give finalizers their own RNG state (JuliaLang#45212)

fixes JuliaLang#42752

src/gc.c

@@ -375,6 +375,15 @@ static void jl_gc_run_finalizers_in_list(jl_task_t *ct, arraylist_t *list)
  ct->sticky = sticky;
 }
 
+static uint64_t finalizer_rngState[4];
+
+void jl_rng_split(uint64_t to[4], uint64_t from[4]);
+
+JL_DLLEXPORT void jl_gc_init_finalizer_rng_state(void)
+{
+ jl_rng_split(finalizer_rngState, jl_current_task->rngState);
+}
+
 static void run_finalizers(jl_task_t *ct)
 {
  // Racy fast path:
@@ -396,9 +405,16 @@ static void run_finalizers(jl_task_t *ct)
  }
  jl_atomic_store_relaxed(&jl_gc_have_pending_finalizers, 0);
  arraylist_new(&to_finalize, 0);
+
+ uint64_t save_rngState[4];
+ memcpy(&save_rngState[0], &ct->rngState[0], sizeof(save_rngState));
+ jl_rng_split(ct->rngState, finalizer_rngState);
+
  // This releases the finalizers lock.
  jl_gc_run_finalizers_in_list(ct, &copied_list);
  arraylist_free(&copied_list);
+
+ memcpy(&ct->rngState[0], &save_rngState[0], sizeof(save_rngState));
 }
 
 JL_DLLEXPORT void jl_gc_run_pending_finalizers(jl_task_t *ct)

src/task.c

@@ -729,7 +729,7 @@ uint64_t jl_genrandom(uint64_t rngState[4]) JL_NOTSAFEPOINT
  return res;
 }
 
-static void rng_split(jl_task_t *from, jl_task_t *to) JL_NOTSAFEPOINT
+void jl_rng_split(uint64_t to[4], uint64_t from[4]) JL_NOTSAFEPOINT
 {
  /* TODO: consider a less ad-hoc construction
  Ideally we could just use the output of the random stream to seed the initial
@@ -747,10 +747,10 @@ static void rng_split(jl_task_t *from, jl_task_t *to) JL_NOTSAFEPOINT
  0x3688cf5d48899fa7 == hash(UInt(3))|0x01
  0x867b4bb4c42e5661 == hash(UInt(4))|0x01
  */
- to->rngState[0] = 0x02011ce34bce797f * jl_genrandom(from->rngState);
- to->rngState[1] = 0x5a94851fb48a6e05 * jl_genrandom(from->rngState);
- to->rngState[2] = 0x3688cf5d48899fa7 * jl_genrandom(from->rngState);
- to->rngState[3] = 0x867b4bb4c42e5661 * jl_genrandom(from->rngState);
+ to[0] = 0x02011ce34bce797f * jl_genrandom(from);
+ to[1] = 0x5a94851fb48a6e05 * jl_genrandom(from);
+ to[2] = 0x3688cf5d48899fa7 * jl_genrandom(from);
+ to[3] = 0x867b4bb4c42e5661 * jl_genrandom(from);
 }
 
 JL_DLLEXPORT jl_task_t *jl_new_task(jl_function_t *start, jl_value_t *completion_future, size_t ssize)
@@ -790,7 +790,7 @@ JL_DLLEXPORT jl_task_t *jl_new_task(jl_function_t *start, jl_value_t *completion
  // Inherit logger state from parent task
  t->logstate = ct->logstate;
  // Fork task-local random state from parent
- rng_split(ct, t);
+ jl_rng_split(t->rngState, ct->rngState);
  // there is no active exception handler available on this stack yet
  t->eh = NULL;
  t->sticky = 1;

stdlib/Random/src/RNGs.jl

@@ -388,6 +388,7 @@ end
 
 function __init__()
  seed!(GLOBAL_RNG)
+ ccall(:jl_gc_init_finalizer_rng_state, Cvoid, ())
 end
 
 

stdlib/Random/test/runtests.jl

@@ -994,3 +994,26 @@ end
  @test minimum(m) >= 0.094
  @test maximum(m) <= 0.106
 end
+
+# issue #42752
+# test that running finalizers that launch tasks doesn't change RNG stream
+function f42752(do_gc::Bool, cell = (()->Any[[]])())
+ a = rand()
+ if do_gc
+ finalizer(cell[1]) do _
+ @async nothing
+ end
+ cell[1] = nothing
+ GC.gc()
+ end
+ b = rand()
+ (a, b)
+end
+guardseed() do
+ for _ in 1:4
+ Random.seed!(1)
+ val = f42752(false)
+ Random.seed!(1)
+ @test f42752(true) === val
+ end
+end