Add jl_active_task_stack to gcext API (#36823)

* Add jl_active_task_stack to gcext API

This is meant as a successor for `jl_task_stack_buffer`, which this
patch marks as deprecated in a comment.

* Let jl_active_task_stack also provide total stack range

This may be useful for debugging and other future applications, while
costing us little here; jl_active_task_stack is not supposed to be
used in hot paths either.

Author: fingolfin

src/julia_gcext.h

@@ -125,8 +125,18 @@ JL_DLLEXPORT jl_value_t *jl_gc_internal_obj_base_ptr(void *p);
 // the size of that stack buffer upon return. Also, if task is a thread's
 // current task, that thread's id will be stored in *tid; otherwise,
 // *tid will be set to -1.
+//
+// DEPRECATED: use jl_active_task_stack() instead.
 JL_DLLEXPORT void *jl_task_stack_buffer(jl_task_t *task, size_t *size, int *tid);
 
+// Query the active and total stack range for the given task, and set
+// *active_start and *active_end respectively *total_start and *total_end
+// accordingly. The range for the active part is a best-effort approximation
+// and may not be tight.
+JL_DLLEXPORT void jl_active_task_stack(jl_task_t *task,
+                                       char **active_start, char **active_end,
+                                       char **total_start, char **total_end);
+
 #ifdef __cplusplus
 }
 #endif

src/task.c

@@ -247,6 +247,57 @@ JL_DLLEXPORT void *jl_task_stack_buffer(jl_task_t *task, size_t *size, int *tid)
     return (void *)((char *)task->stkbuf + off);
 }
 
+JL_DLLEXPORT void jl_active_task_stack(jl_task_t *task,
+                                       char **active_start, char **active_end,
+                                       char **total_start, char **total_end)
+{
+    if (!task->started) {
+        *total_start = *active_start = 0;
+        *total_end = *active_end = 0;
+        return;
+    }
+
+    int16_t tid = task->tid;
+    jl_ptls_t ptls2 = (tid != -1) ? jl_all_tls_states[tid] : 0;
+
+    if (task->copy_stack && ptls2 && task == ptls2->current_task) {
+        *total_start = *active_start = (char*)ptls2->stackbase - ptls2->stacksize;
+        *total_end = *active_end = (char*)ptls2->stackbase;
+    }
+    else if (task->stkbuf) {
+        *total_start = *active_start = (char*)task->stkbuf;
+#ifndef _OS_WINDOWS_
+        if (jl_all_tls_states[0]->root_task == task) {
+            // See jl_init_root_task(). The root task of the main thread
+            // has its buffer enlarged by an artificial 3000000 bytes, but
+            // that means that the start of the buffer usually points to
+            // inaccessible memory. We need to correct for this.
+            *active_start += ROOT_TASK_STACK_ADJUSTMENT;
+            *total_start += ROOT_TASK_STACK_ADJUSTMENT;
+        }
+#endif
+
+        *total_end = *active_end = (char*)task->stkbuf + task->bufsz;
+#ifdef COPY_STACKS
+        // save_stack stores the stack of an inactive task in stkbuf, and the
+        // actual number of used bytes in copy_stack.
+        if (task->copy_stack > 1)
+            *active_end = (char*)task->stkbuf + task->copy_stack;
+#endif
+    }
+    else {
+        // no stack allocated yet
+        *total_start = *active_start = 0;
+        *total_end = *active_end = 0;
+        return;
+    }
+
+    if (task == jl_current_task) {
+        // scan up to current `sp` for current thread and task
+        *active_start = (char*)jl_get_frame_addr();
+    }
+}
+
 // Marked noinline so we can consistently skip the associated frame.
 // `skip` is number of additional frames to skip.
 NOINLINE static void record_backtrace(jl_ptls_t ptls, int skip) JL_NOTSAFEPOINT

test/gcext/gcext.c

@@ -478,37 +478,29 @@ static int stack_grows_down(void) {
 
 void task_scanner(jl_task_t *task, int root_task)
 {
+    int var_on_frame;
+
     // The task scanner is not necessary for liveness, as the
     // corresponding task stack is already part of the stack.
     // Its purpose is simply to test that the task scanner
     // doing actual work does not trigger a problem.
-    size_t size;
-    int tid;
-    void *stack = jl_task_stack_buffer(task, &size, &tid);
-    if (tid >= 0) {
-        // this is the live stack of a thread. Is it ours?
-        if (stack && tid == jl_threadid()) {
-            // only scan the live portion of the stack.
-            char *end_stack = (char *) stack + size;
-            if (lt_ptr(stack, &size) && lt_ptr(&size, (char *)stack + size)) {
-                if (stack_grows_down()) {
-                    size = end_stack - (char *)&size;
-                    stack = (void *)&size;
-                }
-                else {
-                    size = (char *) end_stack - (char *) &size;
-                }
-            } else {
-                // error, current stack frame must be on the live stack.
-                jl_error("stack frame not part of the current task");
-            }
+    char *start_stack;
+    char *end_stack;
+    char *total_start_stack;
+    char *total_end_stack;
+    jl_active_task_stack(task, &start_stack, &end_stack, &total_start_stack, &total_end_stack);
+
+    // this is the live stack of a thread. Is it ours?
+    if (start_stack && task == (jl_task_t *)jl_get_current_task()) {
+        if (!(lt_ptr(start_stack, &var_on_frame) && lt_ptr(&var_on_frame, end_stack))) {
+            // error, current stack frame must be on the live stack.
+            jl_error("stack frame not part of the current task");
         }
-        else
-            stack = NULL;
     }
-    if (stack) {
-        void **start = (void **) stack;
-        void **end = start + size / sizeof(void *);
+
+    if (start_stack) {
+        void **start = (void **)start_stack;
+        void **end = (void **)end_stack;
         while (start < end) {
             void *p = *start++;
             void *q = jl_gc_internal_obj_base_ptr(p);