always run on original process stack in ALWAYS_COPY_STACKS mode (#31697)

this makes it possible to work around #31104

(cherry picked from commit 48634f9)

Author: JeffBezanson

src/task.c

@@ -74,6 +74,15 @@ static void jl_swap_fiber(jl_ucontext_t *lastt, jl_ucontext_t *t);
 static JL_THREAD unw_cursor_t jl_basecursor;
 #endif
 
+#ifdef ALWAYS_COPY_STACKS
+# ifndef COPY_STACKS
+# error "ALWAYS_COPY_STACKS requires COPY_STACKS"
+# endif
+static int always_copy_stacks = 1;
+#else
+static int always_copy_stacks = 0;
+#endif
+
 #ifdef COPY_STACKS
 
 static void memcpy_a16(uint64_t *to, uint64_t *from, size_t nb)
@@ -323,7 +332,10 @@ static void ctx_switch(jl_ptls_t ptls, jl_task_t **pt)
             jl_swap_fiber(lastt_ctx, &t->ctx);
     }
     else {
-        jl_start_fiber(lastt_ctx, &t->ctx);
+        if (always_copy_stacks)
+            jl_longjmp(ptls->base_ctx.uc_mcontext, 1);
+        else
+            jl_start_fiber(lastt_ctx, &t->ctx);
     }
     // TODO: mutex unlock the thread we just switched from
 #ifdef ENABLE_TIMINGS
@@ -459,12 +471,13 @@ JL_DLLEXPORT jl_task_t *jl_new_task(jl_function_t *start, jl_value_t *completion
     t->copy_stack = 0;
     if (ssize == 0) {
         // stack size unspecified; use default
-#if defined(COPY_STACKS) && defined(ALWAYS_COPY_STACKS)
-        t->copy_stack = 1;
-        t->bufsz = 0;
-#else
-        t->bufsz = JL_STACK_SIZE;
-#endif
+        if (always_copy_stacks) {
+            t->copy_stack = 1;
+            t->bufsz = 0;
+        }
+        else {
+            t->bufsz = JL_STACK_SIZE;
+        }
     }
     else {
         // user requested dedicated stack of a certain size
@@ -922,7 +935,6 @@ void jl_init_root_task(void *stack_lo, void *stack_hi)
     jl_ptls_t ptls = jl_get_ptls_states();
     ptls->current_task = (jl_task_t*)jl_gc_alloc(ptls, sizeof(jl_task_t),
                                                  jl_task_type);
-    ptls->current_task->copy_stack = 0;
     void *stack = stack_lo;
     size_t ssize = (char*)stack_hi - (char*)stack_lo;
 #ifndef _OS_WINDOWS_
@@ -931,8 +943,16 @@ void jl_init_root_task(void *stack_lo, void *stack_hi)
         ssize += ROOT_TASK_STACK_ADJUSTMENT; // sizeof stack is known exactly, but not where we are in that stack
     }
 #endif
-    ptls->current_task->stkbuf = stack;
-    ptls->current_task->bufsz = ssize;
+    if (always_copy_stacks) {
+        ptls->current_task->copy_stack = 1;
+        ptls->current_task->stkbuf = NULL;
+        ptls->current_task->bufsz = 0;
+    }
+    else {
+        ptls->current_task->copy_stack = 0;
+        ptls->current_task->stkbuf = stack;
+        ptls->current_task->bufsz = ssize;
+    }
     ptls->current_task->started = 1;
     ptls->current_task->next = jl_nothing;
     ptls->current_task->queue = jl_nothing;
@@ -955,7 +975,15 @@ void jl_init_root_task(void *stack_lo, void *stack_hi)
 
     ptls->root_task = ptls->current_task;
 
-    jl_init_basefiber(JL_STACK_SIZE);
+    if (always_copy_stacks) {
+        ptls->stackbase = stack_hi;
+        ptls->stacksize = ssize;
+        if (jl_setjmp(ptls->base_ctx.uc_mcontext, 0))
+            start_task();
+    }
+    else {
+        jl_init_basefiber(JL_STACK_SIZE);
+    }
 }
 
 JL_DLLEXPORT int jl_is_task_started(jl_task_t *t)