task_manager: module: make_task: enter gate when the task is created

Passing the gate_closed_exception to the task promise in start()
ends up with abandoned exception since no-one is waiting
for it.

Instead, enter the gate when the task is made
so it will fail make_task if the gate is already closed.

Fixes scylladb#15211

Signed-off-by: Benny Halevy <bhalevy@scylladb.com>

tasks/task_manager.cc

@@ -9,6 +9,7 @@
 #include <seastar/core/on_internal_error.hh>
 #include <seastar/coroutine/parallel_for_each.hh>
 #include <seastar/core/abort_source.hh>
+#include <seastar/core/gate.hh>
 
 #include "task_manager.hh"
 #include "test_module.hh"
@@ -145,7 +146,7 @@ void task_manager::task::impl::finish_failed(std::exception_ptr ex) {
     finish_failed(ex, fmt::format("{}", ex));
 }
 
-task_manager::task::task(task_impl_ptr&& impl) noexcept : _impl(std::move(impl)) {
+task_manager::task::task(task_impl_ptr&& impl, gate::holder gh) noexcept : _impl(std::move(impl)), _gate_holder(std::move(gh)) {
     register_task();
 }
 
@@ -186,14 +187,13 @@ void task_manager::task::start() {
     try {
         // Background fiber does not capture task ptr, so the task can be unregistered and destroyed independently in the foreground.
         // After the ttl expires, the task id will be used to unregister the task if that didn't happen in any other way.
-        (void)with_gate(_impl->_module->async_gate(), [f = done(), module = _impl->_module, id = id()] () mutable {
-            return std::move(f).finally([module] {
+        auto module = _impl->_module;
+            (void)done().finally([module] {
                 return sleep_abortable(module->get_task_manager().get_task_ttl(), module->abort_source());
-            }).then_wrapped([module, id] (auto f) {
+            }).then_wrapped([module, id = id()] (auto f) {
                 f.ignore_ready_future();
                 module->unregister_task(id);
             });
-        });
         _impl->_as.check();
         _impl->_status.state = task_manager::task_state::running;
         _impl->run_to_completion();
@@ -307,7 +307,7 @@ future<> task_manager::module::stop() noexcept {
 }
 
 future<task_manager::task_ptr> task_manager::module::make_task(task::task_impl_ptr task_impl_ptr, task_info parent_d) {
-    auto task = make_lw_shared<task_manager::task>(std::move(task_impl_ptr));
+    auto task = make_lw_shared<task_manager::task>(std::move(task_impl_ptr), async_gate().hold());
     bool abort = false;
     if (parent_d) {
         task->get_status().sequence_number = co_await _tm.container().invoke_on(parent_d.shard, [id = parent_d.id, task = make_foreign(task), &abort] (task_manager& tm) mutable {

tasks/task_manager.hh

@@ -139,8 +139,10 @@ public:
         using task_impl_ptr = shared_ptr<impl>;
     protected:
         task_impl_ptr _impl;
+    private:
+        gate::holder _gate_holder;
     public:
-        task(task_impl_ptr&& impl) noexcept;
+        task(task_impl_ptr&& impl, gate::holder) noexcept;
 
         task_id id();
         std::string type() const;