[syslinux] RFC: Feature: MENU HIDDEN behaviour on ANY key

[Dany St-Amant]

Hello,

Here's a small enhancement feature for menu.c32. Looking for feedback before starting any coding (not sure when I'll have time for implementation)

Regards,
Dany St-Amant


*Problem statement*

When MENU HIDDEN is used on an headless server, any noise on the serial console can interrupt the boot; it would be preferable if there only specific keys were allowed to interrupt, reducing the probability of unwanted interruption. Even with TOTALTIMEOUT present, there's a delay present as TOTALTIMEOUT has to be large enough to make it useable during intentional boot interruption.

*Basic behaviour change*

Provide a way to assign "reveal menu" to specific key(s)
Provide a default behaviour for unassigned keys (other than current "reveal menu")

*Possible user interface change*

MENU HIDDENMENUKEY key[,key]
MENU HIDDDENKEY key[,key] [command...]
Add ANY as an acceptable value for key
Make command optional, defaulting to boot default.

*Behaviour*

If pressed key is defined by HIDDENKEY, that action is always taken.
no HIDDENMENUKEY, no HIDDENKEY ANY: all keys bring the menu (current behaviour)
HIDDENMENUKEY ANY: all keys bring the menu (current behaviour)
HIDDENMENUKEY Esc, HIDDENKEY ANY (without commands): Esc brings the menu, all keys autoboot
HIDDENMENUKEY Esc, no HIDDENKEY ANY: Esc brings the menu, all keys trigger nothing.

*Exclusion*

No similar support for boot prompt.
No code/behaviour analysis made for cmenu.c32

*Affected code* (outside config parsing)

menumain.c:do_hidden_menu()
Extra comparison when handling mygetkey(),ignoring unbound key if ANY not assigned (or defaulted). Return NULL, if hide_key[key] matches HIDDENMENUKEY. A few extra globals likely required to track/reference pseudo-commands "reveal menu" and "boot default" inside hide_key[] table. Need to use/steal an unused entry of hide_key[] as the ANY key.

*Rejected/discarded interface option*

MENU HIDDENSECUREKEY key[,key] [STRICT]
MENU HIDDENDEFAULTKEY 0|1
MENU HIDDENANYKEY MENU|IGNORE|BOOT
MENU HIDDENKEY ANY|key[,key] MENU|IGNORE|BOOT|command...

Last one is rejected to avoid collision with exiting configuration which might be using label matching the keywords.