Ticket #4599: add support for bash PROMPT_COMMAND being an array

Starting with bash 5.1, PROMPT_COMMAND can be an array. Detect this
case and append an entry to the array instead of appending to a string.

Testing for bash >= 5 is sufficient, because the @A operator exists in
5.0. We need eval on the "then" branch because bash 1.x cannot even
parse that line (it does not support arrays).

Bug: https://bugs.gentoo.org/930401
Suggested-by: kfm@plushkava.net
Signed-off-by: Viorel Munteanu <ceamac@gentoo.org>
Signed-off-by: Yury V. Zaytsev <yury@shurup.com>

src/subshell/common.c

@@ -1100,9 +1100,13 @@ init_subshell_precmd (char *precmd, size_t buff_size)
                     " bind -x '\"\\e" SHELL_BUFFER_KEYBINDING "\":\"mc_print_command_buffer\"'\n"
                     " bind -x '\"\\e" SHELL_CURSOR_KEYBINDING
                     "\":\"echo $BASH_VERSINFO:$READLINE_POINT >&%d\"'\n"
-                    " PROMPT_COMMAND=${PROMPT_COMMAND:+$PROMPT_COMMAND\n}'pwd>&%d;kill -STOP $$'\n"
+                    " if test ${BASH_VERSION%%%%.*} -ge 5 && [[ ${PROMPT_COMMAND@a} == *a* ]] 2> /dev/null; then\n"
+                    "   eval \"PROMPT_COMMAND+=( 'pwd>&%d;kill -STOP $$' )\"\n"
+                    " else\n"
+                    "   PROMPT_COMMAND=${PROMPT_COMMAND:+$PROMPT_COMMAND\n}'pwd>&%d;kill -STOP $$'\n"
+                    " fi\n"
                     "PS1='\\u@\\h:\\w\\$ '\n", command_buffer_pipe[WRITE],
-                    command_buffer_pipe[WRITE], subshell_pipe[WRITE]);
+                    command_buffer_pipe[WRITE], subshell_pipe[WRITE], subshell_pipe[WRITE]);
         break;
 
     case SHELL_ASH_BUSYBOX: