Preferences and transitive dependencies

[vchuravy]

It seems to me that LocalPreferences.toml don't have any effect if the package I am setting the dependency in isn't listed as a direct dependency.

I prepared a small example: prefs.tar.gz

• A is the package using Preferences.jl to manage a "backend"
• B is using A
• C is using B

vchuravy@odin ~/s/prefs> cat B/LocalPreferences.toml 
[A]
backend = "CUDA"
vchuravy@odin ~/s/prefs> cat C/LocalPreferences.toml
[A]
backend = "CUDA"
vchuravy@odin ~/s/prefs> julia --project=B -e "using B; @show B.active_backend()"
B.active_backend() = "CUDA"
vchuravy@odin ~/s/prefs> julia --project=C -e "using B; @show B.active_backend()"
B.active_backend() = "OpenCL"

Both have the same LocalPreferences.toml

After adding A to C as a dependency things seem to work. But this is problematic for several use-cases of Preferences. I found this the first time when I wanted to change the path of a library in a jll package.

cc: @staticfloat @simonbyrne

[vchuravy]

From JuliaLang/julia#37791 (comment)

but the basic idea would be that if Flux attempts to set a preference in NNlib, today this would cause NNlib to get listed as an extra in the top-level Project.toml

vchuravy@odin ~/s/prefs> cat C/Project.toml 
name = "C"
uuid = "df71c7c7-715d-4a3c-a5e1-ea880813ce77"
authors = ["Valentin Churavy <[email protected]>"]
version = "0.1.0"

[deps]
B = "8913e2bf-78c8-4bed-b0db-99fb783f5b1d"

[extras]
A = "eadbc8b5-6f2c-475d-9756-0c67454fc69b"

vchuravy@odin ~/s/prefs> cat C/LocalPreferences.toml 
[A]
backend = "CUDA"

vchuravy@odin ~/s/prefs> julia --project=C -e "using B; @show B.active_backend()"
B.active_backend() = "OpenCL"

So it did indeed get listed in [extras].