[client_channel] ensure that subchannels are always destroyed inside the WorkSerializer (#34077)

- add debug-only `WorkSerializer::IsRunningInWorkSerializer()` method
and use it in client_channel to verify that subchannels are destroyed in
the `WorkSerializer`
  - note: this mechanism uses `std:🧵:id`, so I had to exclude
    work_serializer.cc from the core_banned_constructs check
- fix `WorkSerializer::Run()` to unref the callback before releasing
ownership of the `WorkSerializer`, so that any refs captured by the
`std::function<>` will be released before releasing ownership
- fix the WRR timer callback to hop into the `WorkSerializer` to release
its ref to the picker, since that transitively releases refs to
subchannels
- fix subchannel connectivity state notifications to unref the watcher
inside the `WorkSerializer`, since the watcher often transitively holds
refs to subchannels

src/core/ext/filters/client_channel/client_channel.cc

@@ -513,6 +513,7 @@ class ClientChannel::SubchannelWrapper : public SubchannelInterface {
         ++it->second;
       }
     }
+    GPR_DEBUG_ASSERT(chand_->work_serializer_->RunningInWorkSerializer());
     chand_->subchannel_wrappers_.insert(this);
   }
 
@@ -522,6 +523,7 @@ class ClientChannel::SubchannelWrapper : public SubchannelInterface {
               "chand=%p: destroying subchannel wrapper %p for subchannel %p",
               chand_, this, subchannel_.get());
     }
+    GPR_DEBUG_ASSERT(chand_->work_serializer_->RunningInWorkSerializer());
     chand_->subchannel_wrappers_.erase(this);
     if (chand_->channelz_node_ != nullptr) {
       auto* subchannel_node = subchannel_->channelz_node();
@@ -615,15 +617,16 @@ class ClientChannel::SubchannelWrapper : public SubchannelInterface {
           DEBUG_LOCATION);
     }
 
-    void OnConnectivityStateChange(grpc_connectivity_state state,
-                                   const absl::Status& status) override {
+    void OnConnectivityStateChange(
+        RefCountedPtr<ConnectivityStateWatcherInterface> self,
+        grpc_connectivity_state state, const absl::Status& status) override {
       if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_trace)) {
         gpr_log(GPR_INFO,
                 "chand=%p: connectivity change for subchannel wrapper %p "
                 "subchannel %p; hopping into work_serializer",
                 parent_->chand_, parent_.get(), parent_->subchannel_.get());
       }
-      Ref().release();  // ref owned by lambda
+      self.release();  // Held by callback.
       parent_->chand_->work_serializer_->Run(
           [this, state, status]() ABSL_EXCLUSIVE_LOCKS_REQUIRED(
               *parent_->chand_->work_serializer_) {

src/core/ext/filters/client_channel/lb_policy/health_check_client.cc

@@ -311,9 +311,11 @@ class HealthProducer::ConnectivityWatcher
   explicit ConnectivityWatcher(WeakRefCountedPtr<HealthProducer> producer)
       : producer_(std::move(producer)) {}
 
-  void OnConnectivityStateChange(grpc_connectivity_state state,
-                                 const absl::Status& status) override {
+  void OnConnectivityStateChange(
+      RefCountedPtr<ConnectivityStateWatcherInterface> self,
+      grpc_connectivity_state state, const absl::Status& status) override {
     producer_->OnConnectivityStateChange(state, status);
+    self.reset();
   }
 
   grpc_pollset_set* interested_parties() override {

src/core/ext/filters/client_channel/lb_policy/oob_backend_metric.cc

@@ -77,9 +77,11 @@ class OrcaProducer::ConnectivityWatcher
     grpc_pollset_set_destroy(interested_parties_);
   }
 
-  void OnConnectivityStateChange(grpc_connectivity_state state,
-                                 const absl::Status&) override {
+  void OnConnectivityStateChange(
+      RefCountedPtr<ConnectivityStateWatcherInterface> self,
+      grpc_connectivity_state state, const absl::Status&) override {
     producer_->OnConnectivityStateChange(state);
+    self.reset();
   }
 
   grpc_pollset_set* interested_parties() override {

src/core/ext/filters/client_channel/lb_policy/weighted_round_robin/weighted_round_robin.cc

@@ -608,7 +608,9 @@ void WeightedRoundRobin::Picker::BuildSchedulerAndStartTimerLocked() {
   // Start timer.
   WeakRefCountedPtr<Picker> self = WeakRef();
   timer_handle_ = wrr_->channel_control_helper()->GetEventEngine()->RunAfter(
-      config_->weight_update_period(), [self = std::move(self)]() mutable {
+      config_->weight_update_period(),
+      [self = std::move(self),
+       work_serializer = wrr_->work_serializer()]() mutable {
         ApplicationCallbackExecCtx callback_exec_ctx;
         ExecCtx exec_ctx;
         {
@@ -621,8 +623,8 @@ void WeightedRoundRobin::Picker::BuildSchedulerAndStartTimerLocked() {
             self->BuildSchedulerAndStartTimerLocked();
           }
         }
-        // Release ref before ExecCtx goes out of scope.
-        self.reset();
+        // Release the picker ref inside the WorkSerializer.
+        work_serializer->Run([self = std::move(self)]() {}, DEBUG_LOCATION);
       });
 }
 

src/core/ext/filters/client_channel/subchannel.cc

@@ -367,8 +367,10 @@ void Subchannel::ConnectivityStateWatcherList::NotifyLocked(
     grpc_connectivity_state state, const absl::Status& status) {
   for (const auto& p : watchers_) {
     subchannel_->work_serializer_.Schedule(
-        [watcher = p.second->Ref(), state, status]() {
-          watcher->OnConnectivityStateChange(state, status);
+        [watcher = p.second->Ref(), state, status]() mutable {
+          auto* watcher_ptr = watcher.get();
+          watcher_ptr->OnConnectivityStateChange(std::move(watcher), state,
+                                                 status);
         },
         DEBUG_LOCATION);
   }
@@ -527,8 +529,10 @@ void Subchannel::WatchConnectivityState(
       grpc_pollset_set_add_pollset_set(pollset_set_, interested_parties);
     }
     work_serializer_.Schedule(
-        [watcher = watcher->Ref(), state = state_, status = status_]() {
-          watcher->OnConnectivityStateChange(state, status);
+        [watcher = watcher->Ref(), state = state_, status = status_]() mutable {
+          auto* watcher_ptr = watcher.get();
+          watcher_ptr->OnConnectivityStateChange(std::move(watcher), state,
+                                                 status);
         },
         DEBUG_LOCATION);
     watcher_list_.AddWatcherLocked(std::move(watcher));

src/core/ext/filters/client_channel/subchannel.h

@@ -173,8 +173,14 @@ class Subchannel : public DualRefCounted<Subchannel> {
     // Invoked whenever the subchannel's connectivity state changes.
     // There will be only one invocation of this method on a given watcher
     // instance at any given time.
-    virtual void OnConnectivityStateChange(grpc_connectivity_state state,
-                                           const absl::Status& status) = 0;
+    // A ref to the watcher is passed in here so that the implementation
+    // can unref it in the appropriate synchronization context (e.g.,
+    // inside a WorkSerializer).
+    // TODO(roth): Figure out a cleaner way to guarantee that the ref is
+    // released in the right context.
+    virtual void OnConnectivityStateChange(
+        RefCountedPtr<ConnectivityStateWatcherInterface> self,
+        grpc_connectivity_state state, const absl::Status& status) = 0;
 
     virtual grpc_pollset_set* interested_parties() = 0;
   };

src/core/lib/gprpp/work_serializer.cc

@@ -23,6 +23,7 @@
 #include <atomic>
 #include <functional>
 #include <memory>
+#include <thread>
 #include <utility>
 
 #include <grpc/support/log.h>
@@ -47,6 +48,12 @@ class WorkSerializer::WorkSerializerImpl : public Orphanable {
   void DrainQueue();
   void Orphan() override;
 
+#ifndef NDEBUG
+  bool RunningInWorkSerializer() const {
+    return std::this_thread::get_id() == current_thread_;
+  }
+#endif
+
  private:
   struct CallbackWrapper {
     CallbackWrapper(std::function<void()> cb, const DebugLocation& loc)
@@ -86,6 +93,9 @@ class WorkSerializer::WorkSerializerImpl : public Orphanable {
   // orphaned.
   std::atomic<uint64_t> refs_{MakeRefPair(0, 1)};
   MultiProducerSingleConsumerQueue queue_;
+#ifndef NDEBUG
+  std::thread::id current_thread_;
+#endif
 };
 
 void WorkSerializer::WorkSerializerImpl::Run(std::function<void()> callback,
@@ -102,10 +112,17 @@ void WorkSerializer::WorkSerializerImpl::Run(std::function<void()> callback,
   GPR_DEBUG_ASSERT(GetSize(prev_ref_pair) > 0);
   if (GetOwners(prev_ref_pair) == 0) {
     // We took ownership of the WorkSerializer. Invoke callback and drain queue.
+#ifndef NDEBUG
+    current_thread_ = std::this_thread::get_id();
+#endif
     if (GRPC_TRACE_FLAG_ENABLED(grpc_work_serializer_trace)) {
       gpr_log(GPR_INFO, "  Executing immediately");
     }
     callback();
+    // Delete the callback while still holding the WorkSerializer, so
+    // that any refs being held by the callback via lambda captures will
+    // be destroyed inside the WorkSerializer.
+    callback = nullptr;
     DrainQueueOwned();
   } else {
     // Another thread is holding the WorkSerializer, so decrement the ownership
@@ -158,6 +175,9 @@ void WorkSerializer::WorkSerializerImpl::DrainQueue() {
   const uint64_t prev_ref_pair =
       refs_.fetch_add(MakeRefPair(1, 1), std::memory_order_acq_rel);
   if (GetOwners(prev_ref_pair) == 0) {
+#ifndef NDEBUG
+    current_thread_ = std::this_thread::get_id();
+#endif
     // We took ownership of the WorkSerializer. Drain the queue.
     DrainQueueOwned();
   } else {
@@ -186,6 +206,12 @@ void WorkSerializer::WorkSerializerImpl::DrainQueueOwned() {
     }
     if (GetSize(prev_ref_pair) == 2) {
       // Queue drained. Give up ownership but only if queue remains empty.
+#ifndef NDEBUG
+      // Reset current_thread_ before giving up ownership to avoid TSAN
+      // race.  If we don't wind up giving up ownership, we'll set this
+      // again below before we pull the next callback out of the queue.
+      current_thread_ = std::thread::id();
+#endif
       uint64_t expected = MakeRefPair(1, 1);
       if (refs_.compare_exchange_strong(expected, MakeRefPair(0, 1),
                                         std::memory_order_acq_rel)) {
@@ -200,6 +226,10 @@ void WorkSerializer::WorkSerializerImpl::DrainQueueOwned() {
         delete this;
         return;
       }
+#ifndef NDEBUG
+      // Didn't wind up giving up ownership, so set current_thread_ again.
+      current_thread_ = std::this_thread::get_id();
+#endif
     }
     // There is at least one callback on the queue. Pop the callback from the
     // queue and execute it.
@@ -244,4 +274,10 @@ void WorkSerializer::Schedule(std::function<void()> callback,
 
 void WorkSerializer::DrainQueue() { impl_->DrainQueue(); }
 
+#ifndef NDEBUG
+bool WorkSerializer::RunningInWorkSerializer() const {
+  return impl_->RunningInWorkSerializer();
+}
+#endif
+
 }  // namespace grpc_core

src/core/lib/gprpp/work_serializer.h

@@ -75,6 +75,11 @@ class ABSL_LOCKABLE WorkSerializer {
   // Drains the queue of callbacks.
   void DrainQueue();
 
+#ifndef NDEBUG
+  // Returns true if the current thread is running in the WorkSerializer.
+  bool RunningInWorkSerializer() const;
+#endif
+
  private:
   class WorkSerializerImpl;
 

test/core/gprpp/work_serializer_test.cc

@@ -235,6 +235,43 @@ TEST(WorkSerializerTest, WorkSerializerDestructionRaceMultipleThreads) {
   }
 }
 
+#ifndef NDEBUG
+TEST(WorkSerializerTest, RunningInWorkSerializer) {
+  grpc_core::WorkSerializer work_serializer1;
+  grpc_core::WorkSerializer work_serializer2;
+  EXPECT_FALSE(work_serializer1.RunningInWorkSerializer());
+  EXPECT_FALSE(work_serializer2.RunningInWorkSerializer());
+  work_serializer1.Run(
+      [&]() {
+        EXPECT_TRUE(work_serializer1.RunningInWorkSerializer());
+        EXPECT_FALSE(work_serializer2.RunningInWorkSerializer());
+        work_serializer2.Run(
+            [&]() {
+              EXPECT_TRUE(work_serializer1.RunningInWorkSerializer());
+              EXPECT_TRUE(work_serializer2.RunningInWorkSerializer());
+            },
+            DEBUG_LOCATION);
+      },
+      DEBUG_LOCATION);
+  EXPECT_FALSE(work_serializer1.RunningInWorkSerializer());
+  EXPECT_FALSE(work_serializer2.RunningInWorkSerializer());
+  work_serializer2.Run(
+      [&]() {
+        EXPECT_FALSE(work_serializer1.RunningInWorkSerializer());
+        EXPECT_TRUE(work_serializer2.RunningInWorkSerializer());
+        work_serializer1.Run(
+            [&]() {
+              EXPECT_TRUE(work_serializer1.RunningInWorkSerializer());
+              EXPECT_TRUE(work_serializer2.RunningInWorkSerializer());
+            },
+            DEBUG_LOCATION);
+      },
+      DEBUG_LOCATION);
+  EXPECT_FALSE(work_serializer1.RunningInWorkSerializer());
+  EXPECT_FALSE(work_serializer2.RunningInWorkSerializer());
+}
+#endif
+
 }  // namespace
 
 int main(int argc, char** argv) {

tools/run_tests/sanity/cpp_banned_constructs.sh

@@ -26,7 +26,7 @@ cd "$(dirname "$0")/../../.."
 grep -EIrn \
     'std::(mutex|condition_variable|lock_guard|unique_lock|thread)' \
     include/grpc include/grpcpp src/core src/cpp | \
-    grep -Ev include/grpcpp/impl/sync.h | \
+    grep -Ev 'include/grpcpp/impl/sync.h|src/core/lib/gprpp/work_serializer.cc' | \
     diff - /dev/null
 
 #
@@ -36,7 +36,7 @@ grep -EIrn \
 grep -EIrn \
     '^#include (<mutex>|<condition_variable>|<thread>|<ratio>|<filesystem>|<future>|<system_error>)' \
     include/grpc include/grpcpp src/core src/cpp | \
-    grep -Ev include/grpcpp/impl/sync.h | \
+    grep -Ev 'include/grpcpp/impl/sync.h|src/core/lib/gprpp/work_serializer.cc' | \
     diff - /dev/null
 
 #