Fix race condition in test introduced in #33119 (oops).

Introduce synchronization (via a `Channel()`) to force spawned tasks to
run after the local variables are updated, showcasing the problem and
the solution with `$`-interpolation.

test/threads_exec.jl

@@ -705,9 +705,10 @@ catch ex
 end
 
 @testset "@spawn interpolation" begin
- # Issue #30896: evaluating argumentss immediately
+ # Issue #30896: evaluating arguments immediately
  begin
  outs = zeros(5)
+ # Use interpolation to fill outs with the values of `i`
  @sync begin
  local i = 1
  while i <= 5
@@ -718,40 +719,50 @@ end
  @test outs == 1:5
  end
 
- # Args
+ # Test macro parsing for interpolating into Args
  @test fetch(Threads.@spawn 2+$2) == 4
  @test fetch(Threads.@spawn Int($(2.0))) == 2
  a = 2
  @test fetch(Threads.@spawn *($a,$a)) == a^2
- # kwargs
+ # Test macro parsing for interpolating into kwargs
  @test fetch(Threads.@spawn sort($([3 2; 1 0]), dims=2)) == [2 3; 0 1]
  @test fetch(Threads.@spawn sort([3 $2; 1 $0]; dims=$2)) == [2 3; 0 1]
 
- # Supports multiple levels of interpolation
- @test fetch(Threads.@spawn "$($a)") == "$a"
- let a = 1
- # Interpolate the current value of `a` vs the value of `a` in the closure
- t = Threads.@spawn :(+($$a, $a, a))
- a = 2 # update `a` after spawning
- @test fetch(t) == Expr(:call, :+, 1, 2, :a)
- end
-
- # Test the difference between different levels of interpolation
- let
- oneinterp = Vector{Any}(undef, 5)
- twointerps = Vector{Any}(undef, 5)
- @sync begin
- local i = 1
- while i <= 5
- Threads.@spawn setindex!(oneinterp, :($i), $i)
- Threads.@spawn setindex!(twointerps, :($($i)), $i)
- i += 1
+ # Test macro parsing supports multiple levels of interpolation
+ @testset "spawn macro multiple levels of interpolation" begin
+ # Use `ch` to synchronize within the tests to run after the local variables are
+ # updated, showcasing the problem and the solution.
+ ch = Channel() # (This synchronization fixes test failure reported in #34141.)
+
+ @test fetch(Threads.@spawn "$($a)") == "$a"
+ let a = 1
+ # Interpolate the current value of `a` vs the value of `a` in the closure
+ t = Threads.@spawn (take!(ch); :(+($$a, $a, a)))
+ a = 2 # update `a` after spawning, before `t` runs
+ put!(ch, nothing) # now run t
+ @test fetch(t) == Expr(:call, :+, 1, 2, :a)
+ end
+
+ # Test the difference between different levels of interpolation
+ # Without interpolation, each spawned task sees the last value of `i` (6);
+ # with interpolation, each spawned task has the value of `i` at time of `@spawn`.
+ let
+ oneinterp = Vector{Any}(undef, 5)
+ twointerps = Vector{Any}(undef, 5)
+ @sync begin
+ local i = 1
+ while i <= 5
+ Threads.@spawn (take!(ch); setindex!(oneinterp, :($i), $i))
+ Threads.@spawn (take!(ch); setindex!(twointerps, :($($i)), $i))
+ i += 1
+ end
+ for _ in 1:10; put!(ch, nothing); end # Now run all the tasks.
  end
+ # The first definition _didn't_ interpolate i
+ @test oneinterp == fill(6, 5)
+ # The second definition _did_ interpolate i
+ @test twointerps == 1:5
  end
- # The first definition _didn't_ escape i
- @test oneinterp == fill(6, 5)
- # The second definition _did_ escape i
- @test twointerps == 1:5
  end
 end
 
@@ -770,19 +781,3 @@ end
  @test fetch(@async :($($a))) == a
  @test fetch(@async "$($a)") == "$a"
 end
-
-# errors inside @threads
-function _atthreads_with_error(a, err)
- Threads.@threads for i in eachindex(a)
- if err
- error("failed")
- end
- a[i] = Threads.threadid()
- end
- a
-end
-@test_throws TaskFailedException _atthreads_with_error(zeros(nthreads()), true)
-let a = zeros(nthreads())
- _atthreads_with_error(a, false)
- @test a == [1:nthreads();]
-end