allow multiple local clients, block latch inside latch

Remove single local client restriction so multiple terminals can
attach to different sessions simultaneously. Set LATCH_SESSION env
var inside pane shells and check it on new/attach/default to prevent
running latch inside a local latch session.

Author: realrasengan

cmd/latch/main.go

@@ -21,6 +21,9 @@ func main() {
 	}
 
 	if len(os.Args) < 2 {
+		if os.Getenv("LATCH_SESSION") != "" {
+			fatal("already inside a latch session")
+		}
 		ensureServer(cfg)
 		if err := client.Attach(server.SocketPath(), "default", true, cfg.PrefixKey); err != nil {
 			fatal("%v", err)
@@ -55,6 +58,9 @@ func main() {
 		serve(cfg, sshAddr, webAddr)
 
 	case "new":
+		if os.Getenv("LATCH_SESSION") != "" {
+			fatal("already inside a latch session")
+		}
 		name := "default"
 		var sshAddr, webAddr string
 		detached := false
@@ -88,6 +94,9 @@ func main() {
 		}
 
 	case "attach", "a":
+		if os.Getenv("LATCH_SESSION") != "" {
+			fatal("already inside a latch session")
+		}
 		name := "default"
 		if len(os.Args) > 2 {
 			name = os.Args[2]

internal/mux/pane.go

@@ -67,7 +67,7 @@ func NewPane(id int, cols, rows int, shell string, scrollbackLines ...int) (*Pan
 	}
 
 	cmd := exec.Command(shell, "-l")
-	cmd.Env = append(os.Environ(), "TERM=xterm-256color")
+	cmd.Env = append(os.Environ(), "TERM=xterm-256color", "LATCH_SESSION=1")
 	if home, err := os.UserHomeDir(); err == nil {
 		cmd.Dir = home
 	}

internal/server/server.go

@@ -251,12 +251,6 @@ func (s *Server) attachSession(conn net.Conn, sess *mux.Session) {
 	meta.Session = sess.Name
 	meta.closer = conn
 
-	// Enforce single local attach.
-	if meta.Source == "local" && s.tracker.hasLocal() {
-		proto.Encode(conn, proto.MsgError, []byte("another local client is attached"))
-		return
-	}
-
 	// Per-session access control for remote clients.
 	reject := func(reason string) {
 		s.audit.emit(AuditEvent{