Add a threadpool parameter to Channel constructor (#50858)

Without this, the task created by a `Channel` will run in the threadpool
of the creating task; in the REPL, this could be the interactive
threadpool.

On 1.8, without threadpools:
```julia
% julia +1.8 -t 8  
               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.8.5 (2023-01-08)
 _/ |\__'_|_|_|\__'_|  |  Official https://julialang.org/ release
|__/                   |

julia> for _ in 1:10
           Channel{Int}(1; spawn=true) do _
               Core.print("threadid=$(Threads.threadid())\n")
           end
       end

threadid=2
threadid=5
threadid=2
threadid=2
threadid=1
threadid=6
threadid=7
threadid=8
threadid=3
threadid=4
```

On 1.9, with no interactive threads:
```julia
% julia +1.9 -t 8
               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.9.2 (2023-07-05)
 _/ |\__'_|_|_|\__'_|  |  Official https://julialang.org/ release
|__/                   |

julia> for _ in 1:10
           Channel{Int}(1; spawn=true) do _
               Core.print("threadid=$(Threads.threadid())\n")
           end
       end

threadid=4
threadid=4
threadid=4
threadid=2
threadid=3
threadid=1
threadid=7
threadid=5
threadid=8
threadid=6
```
On 1.9, with an interactive thread:
```julia
% julia +1.9 -t 7,1
               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.9.2 (2023-07-05)
 _/ |\__'_|_|_|\__'_|  |  Official https://julialang.org/ release
|__/                   |

julia> for _ in 1:10
           Channel{Int}(1; spawn=true) do _
               Core.print("threadid=$(Threads.threadid())\n")
           end
       end
threadid=1
threadid=1
threadid=1
threadid=1
threadid=1
threadid=1
threadid=1
threadid=1
threadid=1
threadid=1
```

With this PR, the `:default` threadpool is used instead.
```julia
% julia +master -t7,1
               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.11.0-DEV.244 (2023-08-09)
 _/ |\__'_|_|_|\__'_|  |  Commit d99f249* (0 days old master)
|__/                   |

julia> for _ in 1:10
           Channel{Int}(1; spawn=true) do _
               Core.print("threadid=$(Threads.threadid())\n")
           end
       end

threadid=7
threadid=6
threadid=7
threadid=7
threadid=6
threadid=3
threadid=5
threadid=2
threadid=4
threadid=8
```
And, the behavior can be overridden.
```julia
% julia +master -t7,1
               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.11.0-DEV.244 (2023-08-09)
 _/ |\__'_|_|_|\__'_|  |  Commit d99f249* (0 days old master)
|__/                   |

julia> for _ in 1:10
           Channel{Int}(1; spawn=true, threadpool=:interactive) do _
               Core.print("threadid=$(Threads.threadid())\n")
           end
       end
threadid=1
threadid=1
threadid=1
threadid=1
threadid=1
threadid=1
threadid=1
threadid=1
threadid=1
threadid=1
```

---------

Co-authored-by: Nathan Daly <NHDaly@gmail.com>

base/channels.jl

@@ -59,7 +59,7 @@ Channel(sz=0) = Channel{Any}(sz)
 
 # special constructors
 """
-    Channel{T=Any}(func::Function, size=0; taskref=nothing, spawn=false)
+    Channel{T=Any}(func::Function, size=0; taskref=nothing, spawn=false, threadpool=nothing)
 
 Create a new task from `func`, bind it to a new channel of type
 `T` and size `size`, and schedule the task, all in a single call.
@@ -70,9 +70,14 @@ The channel is automatically closed when the task terminates.
 If you need a reference to the created task, pass a `Ref{Task}` object via
 the keyword argument `taskref`.
 
-If `spawn = true`, the Task created for `func` may be scheduled on another thread
+If `spawn=true`, the `Task` created for `func` may be scheduled on another thread
 in parallel, equivalent to creating a task via [`Threads.@spawn`](@ref).
 
+If `spawn=true` and the `threadpool` argument is not set, it defaults to `:default`.
+
+If the `threadpool` argument is set (to `:default` or `:interactive`), this implies
+that `spawn=true` and the new Task is spawned to the specified threadpool.
+
 Return a `Channel`.
 
 # Examples
@@ -117,6 +122,9 @@ true
     In earlier versions of Julia, Channel used keyword arguments to set `size` and `T`, but
     those constructors are deprecated.
 
+!!! compat "Julia 1.9"
+    The `threadpool=` argument was added in Julia 1.9.
+
 ```jldoctest
 julia> chnl = Channel{Char}(1, spawn=true) do ch
            for c in "hello world"
@@ -129,12 +137,18 @@ julia> String(collect(chnl))
 "hello world"
 ```
 """
-function Channel{T}(func::Function, size=0; taskref=nothing, spawn=false) where T
+function Channel{T}(func::Function, size=0; taskref=nothing, spawn=false, threadpool=nothing) where T
     chnl = Channel{T}(size)
     task = Task(() -> func(chnl))
+    if threadpool === nothing
+        threadpool = :default
+    else
+        spawn = true
+    end
     task.sticky = !spawn
     bind(chnl, task)
     if spawn
+        Threads._spawn_set_thrpool(task, threadpool)
         schedule(task) # start it on (potentially) another thread
     else
         yield(task) # immediately start it, yielding the current thread
@@ -149,17 +163,17 @@ Channel(func::Function, args...; kwargs...) = Channel{Any}(func, args...; kwargs
 # of course not deprecated.)
 # We use `nothing` default values to check which arguments were set in order to throw the
 # deprecation warning if users try to use `spawn=` with `ctype=` or `csize=`.
-function Channel(func::Function; ctype=nothing, csize=nothing, taskref=nothing, spawn=nothing)
+function Channel(func::Function; ctype=nothing, csize=nothing, taskref=nothing, spawn=nothing, threadpool=nothing)
     # The spawn= keyword argument was added in Julia v1.3, and cannot be used with the
     # deprecated keyword arguments `ctype=` or `csize=`.
-    if (ctype !== nothing || csize !== nothing) && spawn !== nothing
-        throw(ArgumentError("Cannot set `spawn=` in the deprecated constructor `Channel(f; ctype=Any, csize=0)`. Please use `Channel{T=Any}(f, size=0; taskref=nothing, spawn=false)` instead!"))
+    if (ctype !== nothing || csize !== nothing) && (spawn !== nothing || threadpool !== nothing)
+        throw(ArgumentError("Cannot set `spawn=` or `threadpool=` in the deprecated constructor `Channel(f; ctype=Any, csize=0)`. Please use `Channel{T=Any}(f, size=0; taskref=nothing, spawn=false, threadpool=nothing)` instead!"))
     end
     # Set the actual default values for the arguments.
     ctype === nothing && (ctype = Any)
     csize === nothing && (csize = 0)
     spawn === nothing && (spawn = false)
-    return Channel{ctype}(func, csize; taskref=taskref, spawn=spawn)
+    return Channel{ctype}(func, csize; taskref=taskref, spawn=spawn, threadpool=threadpool)
 end
 
 closed_exception() = InvalidStateException("Channel is closed.", :closed)

test/channel_threadpool.jl

@@ -0,0 +1,14 @@
+# This file is a part of Julia. License is MIT: https://julialang.org/license
+
+using Test
+using Base.Threads
+
+@testset "Task threadpools" begin
+    c = Channel{Symbol}() do c; put!(c, threadpool(current_task())); end
+    @test take!(c) === threadpool(current_task())
+    c = Channel{Symbol}(spawn = true) do c; put!(c, threadpool(current_task())); end
+    @test take!(c) === :default
+    c = Channel{Symbol}(threadpool = :interactive) do c; put!(c, threadpool(current_task())); end
+    @test take!(c) === :interactive
+    @test_throws ArgumentError Channel{Symbol}(threadpool = :foo) do c; put!(c, :foo); end
+end

test/channels.jl

@@ -107,6 +107,11 @@ end
     @test taskref[].sticky == false
     @test collect(c) == [0]
 end
+let cmd = `$(Base.julia_cmd()) --depwarn=error --rr-detach --startup-file=no channel_threadpool.jl`
+    new_env = copy(ENV)
+    new_env["JULIA_NUM_THREADS"] = "1,1"
+    run(pipeline(setenv(cmd, new_env), stdout = stdout, stderr = stderr))
+end
 
 @testset "multiple concurrent put!/take! on a channel for different sizes" begin
     function testcpt(sz)