Add output of stderr in case of error to exec log message (#3862)

If the command failed with a non-zero exit status there might be an error
message on stderr. Append the first line to the error message to ease the
search for its cause.

plugins/inputs/exec/exec.go

@@ -41,6 +41,8 @@ const sampleConfig = `
   data_format = "influx"
 `
 
+const MaxStderrBytes = 512
+
 type Exec struct {
 	Commands []string
 	Command  string
@@ -96,15 +98,41 @@ func (c CommandRunner) Run(
 
 	cmd := exec.Command(split_cmd[0], split_cmd[1:]...)
 
-	var out bytes.Buffer
+	var (
+		out    bytes.Buffer
+		stderr bytes.Buffer
+	)
 	cmd.Stdout = &out
+	cmd.Stderr = &stderr
 
 	if err := internal.RunTimeout(cmd, e.Timeout.Duration); err != nil {
 		switch e.parser.(type) {
 		case *nagios.NagiosParser:
 			AddNagiosState(err, acc)
 		default:
-			return nil, fmt.Errorf("exec: %s for command '%s'", err, command)
+			var errMessage = ""
+			if stderr.Len() > 0 {
+				stderr = removeCarriageReturns(stderr)
+				// Limit the number of bytes.
+				didTruncate := false
+				if stderr.Len() > MaxStderrBytes {
+					stderr.Truncate(MaxStderrBytes)
+					didTruncate = true
+				}
+				if i := bytes.IndexByte(stderr.Bytes(), '\n'); i > 0 {
+					// Only show truncation if the newline wasn't the last character.
+					if i < stderr.Len()-1 {
+						didTruncate = true
+					}
+					stderr.Truncate(i)
+				}
+				if didTruncate {
+					stderr.WriteString("...")
+				}
+
+				errMessage = fmt.Sprintf(": %s", stderr.String())
+			}
+			return nil, fmt.Errorf("exec: %s for command '%s'%s", err, command, errMessage)
 		}
 	} else {
 		switch e.parser.(type) {