protect system tables from (accidental) overwrite

[pq]

follow-up from (#559), write-protect specific system tables:

• engine Accidentally setting engine method breaks engine until matron restart #656, Added a __newindex method to protect Engine #657

...

/cc @tehn

[tehn]

@pq (i know you're away, so no rush!) wondering how to best approach this with other tables

ie, main user error that will happen is something like:

metro = 5

which plows the metro lib. but of course this gets reset on script reload, so maybe this isn't an issue beyond throwing a confusing error to the user.

however

norns = "rad"

will plow the script restart function.

probably not worth the effort to protect the depths of the table, but maybe worth protecting (with a nice error message) just the top level table for various things that may be accidentally overwritten.

what do you think?

[pq]

i'm all for write-protecting common top-level tables (at least). sub-tables should be just as easy and probably worth guarding a few too.

have we seen any issues w/ this approach? (sorry: can't recall!)

[tehn]

main issue with protecting some subtables is if they have elements that are supposed to be writable by design (i realize this is probably bad style)--- i'd need to go through and identify these.

perhaps we just make another function that read-only's the top-level table?

[pq]

main issue with protecting some subtables is if they have elements that are supposed to be writable by design (i realize this is probably bad style)--- i'd need to go through and identify these.

it looks like we anticipated this 🎉 and there's actually precedent w/ engine.name

norns/lua/core/startup.lua

Line 21 in 96311a8

engine = tab.readonly{table = require 'core/engine', except = {'name'}}

exception logic is here:

norns/lua/lib/tabutil.lua

Lines 206 to 210 in 96311a8

	if (tab.contains(exceptions, k)) then
	t[k] = v
	else
	error("'"..k.."', a read-only key, cannot be re-assigned.")
	end

perhaps we just make another function that read-only's the top-level table?

sure. or if the list of exceptions is short, maybe just explicitly list them like we did for engine?

happy to help with either approach. 👍