Implement a hook in base for disabling threading

of libraries, use it to decouple Distributed from LinearAlgebra.

base/initdefs.jl

@@ -236,3 +236,30 @@ function _atexit()
         end
     end
 end
+
+## hook for disabling threaded libraries ##
+
+library_threading_enabled = true
+const disable_library_threading_hooks = []
+
+function at_disable_library_threading(f)
+    push!(disable_library_threading_hooks, f)
+    if !library_threading_enabled
+        disable_library_threading()
+    end
+    return
+end
+
+function disable_library_threading()
+    global library_threading_enabled = false
+    while !isempty(disable_library_threading_hooks)
+        f = pop!(disable_library_threading_hooks)
+        try
+            f()
+        catch err
+            @warn("a hook from a library to disable threading failed:",
+                  exception = (err, catch_backtrace()))
+        end
+    end
+    return
+end

stdlib/Distributed/Project.toml

@@ -2,7 +2,6 @@ name = "Distributed"
 uuid = "8ba89e20-285c-5b6f-9357-94700520ee1b"
 
 [deps]
-LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"
 Serialization = "9e88b42a-f829-5b0c-bbe9-9e923198166b"
 Random = "9a3f8284-a2c9-5f02-9a11-845980a1fd5c"
 Sockets = "6462fe0b-24de-5631-8697-dd941f90decc"

stdlib/Distributed/src/cluster.jl

@@ -163,12 +163,6 @@ mutable struct LocalProcess
     LocalProcess() = new(1)
 end
 
-
-import LinearAlgebra
-function disable_threaded_libs()
-    LinearAlgebra.BLAS.set_num_threads(1)
-end
-
 worker_timeout() = parse(Float64, get(ENV, "JULIA_WORKER_TIMEOUT", "60.0"))
 
 

stdlib/Distributed/src/process_messages.jl

@@ -307,7 +307,7 @@ function handle_msg(msg::JoinPGRPMsg, header, r_stream, w_stream, version)
     topology(msg.topology)
 
     if !msg.enable_threaded_blas
-        disable_threaded_libs()
+        Base.disable_library_threading()
     end
 
     lazy = msg.lazy

stdlib/LinearAlgebra/src/LinearAlgebra.jl

@@ -437,6 +437,8 @@ function __init__()
         Base.showerror_nostdio(ex,
             "WARNING: Error during initialization of module LinearAlgebra")
     end
+    # register a hook to disable BLAS threading
+    Base.at_disable_library_threading(() -> BLAS.set_num_threads(1))
 end
 
 end # module LinearAlgebra