Add support for extra sendmail arguments (#2731)

* Add support for extra sendmail arguments

* Sendmail args to exec.command should be a list

* Add go-shellquote package

* Use go-shellquote lib for parsing Sendmail args

* Only parse if sendmail is configured

vendor/github.com/kballard/go-shellquote/unquote.go

@@ -0,0 +1,144 @@
+package shellquote
+
+import (
+	"bytes"
+	"errors"
+	"strings"
+	"unicode/utf8"
+)
+
+var (
+	UnterminatedSingleQuoteError = errors.New("Unterminated single-quoted string")
+	UnterminatedDoubleQuoteError = errors.New("Unterminated double-quoted string")
+	UnterminatedEscapeError      = errors.New("Unterminated backslash-escape")
+)
+
+var (
+	splitChars        = " \n\t"
+	singleChar        = '\''
+	doubleChar        = '"'
+	escapeChar        = '\\'
+	doubleEscapeChars = "$`\"\n\\"
+)
+
+// Split splits a string according to /bin/sh's word-splitting rules. It
+// supports backslash-escapes, single-quotes, and double-quotes. Notably it does
+// not support the $'' style of quoting. It also doesn't attempt to perform any
+// other sort of expansion, including brace expansion, shell expansion, or
+// pathname expansion.
+//
+// If the given input has an unterminated quoted string or ends in a
+// backslash-escape, one of UnterminatedSingleQuoteError,
+// UnterminatedDoubleQuoteError, or UnterminatedEscapeError is returned.
+func Split(input string) (words []string, err error) {
+	var buf bytes.Buffer
+	words = make([]string, 0)
+
+	for len(input) > 0 {
+		// skip any splitChars at the start
+		c, l := utf8.DecodeRuneInString(input)
+		if strings.ContainsRune(splitChars, c) {
+			input = input[l:]
+			continue
+		}
+
+		var word string
+		word, input, err = splitWord(input, &buf)
+		if err != nil {
+			return
+		}
+		words = append(words, word)
+	}
+	return
+}
+
+func splitWord(input string, buf *bytes.Buffer) (word string, remainder string, err error) {
+	buf.Reset()
+
+raw:
+	{
+		cur := input
+		for len(cur) > 0 {
+			c, l := utf8.DecodeRuneInString(cur)
+			cur = cur[l:]
+			if c == singleChar {
+				buf.WriteString(input[0 : len(input)-len(cur)-l])
+				input = cur
+				goto single
+			} else if c == doubleChar {
+				buf.WriteString(input[0 : len(input)-len(cur)-l])
+				input = cur
+				goto double
+			} else if c == escapeChar {
+				buf.WriteString(input[0 : len(input)-len(cur)-l])
+				input = cur
+				goto escape
+			} else if strings.ContainsRune(splitChars, c) {
+				buf.WriteString(input[0 : len(input)-len(cur)-l])
+				return buf.String(), cur, nil
+			}
+		}
+		if len(input) > 0 {
+			buf.WriteString(input)
+			input = ""
+		}
+		goto done
+	}
+
+escape:
+	{
+		if len(input) == 0 {
+			return "", "", UnterminatedEscapeError
+		}
+		c, l := utf8.DecodeRuneInString(input)
+		if c == '\n' {
+			// a backslash-escaped newline is elided from the output entirely
+		} else {
+			buf.WriteString(input[:l])
+		}
+		input = input[l:]
+	}
+	goto raw
+
+single:
+	{
+		i := strings.IndexRune(input, singleChar)
+		if i == -1 {
+			return "", "", UnterminatedSingleQuoteError
+		}
+		buf.WriteString(input[0:i])
+		input = input[i+1:]
+		goto raw
+	}
+
+double:
+	{
+		cur := input
+		for len(cur) > 0 {
+			c, l := utf8.DecodeRuneInString(cur)
+			cur = cur[l:]
+			if c == doubleChar {
+				buf.WriteString(input[0 : len(input)-len(cur)-l])
+				input = cur
+				goto raw
+			} else if c == escapeChar {
+				// bash only supports certain escapes in double-quoted strings
+				c2, l2 := utf8.DecodeRuneInString(cur)
+				cur = cur[l2:]
+				if strings.ContainsRune(doubleEscapeChars, c2) {
+					buf.WriteString(input[0 : len(input)-len(cur)-l-l2])
+					if c2 == '\n' {
+						// newline is special, skip the backslash entirely
+					} else {
+						buf.WriteRune(c2)
+					}
+					input = cur
+				}
+			}
+		}
+		return "", "", UnterminatedDoubleQuoteError
+	}
+
+done:
+	return buf.String(), input, nil
+}