backport 1.10: create phantom task for GC threads (#53815) (#54632)

A common idiom used throughout the codebase is to get a pointer to
thread-local-state through `jl_current_task->ptls`.

Create a phantom task for GC threads so that we can make use of this
idiom when running in the GC threads as well.

Idea originally suggested by @vchuravy, bugs are mine.

(cherry picked from commit 9636ef7)

Co-authored-by: Diogo Netto <[email protected]>
Co-authored-by: Max Horn <[email protected]>

src/partr.c

@@ -121,7 +121,11 @@ void jl_gc_mark_threadfun(void *arg)
 
  // initialize this thread (set tid and create heap)
  jl_ptls_t ptls = jl_init_threadtls(targ->tid);
-
+ void *stack_lo, *stack_hi;
+ jl_init_stack_limits(0, &stack_lo, &stack_hi);
+ // warning: this changes `jl_current_task`, so be careful not to call that from this function
+ jl_task_t *ct = jl_init_root_task(ptls, stack_lo, stack_hi);
+ JL_GC_PROMISE_ROOTED(ct);
  // wait for all threads
  jl_gc_state_set(ptls, JL_GC_STATE_WAITING, 0);
  uv_barrier_wait(targ->barrier);
@@ -146,7 +150,11 @@ void jl_gc_sweep_threadfun(void *arg)
 
  // initialize this thread (set tid and create heap)
  jl_ptls_t ptls = jl_init_threadtls(targ->tid);
-
+ void *stack_lo, *stack_hi;
+ jl_init_stack_limits(0, &stack_lo, &stack_hi);
+ // warning: this changes `jl_current_task`, so be careful not to call that from this function
+ jl_task_t *ct = jl_init_root_task(ptls, stack_lo, stack_hi);
+ JL_GC_PROMISE_ROOTED(ct);
  // wait for all threads
  jl_gc_state_set(ptls, JL_GC_STATE_WAITING, 0);
  uv_barrier_wait(targ->barrier);

src/task.c

@@ -1685,6 +1685,7 @@ jl_task_t *jl_init_root_task(jl_ptls_t ptls, void *stack_lo, void *stack_hi)
  JL_GC_PROMISE_ROOTED(ct);
  jl_set_pgcstack(&ct->gcstack);
  assert(jl_current_task == ct);
+ assert(jl_current_task->ptls == ptls);
 
 #ifdef _COMPILER_TSAN_ENABLED_
  ct->ctx.tsan_state = __tsan_get_current_fiber();