lang: funcs: Add a new readfile function

This adds a new function that reads files from the local host.

examples/lang/readfile1.mcl

@@ -0,0 +1,6 @@
+import "os"
+
+# this copies the contents from /tmp/foo and puts them in /tmp/output
+file "/tmp/output" {
+	content => os.readfile("/tmp/foo"),
+}

lang/funcs/core/core.go

@@ -23,5 +23,6 @@ import (
 	_ "github.com/purpleidea/mgmt/lang/funcs/core/coreexample"
 	_ "github.com/purpleidea/mgmt/lang/funcs/core/corefmt"
 	_ "github.com/purpleidea/mgmt/lang/funcs/core/coremath"
+	_ "github.com/purpleidea/mgmt/lang/funcs/core/coreos"
 	_ "github.com/purpleidea/mgmt/lang/funcs/core/coresys"
 )

lang/funcs/core/coreos/coreos.go

@@ -0,0 +1,23 @@
+// Mgmt
+// Copyright (C) 2013-2018+ 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 <http://www.gnu.org/licenses/>.
+
+package coreos
+
+const (
+	// moduleName is the prefix given to all the functions in this module.
+	moduleName = "os"
+)

lang/funcs/core/coreos/readfile_func.go

@@ -0,0 +1,211 @@
+// Mgmt
+// Copyright (C) 2013-2018+ 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 <http://www.gnu.org/licenses/>.
+
+package coreos
+
+import (
+	"fmt"
+	"io/ioutil"
+	"sync"
+
+	"github.com/purpleidea/mgmt/lang/funcs"
+	"github.com/purpleidea/mgmt/lang/interfaces"
+	"github.com/purpleidea/mgmt/lang/types"
+	"github.com/purpleidea/mgmt/recwatch"
+
+	errwrap "github.com/pkg/errors"
+)
+
+func init() {
+	funcs.ModuleRegister(moduleName, "readfile", func() interfaces.Func { return &ReadFileFunc{} }) // must register the func and name
+}
+
+// ReadFileFunc 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.
+type ReadFileFunc struct {
+	init *interfaces.Init
+	last types.Value // last value received to use for diff
+
+	filename   string // the active filename
+	recWatcher *recwatch.RecWatcher
+	events     chan error // internal events
+	wg         *sync.WaitGroup
+
+	result string // last calculated output
+
+	closeChan chan struct{}
+}
+
+// Validate makes sure we've built our struct properly. It is usually unused for
+// normal functions that users can use directly.
+func (obj *ReadFileFunc) Validate() error {
+	return nil
+}
+
+// Info returns some static info about itself.
+func (obj *ReadFileFunc) Info() *interfaces.Info {
+	return &interfaces.Info{
+		Pure: false, // maybe false because the file contents can change
+		Memo: false,
+		Sig:  types.NewType("func(filename str) str"),
+	}
+}
+
+// Init runs some startup code for this function.
+func (obj *ReadFileFunc) Init(init *interfaces.Init) error {
+	obj.init = init
+	obj.events = make(chan error)
+	obj.wg = &sync.WaitGroup{}
+	obj.closeChan = make(chan struct{})
+	return nil
+}
+
+// Stream returns the changing values that this func has over time.
+func (obj *ReadFileFunc) Stream() error {
+	defer close(obj.init.Output) // the sender closes
+	defer obj.wg.Wait()
+	defer func() {
+		if obj.recWatcher != nil {
+			obj.recWatcher.Close() // close previous watcher
+			obj.wg.Wait()
+		}
+	}()
+	for {
+		select {
+		case input, ok := <-obj.init.Input:
+			if !ok {
+				obj.init.Input = nil // don't infinite loop back
+				continue             // no more inputs, but don't return!
+			}
+			//if err := input.Type().Cmp(obj.Info().Sig.Input); err != nil {
+			//	return errwrap.Wrapf(err, "wrong function input")
+			//}
+
+			if obj.last != nil && input.Cmp(obj.last) == nil {
+				continue // value didn't change, skip it
+			}
+			obj.last = input // store for next
+
+			filename := input.Struct()["filename"].Str()
+			// TODO: add validation for absolute path?
+			if filename == obj.filename {
+				continue // nothing changed
+			}
+			obj.filename = filename
+
+			if obj.recWatcher != nil {
+				obj.recWatcher.Close() // close previous watcher
+				obj.wg.Wait()
+			}
+			// create new watcher
+			obj.recWatcher = &recwatch.RecWatcher{
+				Path:    obj.filename,
+				Recurse: false,
+				Flags: recwatch.Flags{
+					// TODO: add Logf
+					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...
+			obj.wg.Add(1)
+			go func() {
+				defer obj.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")
+						}
+					}
+
+					select {
+					case obj.events <- err:
+						// send event...
+
+					case <-obj.closeChan:
+						// don't block here on shutdown
+						return
+					}
+					//err = nil // reset
+				}
+			}()
+
+		case err, ok := <-obj.events:
+			if !ok {
+				return fmt.Errorf("no more events")
+			}
+			if err != nil {
+				return errwrap.Wrapf(err, "error event received")
+			}
+
+			if obj.last == nil {
+				continue // still waiting for input values
+			}
+
+			// read file...
+			content, err := ioutil.ReadFile(obj.filename)
+			if err != nil {
+				return errwrap.Wrapf(err, "error reading file")
+			}
+			result := string(content) // convert to string
+
+			if obj.result == result {
+				continue // result didn't change
+			}
+			obj.result = result // store new result
+
+		case <-obj.closeChan:
+			return nil
+		}
+
+		select {
+		case obj.init.Output <- &types.StrValue{
+			V: obj.result,
+		}:
+		case <-obj.closeChan:
+			return nil
+		}
+	}
+}
+
+// Close runs some shutdown code for this function and turns off the stream.
+func (obj *ReadFileFunc) Close() error {
+	close(obj.events) // clean up for fun
+	close(obj.closeChan)
+	return nil
+}