Allow access to exception stack of a failed task

base/error.jl

@@ -92,16 +92,20 @@ function catch_backtrace()
 end
 
 """
-    catch_stack(; [inclue_bt=true])
+    catch_stack(task=current_task(); [inclue_bt=true])
 
 Get the stack of exceptions currently being handled. For nested catch blocks
-there may be more than one current exception in which case the most recent
-exception is last in the stack. The stack is returned as a Vector of
-`(exception,backtrace)` pairs, or a Vector of exceptions only if `include_bt`
-is false.
+there may be more than one current exception in which case the most recently
+thrown exception is last in the stack. The stack is returned as a Vector of
+`(exception,backtrace)` pairs, or a Vector of exceptions if `include_bt` is
+false.
+
+Explicitly passing `task` will return the current exception stack on an
+arbitrary task. This is useful for inspecting tasks which have failed due to
+uncaught exceptions.
 """
-function catch_stack(; include_bt=true)
-    raw = ccall(:jl_get_exc_stack, Any, (Cint,Cint), include_bt, typemax(Cint))
+function catch_stack(task=current_task(); include_bt=true)
+    raw = ccall(:jl_get_exc_stack, Any, (Any,Cint,Cint), task, include_bt, typemax(Cint))
     formatted = Any[]
     stride = include_bt ? 3 : 1
     for i = reverse(1:stride:length(raw))

src/stackwalk.c

@@ -177,17 +177,20 @@ JL_DLLEXPORT void jl_get_backtrace(jl_array_t **btout, jl_array_t **bt2out)
     decode_backtrace(bt_data, bt_size, btout, bt2out);
 }
 
-// Return data from the exception stack as an array of Any, starting 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_exc_stack(int include_bt, int max_entries)
+// Return data from the exception stack for `task` as an array of Any, starting
+// 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_exc_stack(jl_value_t* task, int include_bt, int max_entries)
 {
     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_exc_stack_t *s = jl_get_ptls_states()->current_task->exc_stack;
+    if (!jl_typeis(task, jl_task_type))
+        jl_error("Cannot get exception stack from a non-Task type");
+    jl_exc_stack_t *s = ((jl_task_t*)task)->exc_stack;
     if (!s)
         return (jl_value_t*)stack;
     size_t itr = s->top;

test/exceptions.jl

@@ -111,7 +111,7 @@ end
     @test length(catch_stack()) == 0
 end
 
-@testset "Exception stacks and Task switching" begin
+@testset "Exception stacks and Tasks" begin
     # See #12485
     try
         error("A")
@@ -174,6 +174,16 @@ end
         @test bt == catch_backtrace()
     end
     @test length(catch_stack()) == 0
+    # Exception stacks on other tasks
+    t = @task try
+        error("A")
+    catch
+        error("B")
+    end
+    yield(t)
+    @test t.state == :failed
+    @test t.result == ErrorException("B")
+    @test catch_stack(t, include_bt=false) == [ErrorException("A"), ErrorException("B")]
 end
 
 @testset "rethrow" begin