Ensure jl_get_excstack can't read from concurrently running tasks

This might not be a major problem right now, but could become a source
of crashes once PARTR lands.

Author: c42f

src/julia_internal.h

@@ -1011,6 +1011,7 @@ extern jl_sym_t *macrocall_sym;  extern jl_sym_t *colon_sym;
 extern jl_sym_t *hygienicscope_sym; extern jl_sym_t *escape_sym;
 extern jl_sym_t *gc_preserve_begin_sym; extern jl_sym_t *gc_preserve_end_sym;
 extern jl_sym_t *throw_undef_if_not_sym; extern jl_sym_t *getfield_undefref_sym;
+extern jl_sym_t *failed_sym; extern jl_sym_t *done_sym; extern jl_sym_t *runnable_sym;
 
 struct _jl_sysimg_fptrs_t;
 

src/stackwalk.c

@@ -181,15 +181,21 @@ JL_DLLEXPORT void jl_get_backtrace(jl_array_t **btout, jl_array_t **bt2out)
 // with the top of the stack and returning up to `max_entries`. If requested by
 // setting the `include_bt` flag, backtrace data in bt,bt2 format is
 // interleaved.
-JL_DLLEXPORT jl_value_t *jl_get_excstack(jl_value_t* task, int include_bt, int max_entries)
+JL_DLLEXPORT jl_value_t *jl_get_excstack(jl_task_t* task, int include_bt, int max_entries)
 {
-    JL_TYPECHK(catch_stack, task, task);
+    JL_TYPECHK(catch_stack, task, (jl_value_t*)task);
+    jl_ptls_t ptls = jl_get_ptls_states();
+    if (task != ptls->current_task &&
+        task->state != failed_sym && task->state != done_sym) {
+        jl_error("Inspecting the exception stack of a task which might "
+                 "be running concurrently isn't allowed.");
+    }
     jl_array_t *stack = NULL;
     jl_array_t *bt = NULL;
     jl_array_t *bt2 = NULL;
     JL_GC_PUSH3(&stack, &bt, &bt2);
     stack = jl_alloc_array_1d(jl_array_any_type, 0);
-    jl_excstack_t *excstack = ((jl_task_t*)task)->excstack;
+    jl_excstack_t *excstack = task->excstack;
     size_t itr = excstack ? excstack->top : 0;
     int i = 0;
     while (itr > 0 && i < max_entries) {

src/task.c

@@ -59,9 +59,9 @@ volatile int jl_in_stackwalk = 0;
 
 #define ROOT_TASK_STACK_ADJUSTMENT 3000000
 
-static jl_sym_t *done_sym;
-static jl_sym_t *failed_sym;
-static jl_sym_t *runnable_sym;
+jl_sym_t *done_sym;
+jl_sym_t *failed_sym;
+jl_sym_t *runnable_sym;
 
 extern size_t jl_page_size;
 jl_datatype_t *jl_task_type;

test/exceptions.jl

@@ -247,6 +247,10 @@ end
     end == ErrorException("expected")
     @test length(catch_stack(t)) == 1
     @test length(catch_stack(t)[1][2]) > 0 # backtrace is nonempty
+    # Exception stacks should not be accessed on concurrently running tasks
+    t = @task ()->nothing
+    @test_throws ErrorException("Inspecting the exception stack of a task which might "*
+                                "be running concurrently isn't allowed.") catch_stack(t)
 end
 
 @testset "rethrow" begin