don't print task backtrace for GC threads (JuliaLang#51413) (#78)

GC threads don't have tasks associated with them.

Co-authored-by: Diogo Netto <61364108+d-netto@users.noreply.github.com>

src/stackwalk.c

@@ -1136,6 +1136,8 @@ JL_DLLEXPORT void jl_print_backtrace(void) JL_NOTSAFEPOINT
     jlbacktrace();
 }
 
+extern int gc_first_tid;
+
 // Print backtraces for all live tasks, for all threads.
 // WARNING: this is dangerous and can crash if used outside of gdb, if
 // all of Julia's threads are not stopped!
@@ -1144,6 +1146,10 @@ JL_DLLEXPORT void jl_print_task_backtraces(int show_done) JL_NOTSAFEPOINT
     size_t nthreads = jl_atomic_load_acquire(&jl_n_threads);
     jl_ptls_t *allstates = jl_atomic_load_relaxed(&jl_all_tls_states);
     for (size_t i = 0; i < nthreads; i++) {
+        // skip GC threads since they don't have tasks
+        if (gc_first_tid <= i && i < gc_first_tid + jl_n_gcthreads) {
+            continue;
+        }
         jl_ptls_t ptls2 = allstates[i];
         arraylist_t *live_tasks = &ptls2->heap.live_tasks;
         size_t n = live_tasks->len;