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

@@ -2,13 +2,16 @@ package internal
 
 import (
 	"bufio"
+	"bytes"
 	"crypto/rand"
 	"crypto/tls"
 	"crypto/x509"
 	"errors"
 	"fmt"
 	"io/ioutil"
+	"log"
 	"os"
+	"os/exec"
 	"strings"
 	"time"
 	"unicode"
@@ -16,6 +19,12 @@ import (
 
 const alphanum string = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
 
+var (
+	TimeoutErr = errors.New("Command timed out.")
+
+	NotImplementedError = errors.New("not implemented yet")
+)
+
 // Duration just wraps time.Duration
 type Duration struct {
 	Duration time.Duration
@@ -33,8 +42,6 @@ func (d *Duration) UnmarshalTOML(b []byte) error {
 	return nil
 }
 
-var NotImplementedError = errors.New("not implemented yet")
-
 // ReadLines reads contents from a file and splits them by new lines.
 // A convenience wrapper to ReadLinesOffsetN(filename, 0, -1).
 func ReadLines(filename string) ([]string, error) {
@@ -139,3 +146,48 @@ 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
+	}
+	err := WaitTimeout(c, timeout)
+	return b.Bytes(), err
+}
+
+// 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 err
+	}
+	return WaitTimeout(c, timeout)
+}
+
+// WaitTimeout waits for the given command to finish with a timeout.
+// It assumes the command has already been started.
+// If the command times out, it attempts to kill the process.
+func WaitTimeout(c *exec.Cmd, timeout time.Duration) error {
+	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 err
+		}
+		// wait for the command to return after killing it
+		<-done
+		return TimeoutErr
+	}
+}