give finalizers their own RNG state

fixes #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(jl_task_t *from, uint64_t *to);
+
+void jl_gc_init_finalizer_rng_state(void)
+{
+    jl_rng_split(jl_current_task, &finalizer_rngState[0]);
+}
+
 static void run_finalizers(jl_task_t *ct)
 {
     // Racy fast path:
@@ -396,9 +405,17 @@ 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));
+    memcpy(&ct->rngState[0], &finalizer_rngState[0], sizeof(save_rngState));
+
     // This releases the finalizers lock.
     jl_gc_run_finalizers_in_list(ct, &copied_list);
     arraylist_free(&copied_list);
+
+    memcpy(&finalizer_rngState[0], &ct->rngState[0], sizeof(save_rngState));
+    memcpy(&ct->rngState[0], &save_rngState[0], sizeof(save_rngState));
 }
 
 JL_DLLEXPORT void jl_gc_run_pending_finalizers(jl_task_t *ct)

src/init.c

@@ -753,6 +753,8 @@ static NOINLINE void _finish_julia_init(JL_IMAGE_SEARCH rel, jl_ptls_t ptls, jl_
         JL_GC_POP();
     }
 
+    jl_gc_init_finalizer_rng_state();
+
     if (jl_options.handle_signals == JL_OPTIONS_HANDLE_SIGNALS_ON)
         jl_install_sigint_handler();
 }

src/julia_internal.h

@@ -758,6 +758,7 @@ void jl_init_stack_limits(int ismaster, void **stack_hi, void **stack_lo);
 jl_task_t *jl_init_root_task(jl_ptls_t ptls, void *stack_lo, void *stack_hi);
 void jl_init_serializer(void);
 void jl_gc_init(void);
+void jl_gc_init_finalizer_rng_state(void);
 void jl_init_uv(void);
 void jl_init_thread_heap(jl_ptls_t ptls);
 void jl_init_int32_int64_cache(void);

src/task.c

@@ -730,7 +730,7 @@ 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(jl_task_t *from, uint64_t *to) JL_NOTSAFEPOINT
 {
     /* TODO: consider a less ad-hoc construction
        Ideally we could just use the output of the random stream to seed the initial
@@ -748,10 +748,10 @@ 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->rngState);
+    to[1] = 0x5a94851fb48a6e05 * jl_genrandom(from->rngState);
+    to[2] = 0x3688cf5d48899fa7 * jl_genrandom(from->rngState);
+    to[3] = 0x867b4bb4c42e5661 * jl_genrandom(from->rngState);
 }
 
 JL_DLLEXPORT jl_task_t *jl_new_task(jl_function_t *start, jl_value_t *completion_future, size_t ssize)
@@ -791,7 +791,7 @@ 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(ct, &t->rngState[0]);
     // there is no active exception handler available on this stack yet
     t->eh = NULL;
     t->sticky = 1;

stdlib/Random/test/runtests.jl

@@ -995,3 +995,26 @@ 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