The alternative would look something like

 function run_extension_callbacks(pkgid::PkgId)
+    distributed_id = PkgId(UUID((0x8ba89e20_285c_5b6f, 0x9357_94700520ee1b)), "
Distributed")
+    Distributed = get(loaded_modules, distributed_id, nothing)
+    if Distributed !== nothing && isdefined(Distributed, :myid)
+        id = @invokelatest Distributed.myid()
+        # Do not trigger extensions to load on workers, they will be loaded by the callback Distributed inserts
+        id != 1 && return
+    end

Thanks for this quick fix. The solution in the PR seems right to me because at least based on your description / my basic understanding (ie I havent actually tested), it will handle this correctly:

using Foo, Distributed
addprocs(2)
using Bar

which, based on current rules, should load Foo, Bar, and BarFooExt on master, and only Bar on workers. Conversely, letting master trigger the worker-loads I don't know if it could really handle that easily.

Shouldn't Base.toplevel_load[] be false already, since the extension is not the toplevel load?

I don't know the exact definition of that but adding

        @show Base.toplevel_load[]
        @show mod

to

shows:

Base.toplevel_load[] = true
mod = BarFooExt [4863ffa8-0e90-52b0-a292-a281b01911ec]

I think an extension loading like this is arguably a top level load, no; it doesn't get loaded into any other package. The "extension callback" here gets run after we have loaded all the triggers and at that point we are at toplevel again calling require_prelocked.

I'll go with this for now, if someone has a cleaner solution it can always be tweaked.