Add MPI.Dist_graph_neighbors

[fredrikekre]

This patch adds MPI.Dist_graph_neighbors as a convenience wrapper around MPI.Dist_graph_neighbors_count and MPI.Dist_graph_neighbors! that allocates the required result vectors automatically.

See #703 (comment).

[giordano]

Probably unrelated to this PR, there are a few weird errors with Julia nightly, for example:

┌ BFloat16s [ab4f0b2a-ad5b-11e8-123f-65d77653426b]
│  WARNING: could not import Printf.ini_hex into BFloat16s
│  WARNING: could not import Printf.ini_HEX into BFloat16s
┌ AMDGPU [21141c5a-9bdb-4563-92ae-f87d6854732e]
│  WARNING: could not import AMDGPU.device_libs_path into Compiler
┌ CUDA [052768ef-5323-5732-b1bb-66c8b64840ba]
│  WARNING: Wrapping `Vararg` directly in UnionAll is deprecated (wrap the tuple instead).
│  WARNING: Wrapping `Vararg` directly in UnionAll is deprecated (wrap the tuple instead).
└  
[...]
┌ Error: Pkg.precompile error
│   exception =
│    fatal error in type inference (type bound)
│    Stacktrace:
│     [1] get!(default::Pkg.API.var"#212#213"{UInt128, Base.PkgId, String, String}, h::Dict{Tuple{Base.PkgId, UInt128, String, String}, Union{Bool, Tuple{Vector{Any}, String}}}, key::Tuple{Base.PkgId, UInt128, String, String})
│       @ Base ./dict.jl:469
│     [2] _is_stale!(stale_cache::Dict{Tuple{Base.PkgId, UInt128, String, String}, Union{Bool, Tuple{Vector{Any}, String}}}, paths::Vector{String}, sourcepath::String)
│       @ Pkg.API ~/hostedtoolcache/julia/nightly/x64/share/julia/stdlib/v1.10/Pkg/src/API.jl:1051
│     [3] macro expansion
│       @ ~/hostedtoolcache/julia/nightly/x64/share/julia/stdlib/v1.10/Pkg/src/API.jl:1372 [inlined]
│     [4] (::Pkg.API.var"#235#264"{Bool, Pkg.Types.Context, Vector{Task}, Dict{Base.PkgId, String}, Pkg.API.var"#handle_interrupt#256"{Base.Event, ReentrantLock, Base.DevNull}, Pkg.API.var"#color_string#254", Base.Event, Base.Event, ReentrantLock, Vector{Base.PkgId}, Vector{Base.PkgId}, Dict{Base.PkgId, String}, Vector{Base.PkgId}, String, Vector{Base.PkgId}, Dict{Base.PkgId, Bool}, Dict{Base.PkgId, Base.Event}, Dict{Base.PkgId, Bool}, Vector{Pkg.Types.PackageSpec}, Dict{Base.PkgId, String}, Dict{Tuple{Base.PkgId, UInt128, String, String}, Union{Bool, Tuple{Vector{Any}, String}}}, Vector{Base.PkgId}, Bool, Base.DevNull, Base.Semaphore, String, Vector{String}, Vector{Base.PkgId}, Base.PkgId})()
│       @ Pkg.API ./task.jl:514
└ @ Pkg.API ~/hostedtoolcache/julia/nightly/x64/share/julia/stdlib/v1.10/Pkg/src/API.jl:1260

Edit: oh, and also a segmentation fault with MVAPICH which looks a lot like #684, but it hasn't crashed before here. We might need to apply the upstream patch...

[fredrikekre]

@KristofferC could that be related to your max_methods=1 PR in Pkg?

[KristofferC]

It could... The stacktrace is for files added in JuliaLang/Pkg.jl#3308 but not sure if it was the max_method thing that caused it to error or it already was an issue after that PR.

Edit: Actually, max_methods=1 change is not on master yet so can't be that.

[simonbyrne]

Thanks!