Julia: Global variable not updated on remote workers in some cases

[marius311]

Looks like in some cases, if I switch a global variable, then switch it back, its value on remote workers when captured is only updated after the first switch. Here is a MWE (Julia 1.1.0):

julia> using Distributed

julia> addprocs(1)
1-element Array{Int64,1}:
 2

julia> x = :a
:a

julia> @fetch x
:a

julia> x = :b
:b

julia> @fetch x
:b

julia> x = :a
:a

julia> @fetch x
:b # <--- should be :a here

This doesn't always happen though. E.g. if x is instead an Int, it works as I would have expected from the docs:

julia> using Distributed

julia> addprocs(1)
1-element Array{Int64,1}:
 2

julia> x = 1
1

julia> @fetch x
1

julia> x = 2
2

julia> @fetch x
2

julia> x = 1
1

julia> @fetch x
1

Note sure what triggers the bug, if it even has anything to do with the type of x, although is seems reproducible every time. I've found that e.g. x::Int and x::Vector work fine, but x::Symbol and x::String do not.

[raminammour]

The logic here is wrong (you can redefine the function with @show s.glbs_sent to see what is going on):

https://github.com/JuliaLang/julia/blob/a1e41b96e71d36ec568440a8ad3430d862f363a4/stdlib/Distributed/src/clusterserialize.jl#L135

In the steps you show before (for a a non-bits type):

x=:a //objectid(x)=hash(:a)
s.globs_sent=Dict(hash(:a)=>hash(x,hash(:a))) //new oid, send x
x=:b //objectid(x)=hash(:b) 
s.globs_sent=Dict(hash(:a)=>hash(x,hash(:a)),hash(:b)=>hash(x,hash(:b))) //new oid, send x
x=:a
s.globs_sent=Dict(hash(:a)=>hash(x,hash(:a)),hash(:b)=>hash(x,hash(:b))) //key and value match from the first run, don't send x

This is a bug and will happen for many types (as long as you go back to a previous value for the same variable), and I am not sure this PR helps either :(

[amitmurthy]

The issue is here -

https://github.com/JuliaLang/julia/blob/a1e41b96e71d36ec568440a8ad3430d862f363a4/stdlib/Distributed/src/clusterserialize.jl#L18

I think glbs_sent::Dict{UInt64, UInt64} should be glbs_sent::Dict{Symbol, Tuple{UInt64, UInt64}} i.e. (key,value) -> (global_symbol,(objectid, hash_value))`.

I'll submit a PR in a couple of days.

[raminammour]

Yes, i figured that glbs_sent should be symbol => (oid,value), so the changing value can be overwritten. I was too scared to submit any PR since this is quite low level and I don't know what else relies on the ClusterSerializer struct.
Thanks for taking care of this!

[amitmurthy]

I was too scared to submit any PR since this is quite low level and I don't know what else relies on the ClusterSerializer struct.

Please don't be! The more contributors the better. Most of us have been new to the Julia codebase at one time.

[Keno]

The linked PR that was merged claims to have fixed this.