If we know the terminal outside tmux is not UTF-8, replace UTF-8 in

error messages and whatnot with underscores the same as we do when we
draw UTF-8 characters as part of the screen.

cmd-load-buffer.c

@@ -133,7 +133,7 @@ void
 cmd_load_buffer_callback(struct client *c, int closed, void *data)
 {
 	const char	*bufname = data;
-	char		*pdata, *cause;
+	char		*pdata, *cause, *saved;
 	size_t		 psize;
 
 	if (!closed)
@@ -154,6 +154,11 @@ cmd_load_buffer_callback(struct client *c, int closed, void *data)
 
 	if (paste_set(pdata, psize, bufname, &cause) != 0) {
 		/* No context so can't use server_client_msg_error. */
+		if (~c->flags & CLIENT_UTF8) {
+			saved = cause;
+			cause = utf8_sanitize(saved);
+			free(saved);
+		}
 		evbuffer_add_printf(c->stderr_data, "%s", cause);
 		server_push_stderr(c);
 		free(pdata);

cmd-queue.c

@@ -69,14 +69,21 @@ cmdq_print(struct cmd_q *cmdq, const char *fmt, ...)
 	struct client	*c = cmdq->client;
 	struct window	*w;
 	va_list		 ap;
+	char		*tmp, *msg;
 
 	va_start(ap, fmt);
 
 	if (c == NULL)
 		/* nothing */;
 	else if (c->session == NULL || (c->flags & CLIENT_CONTROL)) {
+		if (~c->flags & CLIENT_UTF8) {
+			vasprintf(&tmp, fmt, ap);
+			msg = utf8_sanitize(tmp);
+			free(tmp);
+			evbuffer_add(c->stdout_data, msg, strlen(msg));
+			free(msg);
+		} else
 			evbuffer_add_vprintf(c->stdout_data, fmt, ap);
-
 		evbuffer_add(c->stdout_data, "\n", 1);
 		server_push_stdout(c);
 	} else {
@@ -101,6 +108,7 @@ cmdq_error(struct cmd_q *cmdq, const char *fmt, ...)
 	va_list		 ap;
 	char		*msg;
 	size_t		 msglen;
+	char		*tmp;
 
 	va_start(ap, fmt);
 	msglen = xvasprintf(&msg, fmt, ap);
@@ -109,9 +117,14 @@ cmdq_error(struct cmd_q *cmdq, const char *fmt, ...)
 	if (c == NULL)
 		cfg_add_cause("%s:%u: %s", cmd->file, cmd->line, msg);
 	else if (c->session == NULL || (c->flags & CLIENT_CONTROL)) {
+		if (~c->flags & CLIENT_UTF8) {
+			tmp = msg;
+			msg = utf8_sanitize(tmp);
+			free(tmp);
+			msglen = strlen(msg);
+		}
 		evbuffer_add(c->stderr_data, msg, msglen);
 		evbuffer_add(c->stderr_data, "\n", 1);
-
 		server_push_stderr(c);
 		c->retval = 1;
 	} else {

tmux.h

@@ -2189,6 +2189,7 @@ u_int		 utf8_combine(const struct utf8_data *);
 int		 utf8_split(u_int, struct utf8_data *);
 u_int		 utf8_split2(u_int, u_char *);
 int		 utf8_strvis(char *, const char *, size_t, int);
+char		*utf8_sanitize(const char *);
 struct utf8_data *utf8_fromcstr(const char *);
 char		*utf8_tocstr(struct utf8_data *);
 u_int		 utf8_cstrwidth(const char *);

utf8.c

@@ -585,6 +585,50 @@ utf8_strvis(char *dst, const char *src, size_t len, int flag)
 	return (dst - start);
 }
 
+/*
+ * Sanitize a string, changing any UTF-8 characters to '_'. Caller should free
+ * the returned string. Anything not valid printable ASCII or UTF-8 is
+ * stripped.
+ */
+char *
+utf8_sanitize(const char *src)
+{
+	char			*dst;
+	size_t			 n;
+	int			 more;
+	struct utf8_data	 utf8data;
+	u_int			 i;
+
+	dst = NULL;
+
+	n = 0;
+	while (*src != '\0') {
+		dst = xreallocarray(dst, n + 1, sizeof *dst);
+		if (utf8_open(&utf8data, *src)) {
+			more = 1;
+			while (*++src != '\0' && more)
+				more = utf8_append(&utf8data, *src);
+			if (!more) {
+				dst = xreallocarray(dst, n + utf8data.width,
+				    sizeof *dst);
+				for (i = 0; i < utf8data.width; i++)
+					dst[n++] = '_';
+				continue;
+			}
+			src -= utf8data.have;
+		}
+		if (*src > 0x1f && *src < 0x7f)
+			dst[n] = *src;
+		src++;
+
+		n++;
+	}
+
+	dst = xreallocarray(dst, n + 1, sizeof *dst);
+	dst[n] = '\0';
+	return (dst);
+}
+
 /*
  * Convert a string into a buffer of UTF-8 characters. Terminated by size == 0.
  * Caller frees.