Export num_stack_mappings to track the number of in-flight stack mappings and tasks in application (JuliaLang#51301)

Author: NHDaly

src/gc-stacks.c

@@ -76,6 +76,10 @@ static void free_stack(void *stkbuf, size_t bufsz)
 }
 #endif
 
+JL_DLLEXPORT uint32_t jl_get_num_stack_mappings(void)
+{
+    return jl_atomic_load_relaxed(&num_stack_mappings);
+}
 
 const unsigned pool_sizes[] = {
     128 * 1024,

test/threads.jl

@@ -327,3 +327,9 @@ end
     @test_throws ArgumentError @macroexpand(@threads 1) # arg isn't an Expr
     @test_throws ArgumentError @macroexpand(@threads if true 1 end) # arg doesn't start with for
 end
+
+@testset "num_stack_mappings metric" begin
+    @test @ccall(jl_get_num_stack_mappings()::Cint) >= 1
+    # There must be at least two: one for the root test task and one for the async task:
+    @test fetch(@async(@ccall(jl_get_num_stack_mappings()::Cint))) >= 2
+end