UTF8/7 - consistent and context aware handling

imapclient/client.go

@@ -371,6 +371,8 @@ func (c *Client) setCaps(caps imap.CapSet) {
 	c.mutex.Lock()
 	c.caps = caps
 	c.pendingCapCh = capCh
+	quotedUTF8 := c.caps.Has(imap.CapIMAP4rev2) || c.enabled.Has(imap.CapUTF8Accept)
+	c.dec.QuotedUTF8 = quotedUTF8
 	c.mutex.Unlock()
 }
 

imapclient/enable.go

@@ -43,6 +43,9 @@ func (c *Client) handleEnabled() error {
 	for name := range caps {
 		c.enabled[name] = struct{}{}
 	}
+
+	quotedUTF8 := c.caps.Has(imap.CapIMAP4rev2) || c.enabled.Has(imap.CapUTF8Accept)
+	c.dec.QuotedUTF8 = quotedUTF8
 	c.mutex.Unlock()
 
 	if cmd := findPendingCmdByType[*EnableCommand](c); cmd != nil {

imapserver/conn.go

@@ -179,6 +179,15 @@ func (c *Conn) serve() {
 		dec.MaxSize = maxCommandSize
 		dec.CheckBufferedLiteralFunc = c.checkBufferedLiteral
 
+		c.mutex.Lock()
+		// IMAP4rev2 is automatically enabled when advertised in capabilities
+		// UTF8=ACCEPT must be explicitly enabled
+		imap4rev2Available := c.server.options.caps().Has(imap.CapIMAP4rev2)
+		utf8AcceptEnabled := c.enabled.Has(imap.CapUTF8Accept)
+		quotedUTF8 := imap4rev2Available || utf8AcceptEnabled
+		c.mutex.Unlock()
+		dec.QuotedUTF8 = quotedUTF8
+
 		if c.state == imap.ConnStateLogout || dec.EOF() {
 			break
 		}
@@ -492,7 +501,11 @@ type responseEncoder struct {
 
 func newResponseEncoder(conn *Conn) *responseEncoder {
 	conn.mutex.Lock()
-	quotedUTF8 := conn.enabled.Has(imap.CapIMAP4rev2) || conn.enabled.Has(imap.CapUTF8Accept)
+	// IMAP4rev2 is automatically enabled when advertised in capabilities
+	// UTF8=ACCEPT must be explicitly enabled
+	imap4rev2Available := conn.server.options.caps().Has(imap.CapIMAP4rev2)
+	utf8AcceptEnabled := conn.enabled.Has(imap.CapUTF8Accept)
+	quotedUTF8 := imap4rev2Available || utf8AcceptEnabled
 	conn.mutex.Unlock()
 
 	wireEnc := imapwire.NewEncoder(conn.bw, imapwire.ConnSideServer)

imapserver/list.go

@@ -206,7 +206,12 @@ func readListMailbox(dec *imapwire.Decoder) (string, error) {
 			return "", dec.Err()
 		}
 	}
-	return utf7.Decode(mailbox)
+
+	if dec.QuotedUTF8 {
+		return utf7.Unescape(mailbox)
+	} else {
+		return utf7.Decode(mailbox)
+	}
 }
 
 func isListChar(ch byte) bool {

internal/imapwire/decoder.go

@@ -55,6 +55,9 @@ type Decoder struct {
 	// MaxSize defines a maximum number of bytes to be read from the input.
 	// Literals are ignored.
 	MaxSize int64
+	// QuotedUTF8 allows raw UTF-8 in quoted strings. This requires IMAP4rev2
+	// to be available, or UTF8=ACCEPT to be enabled.
+	QuotedUTF8 bool
 
 	r         *bufio.Reader
 	side      ConnSide
@@ -517,7 +520,13 @@ func (dec *Decoder) ExpectMailbox(ptr *string) bool {
 		*ptr = "INBOX"
 		return true
 	}
-	name, err := utf7.Decode(name)
+
+	var err error
+	if dec.QuotedUTF8 {
+		name, err = utf7.Unescape(name)
+	} else {
+		name, err = utf7.Decode(name)
+	}
 	if err == nil {
 		*ptr = name
 	}

internal/utf7/decoder.go

@@ -116,3 +116,34 @@ func decode(b64 []byte) []byte {
 	}
 	return s[:j]
 }
+
+// Unescape passes through raw UTF-8 as-is and unescapes the special UTF-7 marker
+// (the "&-" sequence back to "&").
+func Unescape(src string) (string, error) {
+	if !utf8.ValidString(src) {
+		return "", errors.New("invalid UTF-8")
+	}
+
+	var sb strings.Builder
+	sb.Grow(len(src))
+
+	for i := 0; i < len(src); i++ {
+		ch := src[i]
+
+		if ch != '&' {
+			sb.WriteByte(ch)
+			continue
+		}
+
+		// Check if this is an escape sequence "&-"
+		if i+1 < len(src) && src[i+1] == '-' {
+			sb.WriteByte('&')
+			i++ // Skip the '-'
+		} else {
+			// This is not an escape sequence, keep the '&' as-is
+			sb.WriteByte(ch)
+		}
+	}
+
+	return sb.String(), nil
+}