fix data race in code for logging login attempts

logging of login attempts happens in the background, because we don't want to
block regular operation with disk since for such logging. however, when a line
is logged, we evaluate some attributes of a connection, notably the username.
but about when we do authentication, we change the username on a connection. so
we were reading and writing at the same time. this is now fixed by evaluating
the attributes before we pass off the logger to the goroutine.

found by the go race detector.

imapserver/server.go

@@ -186,8 +186,8 @@ type conn struct {
 	cmd               string // Currently executing, for deciding to applyChanges and logging.
 	cmdMetric         string // Currently executing, for metrics.
 	cmdStart          time.Time
-	ncmds             int // Number of commands processed. Used to abort connection when first incoming command is unknown/invalid.
-	log               mlog.Log
+	ncmds             int                 // Number of commands processed. Used to abort connection when first incoming command is unknown/invalid.
+	log               mlog.Log            // Used for all synchronous logging on this connection, see logbg for logging in a separate goroutine.
 	enabled           map[capability]bool // All upper-case.
 
 	// Set by SEARCH with SAVE. Can be used by commands accepting a sequence-set with
@@ -415,6 +415,18 @@ func Serve() {
 	servers = nil
 }
 
+// Logbg returns a logger for logging in the background (in a goroutine), eg for
+// logging LoginAttempts. The regular c.log has a handler that evaluates fields on
+// the connection at time of logging, which may happen at the same time as
+// modifications to those fields.
+func (c *conn) logbg() mlog.Log {
+	log := mlog.New("imapserver", nil).WithCid(c.cid)
+	if c.username != "" {
+		log = log.With(slog.String("username", c.username))
+	}
+	return log
+}
+
 // returns whether this connection accepts utf-8 in strings.
 func (c *conn) utf8strings() bool {
 	return c.enabled[capIMAP4rev2] || c.enabled[capUTF8Accept]
@@ -692,6 +704,7 @@ func serve(listenerName string, cid int64, tlsConfig *tls.Config, nc net.Conn, x
 		cmdStart:          time.Now(),
 	}
 	var logmutex sync.Mutex
+	// Also see (and possibly update) c.logbg, for logging in a goroutine.
 	c.log = mlog.New("imapserver", nil).WithFunc(func() []slog.Attr {
 		logmutex.Lock()
 		defer logmutex.Unlock()
@@ -818,7 +831,7 @@ func serve(listenerName string, cid int64, tlsConfig *tls.Config, nc net.Conn, x
 	// Ensure any pending loginAttempt is written before we stop.
 	defer func() {
 		if c.loginAttempt != nil {
-			store.LoginAttemptAdd(context.Background(), c.log, *c.loginAttempt)
+			store.LoginAttemptAdd(context.Background(), c.logbg(), *c.loginAttempt)
 			c.loginAttempt = nil
 		}
 	}()
@@ -834,7 +847,7 @@ func serve(listenerName string, cid int64, tlsConfig *tls.Config, nc net.Conn, x
 		if storeLoginAttempt {
 			storeLoginAttempt = false
 			if c.loginAttempt != nil {
-				store.LoginAttemptAdd(context.Background(), c.log, *c.loginAttempt)
+				store.LoginAttemptAdd(context.Background(), c.logbg(), *c.loginAttempt)
 				c.loginAttempt = nil
 			}
 		} else if c.loginAttempt != nil {
@@ -853,7 +866,7 @@ func isClosed(err error) bool {
 // filling in the results and other details.
 func (c *conn) newLoginAttempt(useTLS bool, authMech string) {
 	if c.loginAttempt != nil {
-		store.LoginAttemptAdd(context.Background(), c.log, *c.loginAttempt)
+		store.LoginAttemptAdd(context.Background(), c.logbg(), *c.loginAttempt)
 		c.loginAttempt = nil
 	}
 
@@ -941,6 +954,7 @@ func (c *conn) tlsClientAuthVerifyPeerCertParsed(cert *x509.Certificate) error {
 		conn := c.conn.(*tls.Conn)
 		la := *c.loginAttempt
 		c.loginAttempt = nil
+		logbg := c.logbg() // Evaluate attributes now, can't do it in goroutine.
 		go func() {
 			defer func() {
 				// In case of panic don't take the whole program down.
@@ -954,7 +968,7 @@ func (c *conn) tlsClientAuthVerifyPeerCertParsed(cert *x509.Certificate) error {
 
 			state := conn.ConnectionState()
 			la.TLS = store.LoginAttemptTLS(&state)
-			store.LoginAttemptAdd(context.Background(), c.log, la)
+			store.LoginAttemptAdd(context.Background(), logbg, la)
 		}()
 
 		if la.Result == store.AuthSuccess {
@@ -1773,7 +1787,7 @@ func (c *conn) cmdID(tag, cmd string, p *parser) {
 	// prepared the LoginAttempt and write it now.
 	if c.loginAttempt != nil {
 		c.loginAttempt.UserAgent = strings.Join(values, " ")
-		store.LoginAttemptAdd(context.Background(), c.log, *c.loginAttempt)
+		store.LoginAttemptAdd(context.Background(), c.logbg(), *c.loginAttempt)
 		c.loginAttempt = nil
 	}