Merge support for CHILDREN extension

Author: foxcpp

README.md

@@ -136,6 +136,7 @@ includes:
 * [LITERAL+](https://tools.ietf.org/html/rfc7888)
 * [SASL-IR](https://tools.ietf.org/html/rfc4959)
 * [SPECIAL-USE](https://tools.ietf.org/html/rfc6154)
+* [CHILDREN](https://tools.ietf.org/html/rfc3348)
 
 Support for other extensions is provided via separate packages. See below.
 

mailbox.go

@@ -58,6 +58,16 @@ const (
 	TrashAttr = "\\Trash"
 )
 
+// Mailbox attributes defined in RFC 3348 (CHILDREN extension)
+const (
+	// The presence of this attribute indicates that the mailbox has child
+	// mailboxes.
+	HasChildrenAttr = "\\HasChildren"
+	// The presence of this attribute indicates that the mailbox has no child
+	// mailboxes.
+	HasNoChildrenAttr = "\\HasNoChildren"
+)
+
 // Basic mailbox info.
 type MailboxInfo struct {
 	// The mailbox attributes.

server/conn.go

@@ -163,7 +163,7 @@ func (c *conn) Close() error {
 }
 
 func (c *conn) Capabilities() []string {
-	caps := []string{"IMAP4rev1", "LITERAL+", "SASL-IR"}
+	caps := []string{"IMAP4rev1", "LITERAL+", "SASL-IR", "CHILDREN"}
 
 	if c.ctx.State == imap.NotAuthenticatedState {
 		if !c.IsTLS() && c.s.TLSConfig != nil {

server/server_test.go

@@ -10,7 +10,7 @@ import (
 )
 
 // Extnesions that are always advertised by go-imap server.
-const builtinExtensions = "LITERAL+ SASL-IR"
+const builtinExtensions = "LITERAL+ SASL-IR CHILDREN"
 
 func testServer(t *testing.T) (s *server.Server, conn net.Conn) {
 	bkd := memory.New()