fix #32970, at-threads disabled after a loop errors (#33034)

(cherry picked from commit 5c42f10)

base/threadingconstructs.jl

@@ -18,9 +18,6 @@ on `threadid()`.
 """
 nthreads() = Int(unsafe_load(cglobal(:jl_n_threads, Cint)))
 
-# Only read/written by the main thread
-const in_threaded_loop = Ref(false)
-
 function _threadsfor(iter,lbody)
     lidx = iter.args[1]         # index
     range = iter.args[2]
@@ -65,15 +62,11 @@ function _threadsfor(iter,lbody)
             end
         end
         end
-        # Hack to make nested threaded loops kinda work
-        if threadid() != 1 || in_threaded_loop[]
-            # We are in a nested threaded loop
+        if threadid() != 1
+            # only thread 1 can enter/exit _threadedregion
             Base.invokelatest(threadsfor_fun, true)
         else
-            in_threaded_loop[] = true
-            # the ccall is not expected to throw
             ccall(:jl_threading_run, Cvoid, (Any,), threadsfor_fun)
-            in_threaded_loop[] = false
         end
         nothing
     end

test/threads_exec.jl

@@ -670,3 +670,19 @@ let ch = Channel{Char}(0), t
     schedule(t)
     @test String(collect(ch)) == "hello"
 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