Starting with the very specific: I do not think it was an accident that the xz backdoor's exploit chain started with a modified version of a third party .m4 file to be compiled into xz's configure script.

It's possible to write incomprehensible, underhanded code in any programming language. There's competitions for it, even. But when you have a programming language, or perhaps a mashup of two languages, that everyone *expects* not to be able to understand — no matter how careful the author is — well, then you have what we might call an attractive nuisance. And when blobs of code in that language are passed around in copy-and-paste fashion without much review or testing or version control, that makes it an even easier target.

So, in my capacity as one of the last few people still keeping autoconf limping along, I'm thinking pretty hard about what could be done to replace its implementation language, and concurrently what could be done to improve development practice for both autoconf and its extensions (the macro archive, gnulib, etc.)

On the subject of implementation language, I have one half-baked idea and one castle in the air.

The half-baked idea is: Suppose ./configure continues to be a shell script, but it ceases to be a *generated* shell script. No more M4. Similarly, the Makefile continues to be a Makefile but it ceases to be generated from Makefile.am. Instead, there is a large library of shell functions and a somewhat smaller library of Make rules that you include and then use.

For ./configure I'm fairly confident it would be possible to do this and remain compatible with POSIX.1-2001 "shell and utilities". (Little known fact: for a long time now, autoconf scripts *do* use shell functions! Internally, wrapped in multiple layers of M4 goo, but still — we haven't insisted on backcompat all the way to System V sh in a long, long time.) For Makefiles I believe it would be necessary to insist on GNU Make.

This would definitely be an improvement on the status quo, but would it be *enough* of one? And would it be less work than migration to something else? (It would be a compatibility break and it would *not* be possible to automate the conversion. Lots of work for everyone no matter what.)

Suppose that's not good enough. Bourne shell is still a shitty programming language, and in particular it is really dang hard to read, especially if you're worried about malicious insiders. Which we are.

Now we have another problem. The #1 selling point for autotools vs all other build orchestrators is "no build dependencies if you're working from tarballs," and the only reason that works is you can count on /bin/sh to exist on anything that purports to be Unix. If we want to stop using /bin/sh, we're going to have to make people install something else first, and that something else needs to be a small and stable Twinkie. Python need not apply (sorry, Meson).

What's small and stable enough? Lua is already too large, and at the same time, too limited.

There's one language that's famous for being tiny, flexible, and pleasantly readable once you wrap your head around it: Forth.

If I had investments to live off, I would be sorely tempted to take the next year or so and write my own Forth that was also a shell language and a build orchestrator, and then have a look at rewriting Autoconf in *that.* This is the castle in the air.

@zwol In #bootstrapping circles, we have GNU Mes and Gash (the combination of which is good enough to run ./configure scripts).

The Racket folks have switched to Zuo as their build system, also based on a minimal Scheme implementation.

Maybe not a universal option, but I can imagine a build system based on Mes/Zuo, at least in the circles I care about.