examples, lang: funcs: core: example: Update vumeter

This patch moves to use the sox package instead of arecord for getting
microphone data, and it also validates that both sox and rec and
installed. We also add a standalone example.

examples/lang/vumeter.mcl

@@ -0,0 +1,8 @@
+import "example"
+
+$vumeter = example.vumeter("====", 10, 0.9)
+
+file "/tmp/vumeter" {
+	state => $const.res.file.state.exists,
+	content => $vumeter,
+}

lang/funcs/core/example/vumeter_func.go

@@ -18,9 +18,11 @@
 package coreexample
 
 import (
+	"context"
 	"fmt"
 	"math"
 	"os/exec"
+	"path/filepath"
 	"strconv"
 	"strings"
 	"sync"
@@ -63,6 +65,43 @@ func (obj *VUMeterFunc) ArgGen(index int) (string, error) {
 // Validate makes sure we've built our struct properly. It is usually unused for
 // normal functions that users can use directly.
 func (obj *VUMeterFunc) Validate() error {
+	check := func(binary string) error {
+		args := []string{"--help"}
+
+		prog := fmt.Sprintf("%s %s", binary, strings.Join(args, " "))
+
+		//obj.init.Logf("running: %s", prog)
+
+		p, err := filepath.EvalSymlinks(binary)
+		if err != nil {
+			return err
+		}
+		// TODO: do we need to do the ^C handling?
+		// XXX: is the ^C context cancellation propagating into this correctly?
+		cmd := exec.CommandContext(context.TODO(), p, args...)
+		cmd.Dir = ""
+		cmd.Env = []string{}
+		cmd.SysProcAttr = &syscall.SysProcAttr{
+			Setpgid: true,
+			Pgid:    0,
+		}
+
+		if err := cmd.Run(); err != nil {
+			if e, ok := err.(*exec.Error); ok && e.Err == exec.ErrNotFound {
+				return fmt.Errorf("is %s in your $PATH ?", binary)
+			}
+
+			return errwrap.Wrapf(err, "error running: %s", prog)
+		}
+		return nil
+	}
+
+	// if rec is a symlink, this will error without the above EvalSymlinks!
+	for _, x := range []string{"/usr/bin/rec", "/usr/bin/sox"} {
+		if err := check(x); err != nil {
+			return err
+		}
+	}
 	return nil
 }
 
@@ -121,9 +160,14 @@ func (obj *VUMeterFunc) Stream() error {
 				continue // still waiting for input values
 			}
 
+			// record for one second to a shared memory file
+			// rec /dev/shm/mgmt_rec.wav trim 0 1 2>/dev/null
+			args1 := []string{"/dev/shm/mgmt_rec.wav", "trim", "0", "1"}
+			cmd1 := exec.Command("/usr/bin/rec", args1...)
+			// XXX: arecord stopped working on newer linux...
 			// arecord -d 1 /dev/shm/mgmt_rec.wav 2>/dev/null
-			args1 := []string{"-d", "1", "/dev/shm/mgmt_rec.wav"}
+			//args1 := []string{"-d", "1", "/dev/shm/mgmt_rec.wav"}
-			cmd1 := exec.Command("/usr/bin/arecord", args1...)
+			//cmd1 := exec.Command("/usr/bin/arecord", args1...)
 			cmd1.SysProcAttr = &syscall.SysProcAttr{
 				Setpgid: true,
 				Pgid:    0,