imapserver: advertise APPENDLIMIT capability

imapserver/capability.go

@@ -1,6 +1,8 @@
 package imapserver
 
 import (
+	"fmt"
+
 	"github.com/emersion/go-imap/v2"
 	"github.com/emersion/go-imap/v2/internal/imapwire"
 )
@@ -86,6 +88,21 @@ func (c *Conn) availableCaps() []imap.Cap {
 
 		// Capabilities which require backend support and apply to both
 		// IMAP4rev1 and IMAP4rev2
+		if appendLimitSession, ok := c.session.(SessionAppendLimit); ok {
+			if appendLimitSession.DiscloseLimit() {
+				limit := appendLimitSession.AppendLimit()
+				caps = append(caps, imap.Cap(fmt.Sprintf("APPENDLIMIT=%d", limit)))
+			} else {
+				caps = append(caps, imap.CapAppendLimit)
+			}
+		} else if limit, ok := available.AppendLimit(); ok {
+			if limit != nil {
+				caps = append(caps, imap.Cap(fmt.Sprintf("APPENDLIMIT=%d", *limit)))
+			} else {
+				caps = append(caps, imap.CapAppendLimit)
+			}
+		}
+
 		addAvailableCaps(&caps, available, []imap.Cap{
 			imap.CapSpecialUse,
 			imap.CapCreateSpecialUse,

imapserver/imapmemserver/server.go

@@ -47,6 +47,25 @@ type serverSession struct {
 }
 
 var _ imapserver.Session = (*serverSession)(nil)
+var _ imapserver.SessionAppendLimit = (*serverSession)(nil)
+
+// AppendLimit implements the SessionAppendLimit interface.
+func (sess *serverSession) AppendLimit() uint32 {
+	if sess.UserSession != nil {
+		return sess.UserSession.AppendLimit()
+	}
+	// Default value for unauthenticated sessions
+	return 104857600 // 100 MiB
+}
+
+// DiscloseLimit implements the SessionAppendLimit interface.
+func (sess *serverSession) DiscloseLimit() bool {
+	if sess.UserSession != nil {
+		return sess.UserSession.DiscloseLimit()
+	}
+	// Default for unauthenticated sessions - true means we show the limit
+	return true
+}
 
 func (sess *serverSession) Login(username, password string) error {
 	u := sess.server.user(username)

imapserver/imapmemserver/session.go

@@ -17,13 +17,35 @@ type (
 type UserSession struct {
 	*user    // immutable
 	*mailbox // may be nil
+
+	// appendLimit is the maximum size in bytes that can be uploaded to this server
+	// in an APPEND command
+	appendLimit uint32
+
+	// discloseLimit indicates whether the append limit should be advertised in the
+	// CAPABILITY response
+	discloseLimit bool
 }
 
 var _ imapserver.SessionIMAP4rev2 = (*UserSession)(nil)
+var _ imapserver.SessionAppendLimit = (*UserSession)(nil)
 
 // NewUserSession creates a new user session.
 func NewUserSession(user *User) *UserSession {
-	return &UserSession{user: user}
+	return &UserSession{
+		user:          user,
+		appendLimit:   104857600, // 100 MiB default
+		discloseLimit: true,      // By default, disclose the limit in CAPABILITY
+	}
+}
+
+// NewUserSessionWithAppendLimit creates a new user session with a custom append limit.
+func NewUserSessionWithAppendLimit(user *User, appendLimit uint32, discloseLimit bool) *UserSession {
+	return &UserSession{
+		user:          user,
+		appendLimit:   appendLimit,
+		discloseLimit: discloseLimit,
+	}
 }
 
 func (sess *UserSession) Close() error {
@@ -138,3 +160,19 @@ func (sess *UserSession) Idle(w *imapserver.UpdateWriter, stop <-chan struct{})
 	}
 	return sess.mailbox.Idle(w, stop)
 }
+
+// AppendLimit implements the SessionAppendLimit interface.
+// It returns the maximum size in bytes that can be uploaded to this server in an APPEND command.
+func (sess *UserSession) AppendLimit() uint32 {
+	// If appendLimit is not set (0), return a default large value
+	if sess.appendLimit == 0 {
+		return 104857600 // 100 MiB default
+	}
+	return sess.appendLimit
+}
+
+// DiscloseLimit implements the SessionAppendLimit interface.
+// It indicates whether the append limit should be advertised in the CAPABILITY response.
+func (sess *UserSession) DiscloseLimit() bool {
+	return sess.discloseLimit
+}

imapserver/session.go

@@ -114,3 +114,18 @@ type SessionUnauthenticate interface {
 	// Authenticated state
 	Unauthenticate() error
 }
+
+// SessionAppendLimit is an IMAP session which has the same APPEND limit for
+// all mailboxes.
+type SessionAppendLimit interface {
+	Session
+
+	// AppendLimit returns the maximum size in bytes that can be uploaded to this server
+	// in an APPEND command.
+	AppendLimit() uint32
+
+	// DiscloseLimit indicates whether the limit should be advertised in the CAPABILITY
+	// response. If false, only "APPENDLIMIT" will be listed, without the actual limit.
+	// If true, "APPENDLIMIT=<limit>" will be listed.
+	DiscloseLimit() bool
+}