Set max_methods for lock and unlock to 1

[JamesWrigley]

This should prevent invalidations, particularly in IJulia:

 inserting lock(io::IJulia.IJuliaStdio) @ IJulia ~/git/IJulia.jl/src/stdio.jl:17 invalidated:
   backedges: 1: superseding lock(::IO) @ Base io.jl:33 with MethodInstance for lock(::IO) (150 children)

 inserting unlock(io::IJulia.IJuliaStdio) @ IJulia ~/git/IJulia.jl/src/stdio.jl:18 invalidated:
   backedges: 1: superseding unlock(::IO) @ Base io.jl:34 with MethodInstance for unlock(::IO) (2275 children)

See for example: JuliaLang/IJulia.jl#1192 (comment)

[nsajko]

xref PR #59377

[JamesWrigley]

(bump)

[nsajko]

What happens if a method of lock gets defined and a method instance gets compiled/inferred before you change max_methods here? Would it cause miscompilation?

[JamesWrigley]

From grepping the code and looking at the include order in Base.jl I think this is where the function is declared the first time, so that should not happen.

[nsajko]

What if some other PR switches the code around a bit, sometime in the future? Will we get miscompilation silently?

[KristofferC]

This seems like a separate concern from this PR since this code pattern is already used in other places already. Maybe move the discussion to a separate issue?

[nsajko]

I don't think it is a separate issue. The pattern of modifying max_methods for a function is only used for a few examples currently:

• copyto!

• merge

• error (PR merge the three methods of error into a single method #60878 attempts to prevent that)

So if it becomes common to modify max_methods for various functions all over the code base, I think then we would actually need to start worrying about stuff like this.

[KristofferC]

Yes, since it is already in the code base, a dedicated issue makes more sense rather than comments on a PR.

[JamesWrigley]

Any chance of this getting backported to 1.13? Would be really nice to have those invalidations fixed in IJulia.

[JamesWrigley]

I took the liberty of adding the backport label, but feel free to remove it if it's too big a change.