Implement timeouts for all exec command runners

First is to write an internal CombinedOutput and Run function with a
timeout.

Second, the following instances of command runners need to have timeouts:

    plugins/inputs/ping/ping.go
    125:	out, err := c.CombinedOutput()

    plugins/inputs/exec/exec.go
    91:	if err := cmd.Run(); err != nil {

    plugins/inputs/ipmi_sensor/command.go
    31:	err := cmd.Run()

    plugins/inputs/sysstat/sysstat.go
    194:	out, err := cmd.CombinedOutput()

    plugins/inputs/leofs/leofs.go
    185:	defer cmd.Wait()

    plugins/inputs/sysstat/sysstat.go
    282:	if err := cmd.Wait(); err != nil {

closes #1067

internal/internal.go

@@ -2,20 +2,25 @@ package internal
 
 import (
 	"bufio"
+	"bytes"
 	"crypto/rand"
 	"crypto/tls"
 	"crypto/x509"
 	"errors"
 	"fmt"
 	"io/ioutil"
+	"log"
 	"os"
+	"os/exec"
 	"strings"
 	"time"
 	"unicode"
 )
 
 const alphanum string = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
 
+var timeoutErr = errors.New("Command timed out.")
+
 // Duration just wraps time.Duration
 type Duration struct {
 	Duration time.Duration
@@ -139,3 +144,49 @@ func SnakeCase(in string) string {
 
 	return string(out)
 }
+
+// CombinedOutputTimeout runs the given command with the given timeout and
+// returns the combined output of stdout and stderr.
+// If the command times out, it attempts to kill the process.
+func CombinedOutputTimeout(c *exec.Cmd, timeout time.Duration) ([]byte, error) {
+	var b bytes.Buffer
+	c.Stdout = &b
+	c.Stderr = &b
+	if err := c.Start(); err != nil {
+		return nil, err
+	}
+	timer := time.NewTimer(timeout)
+	done := make(chan error)
+	go func() { done <- c.Wait() }()
+	select {
+	case err := <-done:
+		timer.Stop()
+		return b.Bytes(), err
+	case <-timer.C:
+		if err := c.Process.Kill(); err != nil {
+			log.Printf("FATAL error killing process: %s", err)
+		}
+		return b.Bytes(), timeoutErr
+	}
+}
+
+// RunTimeout runs the given command with the given timeout.
+// If the command times out, it attempts to kill the process.
+func RunTimeout(c *exec.Cmd, timeout time.Duration) error {
+	if err := c.Start(); err != nil {
+		return nil, err
+	}
+	timer := time.NewTimer(timeout)
+	done := make(chan error)
+	go func() { done <- c.Wait() }()
+	select {
+	case err := <-done:
+		timer.Stop()
+		return err
+	case <-timer.C:
+		if err := c.Process.Kill(); err != nil {
+			log.Printf("FATAL error killing process: %s", err)
+		}
+		return timeoutErr
+	}
+}

plugins/inputs/exec/exec.go

@@ -73,7 +73,11 @@ func AddNagiosState(exitCode error, acc telegraf.Accumulator) error {
 	return nil
 }
 
-func (c CommandRunner) Run(e *Exec, command string, acc telegraf.Accumulator) ([]byte, error) {
+func (c CommandRunner) Run(
+	e *Exec,
+	command string,
+	acc telegraf.Accumulator,
+) ([]byte, error) {
 	split_cmd, err := shellquote.Split(command)
 	if err != nil || len(split_cmd) == 0 {
 		return nil, fmt.Errorf("exec: unable to parse command, %s", err)