lang: Initial implementation of the mgmt language

This is an initial implementation of the mgmt language. It is a
declarative (immutable) functional, reactive, domain specific
programming language. It is intended to be a language that is:

* safe
* powerful
* easy to reason about

With these properties, we hope this language, and the mgmt engine will
allow you to model the real-time systems that you'd like to automate.

This also includes a number of other associated changes. Sorry for the
large size of this patch.

lang/funcs/facts/core/loadfact.go

@@ -0,0 +1,131 @@
+// 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 core // TODO: should this be in its own individual package?
+
+import (
+	"syscall"
+	"time"
+
+	"github.com/purpleidea/mgmt/lang/funcs/facts"
+	"github.com/purpleidea/mgmt/lang/types"
+
+	errwrap "github.com/pkg/errors"
+)
+
+const (
+	// LoadScale factor scales the output from sysinfo to the correct float
+	// value.
+	LoadScale = 65536 // XXX: is this correct or should it be 65535?
+
+	loadSignature = "struct{x1 float; x5 float; x15 float}"
+)
+
+func init() {
+	facts.Register("load", func() facts.Fact { return &LoadFact{} }) // must register the fact and name
+}
+
+// LoadFact is a fact which returns the current system load.
+type LoadFact struct {
+	init      *facts.Init
+	closeChan chan struct{}
+}
+
+// Validate makes sure we've built our struct properly. It is usually unused for
+// normal facts that users can use directly.
+//func (obj *LoadFact) Validate() error {
+//	return nil
+//}
+
+// Info returns some static info about itself.
+func (obj *LoadFact) Info() *facts.Info {
+	return &facts.Info{
+		Output: types.NewType(loadSignature),
+	}
+}
+
+// Init runs some startup code for this fact.
+func (obj *LoadFact) Init(init *facts.Init) error {
+	obj.init = init
+	obj.closeChan = make(chan struct{})
+	return nil
+}
+
+// Stream returns the changing values that this fact has over time.
+func (obj *LoadFact) Stream() error {
+	defer close(obj.init.Output) // always signal when we're done
+
+	// it seems the different values only update once every 5
+	// seconds, so that's as often as we need to refresh this!
+	// TODO: lookup this value if it's something configurable
+	ticker := time.NewTicker(time.Duration(5) * time.Second)
+
+	// streams must generate an initial event on startup
+	startChan := make(chan struct{}) // start signal
+	close(startChan)                 // kick it off!
+	defer ticker.Stop()
+	for {
+		select {
+		case <-startChan: // kick the loop once at start
+			startChan = nil // disable
+		case <-ticker.C: // received the timer event
+			// pass
+		case <-obj.closeChan:
+			return nil
+		}
+
+		x1, x5, x15, err := load()
+		if err != nil {
+			return errwrap.Wrapf(err, "could not read load values")
+		}
+
+		st := types.NewStruct(types.NewType(loadSignature))
+		for k, v := range map[string]float64{"x1": x1, "x5": x5, "x15": x15} {
+			if err := st.Set(k, &types.FloatValue{V: v}); err != nil {
+				return errwrap.Wrapf(err, "struct could not set key: `%s`", k)
+			}
+		}
+
+		select {
+		case obj.init.Output <- st:
+			// send
+		case <-obj.closeChan:
+			return nil
+		}
+	}
+}
+
+// Close runs some shutdown code for this fact and turns off the stream.
+func (obj *LoadFact) Close() error {
+	close(obj.closeChan)
+	return nil
+}
+
+// load returns the system load averages for the last minute, five minutes and
+// fifteen minutes. Calling this more often than once every five seconds seems
+// to be unnecessary, since the kernel only updates these values that often.
+// TODO: is the kernel update interval configurable?
+func load() (one, five, fifteen float64, err error) {
+	var sysinfo syscall.Sysinfo_t
+	if err = syscall.Sysinfo(&sysinfo); err != nil {
+		return
+	}
+	one = float64(sysinfo.Loads[0]) / LoadScale
+	five = float64(sysinfo.Loads[1]) / LoadScale
+	fifteen = float64(sysinfo.Loads[2]) / LoadScale
+	return
+}