REPL: add lock to orchestrate adding repl modes etc.

[IanButterworth]

Fixes #54889
Fixes JuliaLang/Pkg.jl#3933

Pkg changes: JuliaLang/Pkg.jl@master...IanButterworth:Pkg.jl:ib/repl_mutate_lock

[vtjnash]

Taking a lock here does not help with the general unsoundness of this being written inside an __init__ method

[IanButterworth]

So you're saying there's no valid way to initialize the repl mode automatically when calling using Pkg?

[vtjnash]

Correct. Arbitrarily mutating some random global state during __init__ in the hopes that it was the correct local state to change is a bad idea

[IanButterworth]

Pkg aside, if we take a 3rd party package that adds a repl mode, what's the valid way for a user to initialize then?

startup.jl

import CustomREPLMode
CustomREPLMode.init()

and in the REPL itself

julia> import CustomREPLMode

julia> CustomREPLMode.init()

In particular, is the latter any better for knowing that the init is happening during the correct repl state?

I thought a lock was a logical fix here. Maybe there are more places in REPL that should take the lock.

cc. @KristofferC given discussion in JuliaLang/Pkg.jl#3933

[IanButterworth]

I do think the lock is an improvement here. Should we move forward with this, or is there some other automatic initializing method other than __init__ that you have in mind (for 1.11+) @vtjnash ?

[IanButterworth]

Bump