The problem is that the programmers there can't make a ratchet: a monotonously improving codebase where each commit is like the click of a ratchet. If you know your invariants hold then it is fairly easy to establish whether or not a change is an improvement.
Testcases can supply this function, as well as counters set up for that purpose (statsd or something to that effect).
Once you have enough of those you can (slowly) start to make changes to observe the effects, and once you understand the codebase add more invariants that you have now determined should exist.
The last project where I took that approach (about 4 years ago now) went from 'intractable' to 'stable' in a relatively short time but it required a lot of thinking and some really hard work to get it there.
Much better if your language/platform supports that sort of thing out of the box, even better if it can be done across subsystems.
Testcases can supply this function, as well as counters set up for that purpose (statsd or something to that effect).
Once you have enough of those you can (slowly) start to make changes to observe the effects, and once you understand the codebase add more invariants that you have now determined should exist.
The last project where I took that approach (about 4 years ago now) went from 'intractable' to 'stable' in a relatively short time but it required a lot of thinking and some really hard work to get it there.
Much better if your language/platform supports that sort of thing out of the box, even better if it can be done across subsystems.