mux: fix path components mutation

[jonathaningram]

References to other Issues or PRs

Fixes #2990

Have you read the Contributing Guidelines?

Yes

Brief description of what is fixed or changed

There's a for loop which looks at handlers that might match. However there's a case where the path components slice was mutated if idx > 0. Subsequent iterations of the for loop might end up with idx <= 0 and they'll be using a version of the path components that were mutated from the previous iteration. Instead, we keep a copy of the path components at the top, and inside each iteration we make a copy, then modify that.

Other comments

[jonathaningram]

@cclien0725 @johanbrandhorst I believe this is the fix we need. If it looks good, you can just squash and merge it.

[johanbrandhorst]

LGTM, though this code is so hairy I'm really pleased we have it well covered by the tests 😅

[johanbrandhorst]

Thanks for your contribution! @cclien0725 could you confirm that this fixes your issue?

[jonathaningram]

@johanbrandhorst yeah it's all a bit hard to understand. Not claiming I have an obvious better way of writing it without looking into it more, but agree that it's hairy.

Interestingly, by changing the code to NOT mutate data, we avoided a bug. Speaks to benefits of immutable data.

[cclien0725]

@jonathaningram @johanbrandhorst thanks for both of your help, this fixes the issue!