Fix: waitall can throw when all tasks are done (#59905)

Closes #59901

Author: jakobnissen

base/task.jl

@@ -435,19 +435,21 @@ function _wait_multiple(waiting_tasks, throwexc=false, all=false, failfast=false
             done_mask[i] = true
             exception |= istaskfailed(t)
             nremaining -= 1
-        else
-            done_mask[i] = false
         end
     end
 
-    if nremaining == 0
-        return tasks, Task[]
-    elseif any(done_mask) && (!all || (failfast && exception))
+    # We can return early all tasks are done, or if any is done and we only
+    # needed to wait for one, or if any task failed and we have failfast
+    if nremaining == 0 || (any(done_mask) && (!all || (failfast && exception)))
         if throwexc && (!all || failfast) && exception
             exceptions = [TaskFailedException(t) for t in tasks[done_mask] if istaskfailed(t)]
             throw(CompositeException(exceptions))
         else
-            return tasks[done_mask], tasks[.~done_mask]
+            if nremaining == 0
+                return tasks, Task[]
+            else
+                return tasks[done_mask], tasks[.~done_mask]
+            end
         end
     end
 
@@ -489,6 +491,10 @@ function _wait_multiple(waiting_tasks, throwexc=false, all=false, failfast=false
     close(chan)
 
     if nremaining == 0
+        if throwexc && exception
+            exceptions = [TaskFailedException(t) for t in tasks if istaskfailed(t)]
+            throw(CompositeException(exceptions))
+        end
         return tasks, Task[]
     else
         remaining_mask = .~done_mask

test/threads_exec.jl

@@ -1364,6 +1364,17 @@ end
                 @test !istaskdone(tasks[3])
 
                 teardown(tasks, event)
+
+                @test_throws CompositeException begin
+                    waitall(Threads.@spawn(div(1, i)) for i = 0:1)
+                end
+
+                tasks = [Threads.@spawn(div(1, i)) for i = 0:1]
+                wait(tasks[1]; throw=false)
+                wait(tasks[2]; throw=false)
+                @test_throws CompositeException begin
+                    waitall(Threads.@spawn(div(1, i)) for i = 0:1)
+                end
             end
         end
     end