fix #41347, better checks for yield and yieldto with finished tasks (#41410)

(cherry picked from commit e2d5f52)

Author: JeffBezanson

base/task.jl

@@ -721,6 +721,7 @@ A fast, unfair-scheduling version of `schedule(t, arg); yield()` which
 immediately yields to `t` before calling the scheduler.
 """
 function yield(t::Task, @nospecialize(x=nothing))
+    (t._state === task_state_runnable && t.queue === nothing) || error("yield: Task not runnable")
     t.result = x
     enq_work(current_task())
     set_next_task(t)
@@ -736,6 +737,13 @@ call to `yieldto`. This is a low-level call that only switches tasks, not consid
 or scheduling in any way. Its use is discouraged.
 """
 function yieldto(t::Task, @nospecialize(x=nothing))
+    # TODO: these are legacy behaviors; these should perhaps be a scheduler
+    # state error instead.
+    if t._state === task_state_done
+        return x
+    elseif t._state === task_state_failed
+        throw(t.result)
+    end
     t.result = x
     set_next_task(t)
     return try_yieldto(identity)

src/task.c

@@ -475,12 +475,8 @@ JL_DLLEXPORT void jl_switch(void)
     if (t == ct) {
         return;
     }
-    if (t->_state != JL_TASK_STATE_RUNNABLE || (t->started && t->stkbuf == NULL)) {
-        ct->_isexception = t->_isexception;
-        ct->result = t->result;
-        jl_gc_wb(ct, ct->result);
-        return;
-    }
+    if (t->started && t->stkbuf == NULL)
+        jl_error("attempt to switch to exited task");
     if (ptls->in_finalizer)
         jl_error("task switch not allowed from inside gc finalizer");
     if (ptls->in_pure_callback)

test/misc.jl

@@ -182,6 +182,17 @@ end
 
 @test_throws ErrorException("deadlock detected: cannot wait on current task") wait(current_task())
 
+# issue #41347
+let t = @async 1
+    wait(t)
+    @test_throws ErrorException yield(t)
+end
+
+let t = @async error(42)
+    Base._wait(t)
+    @test_throws ErrorException("42") yieldto(t)
+end
+
 # test that @sync is lexical (PR #27164)
 
 const x27164 = Ref(0)