[release-branch.go1.15] net/mail: return error on empty address list

This restores the handling accidentally changed in CL 217377.

Fixes #40804
For #40803
For #36959

Change-Id: If77fbc0c2a1dde4799f760affdfb8dde9bcaf458
Reviewed-on: https://go-review.googlesource.com/c/go/+/248598
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Jeremy Fishman <jfishman@cloudflare.com>
(cherry picked from commit 3e636ab9ad31040aff2d484237808907a776cec6)
Reviewed-on: https://go-review.googlesource.com/c/go/+/251167

2 files changed, 41 insertions, 4 deletions

diff --git a/src/net/mail/message.go b/src/net/mail/message.go
index 6833cfaec1..09fb794005 100644
--- a/src/net/mail/message.go
+++ b/src/net/mail/message.go
@@ -279,9 +279,6 @@ func (p *addrParser) parseAddressList() ([]*Address, error) {
 		if p.consume(',') {
 			continue
 		}
-		if p.empty() {
-			break
-		}
 
 		addrs, err := p.parseAddress(true)
 		if err != nil {
@@ -295,9 +292,17 @@ func (p *addrParser) parseAddressList() ([]*Address, error) {
 		if p.empty() {
 			break
 		}
-		if !p.consume(',') {
+		if p.peek() != ',' {
 			return nil, errors.New("mail: expected comma")
 		}
+
+		// Skip empty entries for obs-addr-list.
+		for p.consume(',') {
+			p.skipSpace()
+		}
+		if p.empty() {
+			break
+		}
 	}
 	return list, nil
 }
diff --git a/src/net/mail/message_test.go b/src/net/mail/message_test.go
index 75db767547..67e3643aeb 100644
--- a/src/net/mail/message_test.go
+++ b/src/net/mail/message_test.go
@@ -446,6 +446,19 @@ func TestAddressParsing(t *testing.T) {
 			},
 		},
 		{
+			` , joe@where.test,,John <jdoe@one.test>,,`,
+			[]*Address{
+				{
+					Name:    "",
+					Address: "joe@where.test",
+				},
+				{
+					Name:    "John",
+					Address: "jdoe@one.test",
+				},
+			},
+		},
+		{
 			`Group1: <addr1@example.com>;, Group 2: addr2@example.com;, John <addr3@example.com>`,
 			[]*Address{
 				{
@@ -1067,3 +1080,22 @@ func TestAddressFormattingAndParsing(t *testing.T) {
 		}
 	}
 }
+
+func TestEmptyAddress(t *testing.T) {
+	parsed, err := ParseAddress("")
+	if parsed != nil || err == nil {
+		t.Errorf(`ParseAddress("") = %v, %v, want nil, error`, parsed, err)
+	}
+	list, err := ParseAddressList("")
+	if len(list) > 0 || err == nil {
+		t.Errorf(`ParseAddressList("") = %v, %v, want nil, error`, list, err)
+	}
+	list, err = ParseAddressList(",")
+	if len(list) > 0 || err == nil {
+		t.Errorf(`ParseAddressList("") = %v, %v, want nil, error`, list, err)
+	}
+	list, err = ParseAddressList("a@b c@d")
+	if len(list) > 0 || err == nil {
+		t.Errorf(`ParseAddressList("") = %v, %v, want nil, error`, list, err)
+	}
+}