Ensure threaded VoA{SArray} regression test actually uses >=2 threads (#570)

The testset added in #564 for `@.. thread=true` on
`VectorOfArray{SArray}` only exercises the regression path when
`Threads.nthreads() >= 2`. FastBroadcast's Polyester-backed threaded
materialize splits work along the last axis via `view(dst, :, r)`; with
a single thread that split is a no-op, so the `setindex!` loop that
originally blew up on the immutable `SVector` element in issue #570 is
never reached. CI currently runs with the default one thread, so the
test has been silently passing without actually covering the regression.

Guard the testset: when running with >=2 threads, assert in-process as
before; otherwise spawn a child `julia --threads=2` process that repeats
the broadcast and fails loudly if the result diverges. This guarantees
the regression is caught regardless of how the outer test process was
launched.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ChrisRackauckas

test/interface_tests.jl

@@ -291,16 +291,37 @@ f3!(z, zz)
 @test z == VA[fill(4, SVector{2, Float64}), fill(2, SVector{2, Float64})]
 @test (@allocated f3!(z, zz)) == 0
 
-# Test threaded FastBroadcast with VectorOfArray of StaticArrays (issue #564)
+# Test threaded FastBroadcast with VectorOfArray of StaticArrays (issues #564, #570).
+# The regression path in #570 only triggers when Threads.nthreads() >= 2, because
+# FastBroadcast's Polyester-backed threaded materialize splits work along the last
+# axis via views — with a single thread that split is a no-op and the failing
+# setindex! path is never reached. If this test process was launched with only one
+# thread (as is currently the default in CI), re-run the testset in a child Julia
+# process with `--threads=2` so the multi-thread batch path is actually exercised.
 @testset "Threaded @.. with VectorOfArray{SArray}" begin
-    u_t = VectorOfArray(fill(SVector(1.0, 1.0), 2, 2))
-    v_t = copy(u_t)
-    @.. thread = true v_t = v_t + u_t
-    @test all(x -> x == SVector(2.0, 2.0), v_t.u)
-
-    # Test that repeated threaded application accumulates correctly
-    @.. thread = true v_t = v_t + u_t
-    @test all(x -> x == SVector(3.0, 3.0), v_t.u)
+    if Threads.nthreads() >= 2
+        u_t = VectorOfArray(fill(SVector(1.0, 1.0), 2, 2))
+        v_t = copy(u_t)
+        @.. thread = true v_t = v_t + u_t
+        @test all(x -> x == SVector(2.0, 2.0), v_t.u)
+
+        # Test that repeated threaded application accumulates correctly
+        @.. thread = true v_t = v_t + u_t
+        @test all(x -> x == SVector(3.0, 3.0), v_t.u)
+    else
+        script = """
+        using RecursiveArrayTools, StaticArrays, FastBroadcast, Polyester
+        Threads.nthreads() >= 2 || error("subprocess did not start with >= 2 threads")
+        u_t = VectorOfArray(fill(SVector(1.0, 1.0), 2, 2))
+        v_t = copy(u_t)
+        @.. thread = true v_t = v_t + u_t
+        all(x -> x == SVector(2.0, 2.0), v_t.u) || error("first threaded broadcast wrong: \$(v_t.u)")
+        @.. thread = true v_t = v_t + u_t
+        all(x -> x == SVector(3.0, 3.0), v_t.u) || error("second threaded broadcast wrong: \$(v_t.u)")
+        """
+        cmd = `$(Base.julia_cmd()) --threads=2 --startup-file=no --project=$(Base.active_project()) -e $script`
+        @test success(run(cmd))
+    end
 end
 
 # Test Polyester-based threaded FastBroadcast extension (issue #564)