net/mail: enhanced Address.String and ParseAddress to match RFC 5322

Updated Address.String so it restores quoted local parts, which wasn't
done before.
When parsing `<" "@example.com>`, the formatted string returned
`< @example>`, which doens't match RFC 5322, since a space is not atext.
Another example is `<"bob@valid"@example.com>` which returned
`<bob@valid@example.com>`, which is completely invalid.
I also added support for quotes and backslashes in a quoted local part.

Besides formatting a parsed Address, the ParseAddress function also
needed more testing and finetuning for special cases.
Things like `<.john.doe@example.com>` and `<john..doe@example.com>`
e.a. were accepted, but are invalid.
I fixed those details and add tests for some other special cases.

Fixes #10768

Change-Id: Ib0caf8ad603eb21e32fcb957a5f1a0fe5d1c6e6e
Reviewed-on: https://go-review.googlesource.com/8724
Reviewed-by: Russ Cox <rsc@golang.org>

1 files changed, 67 insertions, 5 deletions

diff --git a/src/net/mail/message.go b/src/net/mail/message.go
index 04cbfd3e8b..2d8e380cd9 100644
--- a/src/net/mail/message.go
+++ b/src/net/mail/message.go
@@ -168,10 +168,42 @@ func (p *AddressParser) ParseList(list string) ([]*Address, error) {
 // If the address's name contains non-ASCII characters
 // the name will be rendered according to RFC 2047.
 func (a *Address) String() string {
-	s := "<" + a.Address + ">"
+
+	// Format address local@domain
+	at := strings.LastIndex(a.Address, "@")
+	local, domain := a.Address[:at], a.Address[at+1:]
+
+	// Add quotes if needed
+	// TODO: rendering quoted local part and rendering printable name
+	//       should be merged in helper function.
+	quoteLocal := false
+	for i := 0; i < len(local); i++ {
+		ch := local[i]
+		if isAtext(ch, false) {
+			continue
+		}
+		if ch == '.' {
+			// Dots are okay if they are surrounded by atext.
+			// We only need to check that the previous byte is
+			// not a dot, and this isn't the end of the string.
+			if i > 0 && local[i-1] != '.' && i < len(local)-1 {
+				continue
+			}
+		}
+		quoteLocal = true
+		break
+	}
+	if quoteLocal {
+		local = quoteString(local)
+
+	}
+
+	s := "<" + local + "@" + domain + ">"
+
 	if a.Name == "" {
 		return s
 	}
+
 	// If every character is printable ASCII, quoting is simple.
 	allPrintable := true
 	for i := 0; i < len(a.Name); i++ {
@@ -301,7 +333,7 @@ func (p *addrParser) consumeAddrSpec() (spec string, err error) {
 	} else {
 		// dot-atom
 		debug.Printf("consumeAddrSpec: parsing dot-atom")
-		localPart, err = p.consumeAtom(true)
+		localPart, err = p.consumeAtom(true, false)
 	}
 	if err != nil {
 		debug.Printf("consumeAddrSpec: failed: %v", err)
@@ -319,7 +351,7 @@ func (p *addrParser) consumeAddrSpec() (spec string, err error) {
 		return "", errors.New("mail: no domain in addr-spec")
 	}
 	// TODO(dsymonds): Handle domain-literal
-	domain, err = p.consumeAtom(true)
+	domain, err = p.consumeAtom(true, false)
 	if err != nil {
 		return "", err
 	}
@@ -346,7 +378,7 @@ func (p *addrParser) consumePhrase() (phrase string, err error) {
 			// atom
 			// We actually parse dot-atom here to be more permissive
 			// than what RFC 5322 specifies.
-			word, err = p.consumeAtom(true)
+			word, err = p.consumeAtom(true, true)
 		}
 
 		if err == nil {
@@ -402,7 +434,9 @@ Loop:
 
 // consumeAtom parses an RFC 5322 atom at the start of p.
 // If dot is true, consumeAtom parses an RFC 5322 dot-atom instead.
-func (p *addrParser) consumeAtom(dot bool) (atom string, err error) {
+// If permissive is true, consumeAtom will not fail on
+// leading/trailing/double dots in the atom (see golang.org/issue/4938).
+func (p *addrParser) consumeAtom(dot bool, permissive bool) (atom string, err error) {
 	if !isAtext(p.peek(), false) {
 		return "", errors.New("mail: invalid string")
 	}
@@ -410,6 +444,17 @@ func (p *addrParser) consumeAtom(dot bool) (atom string, err error) {
 	for ; i < p.len() && isAtext(p.s[i], dot); i++ {
 	}
 	atom, p.s = string(p.s[:i]), p.s[i:]
+	if !permissive {
+		if strings.HasPrefix(atom, ".") {
+			return "", errors.New("mail: leading dot in atom")
+		}
+		if strings.Contains(atom, "..") {
+			return "", errors.New("mail: double dot in atom")
+		}
+		if strings.HasSuffix(atom, ".") {
+			return "", errors.New("mail: trailing dot in atom")
+		}
+	}
 	return atom, nil
 }
 
@@ -491,6 +536,23 @@ func isQtext(c byte) bool {
 	return '!' <= c && c <= '~'
 }
 
+// quoteString renders a string as a RFC5322 quoted-string.
+func quoteString(s string) string {
+	var buf bytes.Buffer
+	buf.WriteByte('"')
+	for _, c := range s {
+		ch := byte(c)
+		if isQtext(ch) || isWSP(ch) {
+			buf.WriteByte(ch)
+		} else if isVchar(ch) {
+			buf.WriteByte('\\')
+			buf.WriteByte(ch)
+		}
+	}
+	buf.WriteByte('"')
+	return buf.String()
+}
+
 // isVchar reports whether c is an RFC 5322 VCHAR character.
 func isVchar(c byte) bool {
 	// Visible (printing) characters.