filename, ok := <-obj.input:
			if !ok {
				obj.input = nil // don't infinite loop back
				return fmt.Errorf("unexpected close")
			}

			if obj.filename != nil && *obj.filename == filename {
				//select {
				//case obj.ack <- struct{}{}:
				//case <-ctx.Done():
				//	// don't block here on shutdown
				//	return
				//default:
				//	// pass, in case we didn't Call()
				//}
				continue // nothing changed
			}
			obj.filename = &filename

			if obj.recWatcher != nil {
				obj.recWatcher.Close() // close previous watcher
				wg.Wait()
			}
			// create new watcher
			obj.recWatcher = &recwatch.RecWatcher{
				Path:    *obj.filename,
				Recurse: false,
				Opts: []recwatch.Option{
					recwatch.Logf(obj.init.Logf),
					recwatch.Debug(obj.init.Debug),
				},
			}
			if err := obj.recWatcher.Init(); err != nil {
				obj.recWatcher = nil
				// TODO: should we ignore the error and send ""?
				return errwrap.Wrapf(err, "could not watch file")
			}

			// FIXME: instead of sending one event here, the recwatch
			// library should send one initial event at startup...
			startup := make(chan struct{})
			close(startup)

			// watch recwatch events in a proxy goroutine, since
			// changing the recwatch object would panic the main
			// select when it's nil...
			wg.Add(1)
			go func() {
				defer wg.Done()
				for {
					var err error
					select {
					case <-startup:
						startup = nil
						// send an initial event

					case event, ok := <-obj.recWatcher.Events():
						if !ok {
							return // file watcher shut down
						}
						if err = event.Error; err != nil {
							err = errwrap.Wrapf(err, "error event received")
						}
					}

					// XXX: should we send an ACK event to
					// Call() right here? This should ideally
					// be from the Startup event of recWatcher
					//select {
					//case obj.ack <- struct{}{}:
					//case <-ctx.Done():
					//	// don't block here on shutdown
					//	return
					//default:
					//	// pass, in case we didn't Call()
					//}
					select {
					case obj.events <- err:
						// send event...

					case <-ctx.Done():
						// don't block here on shutdown
						return
					}
					//err = nil // reset
				}
			}()
			continue // wait for an actual event or we'd send empty!

		case err, ok := <-obj.events:
			if !ok {
				return fmt.Errorf("no more events")
			}
			if err != nil {
				return errwrap.Wrapf(err, "error event received")
			}

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

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

// Call this function with the input args and return the value if it is possible
// to do so at this time.
func (obj *ReadFileWaitFunc) Call(ctx context.Context, args []types.Value) (types.Value, error) {
	if len(args) < 1 {
		return nil, fmt.Errorf("not enough args")
	}
	filename := args[0].Str()

	// TODO: add validation for absolute path?
	// TODO: add check for empty string

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

	// Tell the Stream what we're watching now... This doesn't block because
	// Stream should always be ready to consume unless it's closing down...
	// If it dies, then a ctx closure should come soon.
	select {
	case obj.input <- filename:
	case <-ctx.Done():
		return nil, ctx.Err()
	}
	// XXX: Should we make sure the Stream is ready before we continue here?
	//select {
	//case <-obj.ack:
	//	// received
	//case <-ctx.Done():
	//	return nil, ctx.Err()
	//}

	// read file...
	content, err := os.ReadFile(filename)
	if err != nil && !os.IsNotExist(err) { // ignore file not found errors
		return nil, errwrap.Wrapf(err, "error reading file")
	}
	s := string(content) // convert to string
	//if err != nil { // file not exists
	//	s = "" // if we errored this should already be empty
	//}

	return &types.StrValue{
		V: s, // convert to string
	}, nil
}

// Cleanup runs after that function was removed from the graph.
func (obj *ReadFileWaitFunc) Cleanup(ctx context.Context) error {
	// Even if the filename stops changing, we never shutdown Stream because
	// those file contents may change. Theoretically if someone sends us an
	// empty string, and then it shuts down we could close.
	//if obj.filename == "" { // we require obj.ack to not have a race here
	//	close(obj.exit) // add a channel into that Stream
	//}
	return nil
}

// Done is a message from the engine to tell us that no more Call's are coming.
func (obj *ReadFileWaitFunc) Done() error {
	close(obj.input) // At this point we know obj.input won't be used.
	return nil
}