If writing a file fails, propagate the error to the server via a new

message. Use a client flag rather than bumping the protocol version.
GitHub issue 5451.

client.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: client.c,v 1.166 2026/07/10 15:45:11 nicm Exp $ */
+/* $OpenBSD: client.c,v 1.167 2026/08/17 07:56:56 nicm Exp $ */
 
 /*
  * Copyright (c) 2007 Nicholas Marriott <nicholas.marriott@gmail.com>
@@ -277,7 +277,7 @@ client_main(struct event_base *base, int argc, char **argv, uint64_t flags,
 	proc_set_signals(client_proc, client_signal);
 
 	/* Save the flags. */
-	client_flags = flags;
+	client_flags = flags|CLIENT_WRITE_ACK;
 	log_debug("flags are %#llx", (unsigned long long)client_flags);
 
 	/* Initialize the client socket and start the server. */

file.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: file.c,v 1.21 2026/07/26 15:08:15 nicm Exp $ */
+/* $OpenBSD: file.c,v 1.22 2026/08/17 07:56:56 nicm Exp $ */
 
 /*
  * Copyright (c) 2019 Nicholas Marriott <nicholas.marriott@gmail.com>
@@ -505,7 +505,8 @@ file_push(struct client_file *cf)
 	} else if (cf->stream > 2) {
 		close.stream = cf->stream;
 		proc_send(cf->peer, MSG_WRITE_CLOSE, -1, &close, sizeof close);
-		file_fire_done(cf);
+		if (cf->c == NULL || (~cf->c->flags & CLIENT_WRITE_ACK))
+			file_fire_done(cf);
 	}
 	free(msg);
 }
@@ -530,14 +531,48 @@ file_write_left(struct client_files *files)
 	return (waiting != 0);
 }
 
+/* Finish writing a client file. */
+static void
+file_write_finished(struct client_file *cf)
+{
+	struct msg_write_done	 msg;
+
+	if (cf->event != NULL) {
+		bufferevent_free(cf->event);
+		cf->event = NULL;
+	}
+	if (cf->fd != -1) {
+		if (close(cf->fd) != 0 && cf->error == 0)
+			cf->error = errno;
+		cf->fd = -1;
+	}
+
+	msg.stream = cf->stream;
+	msg.error = cf->error;
+	proc_send(cf->peer, MSG_WRITE_DONE, -1, &msg, sizeof msg);
+
+	if (cf->cb != NULL)
+		cf->cb(NULL, NULL, 0, -1, NULL, cf->data);
+	file_free(cf);
+}
+
 /* Client file write error callback. */
 static void
-file_write_error_callback(__unused struct bufferevent *bev, __unused short what,
+file_write_error_callback(__unused struct bufferevent *bev, short what,
     void *arg)
 {
 	struct client_file	*cf = arg;
+	int			 error;
+
+	if (what & EVBUFFER_ERROR)
+		error = errno;
+	else
+		error = EIO;
+	if (error == 0)
+		error = EIO;
 
 	log_debug("write error file %d", cf->stream);
+	cf->error = error;
 
 	bufferevent_free(cf->event);
 	cf->event = NULL;
@@ -545,7 +580,9 @@ file_write_error_callback(__unused struct bufferevent *bev, __unused short what,
 	close(cf->fd);
 	cf->fd = -1;
 
-	if (cf->cb != NULL)
+	if (cf->closed)
+		file_write_finished(cf);
+	else if (cf->cb != NULL)
 		cf->cb(NULL, NULL, 0, -1, NULL, cf->data);
 }
 
@@ -557,15 +594,10 @@ file_write_callback(__unused struct bufferevent *bev, void *arg)
 
 	log_debug("write check file %d", cf->stream);
 
-	if (cf->cb != NULL)
+	if (cf->closed && EVBUFFER_LENGTH(cf->event->output) == 0)
+		file_write_finished(cf);
+	else if (cf->cb != NULL)
 		cf->cb(NULL, NULL, 0, -1, NULL, cf->data);
-
-	if (cf->closed && EVBUFFER_LENGTH(cf->event->output) == 0) {
-		bufferevent_free(cf->event);
-		close(cf->fd);
-		RB_REMOVE(client_files, cf->tree, cf);
-		file_free(cf);
-	}
 }
 
 /* Handle a file write open message (client). */
@@ -666,14 +698,10 @@ file_write_close(struct client_files *files, struct imsg *imsg)
 	if ((cf = RB_FIND(client_files, files, &find)) == NULL)
 		fatalx("unknown stream number");
 	log_debug("close file %d", cf->stream);
+	cf->closed = 1;
 
 	if (cf->event == NULL || EVBUFFER_LENGTH(cf->event->output) == 0) {
-		if (cf->event != NULL)
-			bufferevent_free(cf->event);
-		if (cf->fd != -1)
-			close(cf->fd);
-		RB_REMOVE(client_files, files, cf);
-		file_free(cf);
+		file_write_finished(cf);
 	}
 }
 
@@ -832,6 +860,28 @@ file_write_ready(struct client_files *files, struct imsg *imsg)
 	return (0);
 }
 
+/* Handle a write done message (server). */
+int
+file_write_done(struct client_files *files, struct imsg *imsg)
+{
+	struct msg_write_done	*msg = imsg->data;
+	size_t			 msglen = imsg->hdr.len - IMSG_HEADER_SIZE;
+	struct client_file	 find, *cf;
+
+	if (msglen != sizeof *msg)
+		return (-1);
+	find.stream = msg->stream;
+	if ((cf = RB_FIND(client_files, files, &find)) == NULL)
+		return (0);
+	if (cf->c == NULL || (~cf->c->flags & CLIENT_WRITE_ACK))
+		return (0);
+
+	log_debug("file %d write done", cf->stream);
+	cf->error = msg->error;
+	file_fire_done(cf);
+	return (0);
+}
+
 /* Handle read data message (server). */
 int
 file_read_data(struct client_files *files, struct imsg *imsg)

server-client.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: server-client.c,v 1.502 2026/08/04 11:18:22 nicm Exp $ */
+/* $OpenBSD: server-client.c,v 1.503 2026/08/17 07:56:56 nicm Exp $ */
 
 /*
  * Copyright (c) 2009 Nicholas Marriott <nicholas.marriott@gmail.com>
@@ -2700,6 +2700,10 @@ server_client_dispatch(struct imsg *imsg, void *arg)
 		if (file_write_ready(&c->files, imsg) != 0)
 			goto bad;
 		break;
+	case MSG_WRITE_DONE:
+		if (file_write_done(&c->files, imsg) != 0)
+			goto bad;
+		break;
 	case MSG_READ:
 		if (file_read_data(&c->files, imsg) != 0)
 			goto bad;

tmux-protocol.h

@@ -1,4 +1,4 @@
-/* $OpenBSD: tmux-protocol.h,v 1.2 2023/01/06 07:09:27 nicm Exp $ */
+/* $OpenBSD: tmux-protocol.h,v 1.3 2026/08/17 07:56:56 nicm Exp $ */
 
 /*
  * Copyright (c) 2021 Nicholas Marriott <nicholas.marriott@gmail.com>
@@ -67,7 +67,8 @@ enum msgtype {
 	MSG_WRITE,
 	MSG_WRITE_READY,
 	MSG_WRITE_CLOSE,
-	MSG_READ_CANCEL
+	MSG_READ_CANCEL,
+	MSG_WRITE_DONE
 };
 
 /*
@@ -116,4 +117,9 @@ struct msg_write_close {
 	int	stream;
 };
 
+struct msg_write_done {
+	int	stream;
+	int	error;
+};
+
 #endif /* TMUX_PROTOCOL_H */

tmux.h

@@ -1,4 +1,4 @@
-/* $OpenBSD: tmux.h,v 1.1422 2026/08/05 08:54:56 nicm Exp $ */
+/* $OpenBSD: tmux.h,v 1.1423 2026/08/17 07:56:56 nicm Exp $ */
 
 /*
  * Copyright (c) 2007 Nicholas Marriott <nicholas.marriott@gmail.com>
@@ -2252,7 +2252,7 @@ struct client {
 /* 0x800000000ULL unused */
 #define CLIENT_BRACKETPASTING 0x1000000000ULL
 #define CLIENT_ASSUMEPASTING 0x2000000000ULL
-/* 0x4000000000ULL unused */
+#define CLIENT_WRITE_ACK 0x4000000000ULL
 #define CLIENT_NO_DETACH_ON_DESTROY 0x8000000000ULL
 #define CLIENT_ALLREDRAWFLAGS		\
 	(CLIENT_REDRAWWINDOW|		\
@@ -3235,6 +3235,7 @@ void	 file_write_close(struct client_files *, struct imsg *);
 void	 file_read_open(struct client_files *, struct tmuxpeer *, struct imsg *,
 	     int, int, client_file_cb, void *);
 int	 file_write_ready(struct client_files *, struct imsg *);
+int	 file_write_done(struct client_files *, struct imsg *);
 int	 file_read_data(struct client_files *, struct imsg *);
 int	 file_read_done(struct client_files *, struct imsg *);
 void	 file_read_cancel(struct client_files *, struct imsg *);