Disable GC during thread adoption.

Otherwise GC might run after the foreign thread exits the spin loop,
but before it is initialized and ready to handle GC safepoints.

src/gc.c

@@ -3068,7 +3068,7 @@ static void sweep_finalizer_list(arraylist_t *list)
 }
 
 // collector entry point and control
-static _Atomic(uint32_t) jl_gc_disable_counter = 1;
+_Atomic(uint32_t) jl_gc_disable_counter = 1;
 
 JL_DLLEXPORT int jl_gc_enable(int on)
 {

src/julia_internal.h

@@ -883,6 +883,7 @@ STATIC_INLINE int jl_addr_is_safepoint(uintptr_t addr)
     return addr >= safepoint_addr && addr < safepoint_addr + jl_page_size * 3;
 }
 extern _Atomic(uint32_t) jl_gc_running;
+extern _Atomic(uint32_t) jl_gc_disable_counter;
 // All the functions are safe to be called from within a signal handler
 // provided that the thread will not be interrupted by another asynchronous
 // signal.

src/threading.c

@@ -414,6 +414,7 @@ JL_DLLEXPORT jl_gcframe_t **jl_adopt_thread(void) JL_NOTSAFEPOINT_LEAVE
     while (jl_atomic_load_relaxed(&jl_gc_running) || jl_atomic_load_acquire(&jl_gc_running)) {
         jl_cpu_pause();
     }
+    jl_atomic_fetch_add(&jl_gc_disable_counter, 1);
 
     // initialize this thread (assign tid, create heap, set up root task)
     jl_ptls_t ptls = jl_init_threadtls(-1);
@@ -424,6 +425,7 @@ JL_DLLEXPORT jl_gcframe_t **jl_adopt_thread(void) JL_NOTSAFEPOINT_LEAVE
     jl_task_t *ct = jl_init_root_task(ptls, stack_lo, stack_hi);
     JL_GC_PROMISE_ROOTED(ct);
     uv_random(NULL, NULL, &ct->rngState, sizeof(ct->rngState), 0, NULL);
+    jl_atomic_fetch_add(&jl_gc_disable_counter, -1);
     return &ct->gcstack;
 }