shell_write_cb: Schedule error message. (#5670)

Closes #5558

src/nvim/message.c

@@ -19,6 +19,7 @@
 #include "nvim/fileio.h"
 #include "nvim/func_attr.h"
 #include "nvim/getchar.h"
+#include "nvim/main.h"
 #include "nvim/mbyte.h"
 #include "nvim/memory.h"
 #include "nvim/misc1.h"
@@ -581,6 +582,24 @@ bool emsgf(const char *const fmt, ...)
   return emsg(IObuff);
 }
 
+static void msg_emsgf_event(void **argv)
+{
+  char *s = argv[0];
+  (void)emsg((char_u *)s);
+  xfree(s);
+}
+
+void msg_schedule_emsgf(const char *const fmt, ...)
+{
+  va_list ap;
+  va_start(ap, fmt);
+  vim_vsnprintf((char *)IObuff, IOSIZE, fmt, ap, NULL);
+  va_end(ap);
+
+  char *s = xstrdup((char *)IObuff);
+  loop_schedule(&main_loop, event_create(1, msg_emsgf_event, 1, s));
+}
+
 /*
  * Like msg(), but truncate to a single line if p_shm contains 't', or when
  * "force" is TRUE.  This truncates in another way as for normal messages.

src/nvim/os/shell.c

@@ -548,8 +548,8 @@ static void shell_write_cb(Stream *stream, void *data, int status)
   if (status) {
     // Can happen if system() tries to send input to a shell command that was
     // backgrounded (:call system("cat - &", "foo")). #3529 #5241
-    EMSG2(_("E5677: Error writing input to shell-command: %s"),
-          uv_err_name(status));
+    msg_schedule_emsgf(_("E5677: Error writing input to shell-command: %s"),
+                       uv_err_name(status));
   }
   if (stream->closed) {  // Process may have exited before this write.
     ELOG("stream was already closed");

src/nvim/ui.c

@@ -177,14 +177,14 @@ void ui_refresh(void)
   pum_set_external(pum_external);
 }
 
-static void ui_refresh_handler(void **argv)
+static void ui_refresh_event(void **argv)
 {
   ui_refresh();
 }
 
 void ui_schedule_refresh(void)
 {
-  loop_schedule(&main_loop, event_create(1, ui_refresh_handler, 0));
+  loop_schedule(&main_loop, event_create(1, ui_refresh_event, 0));
 }
 
 void ui_resize(int new_width, int new_height)