[rclcpp] adding a "static" single threaded executor (#1034)

* Added static single threaded executor functionality

Signed-off-by: Ishu Goel <ishu.goel@nobleo.nl>

executor enhanced to run clients and waitable

Signed-off-by: Ishu Goel <ishu.goel@nobleo.nl>

tested executor

Signed-off-by: Ishu Goel <ishu.goel@nobleo.nl>

added semi-dynamic feature to the executor

Signed-off-by: Ishu Goel <ishu.goel@nobleo.nl>

Jenkins error fixes

Signed-off-by: Ishu Goel <ishu.goel@nobleo.nl>

Added static single threaded executor functionality

Signed-off-by: Ishu Goel <ishu.goel@nobleo.nl>

* Added semi-dynamic feature and made changes based on review comments

Signed-off-by: Ishu Goel <ishu.goel@nobleo.nl>

* re-added accidentally deleted code in node.hpp, fixed static_single_threaded_executor.cpp w.r.t. intra-process change since last commit

Signed-off-by: MartinCornelis2 <martin.cornelis@nobleo.nl>

* Remove not needed comparison

wait_set_.size_of_* is always different than '0'
if we are inside the for loop

Signed-off-by: Mauro <mpasserino@irobot.com>

* If new entity added to a node: re-collect entities

Now we check ONLY node guard_conditions_

Some possible guard conditions to be triggered HERE are:
1. Ctrl+C guard condition
2. Executor interrupt_guard_condition_
3. Node guard_conditions_
4. Waitables guard conditions
5. ..more

The previous approach was only checking if NOT (1 & 2),
so if a Waitable was triggered, it would re-collect all
entities, even if no new node entity was added. This was the case
of the intra process manager, who relies on waitables.
Every time a subscriber got a message, all the entities
were collected.

Signed-off-by: Mauro <mpasserino@irobot.com>

* Implement static executor entities collector

Signed-off-by: Mauro <mpasserino@irobot.com>

* fixup and style

Signed-off-by: William Woodall <william@osrfoundation.org>

* mark new classes as final, with non-virtual destructors

Signed-off-by: William Woodall <william@osrfoundation.org>

* adding copyright to static executor files

Signed-off-by: Ishu Goel <ishu.goel@nobleo.nl>

* fixup

Signed-off-by: William Woodall <william@osrfoundation.org>

* cpplint fixes

Signed-off-by: William Woodall <william@osrfoundation.org>

Co-authored-by: Ishu Goel <ishu.goel@nobleo.nl>
Co-authored-by: MartinCornelis2 <martin.cornelis@nobleo.nl>
Co-authored-by: Mauro <mpasserino@irobot.com>

rclcpp/CMakeLists.txt

@@ -40,11 +40,14 @@ set(${PROJECT_NAME}_SRCS
   src/rclcpp/duration.cpp
   src/rclcpp/event.cpp
   src/rclcpp/exceptions.cpp
+  src/rclcpp/executable_list.cpp
   src/rclcpp/executor.cpp
   src/rclcpp/executors.cpp
   src/rclcpp/expand_topic_or_service_name.cpp
   src/rclcpp/executors/multi_threaded_executor.cpp
   src/rclcpp/executors/single_threaded_executor.cpp
+  src/rclcpp/executors/static_executor_entities_collector.cpp
+  src/rclcpp/executors/static_single_threaded_executor.cpp
   src/rclcpp/graph_listener.cpp
   src/rclcpp/init_options.cpp
   src/rclcpp/intra_process_manager.cpp

rclcpp/include/rclcpp/executable_list.hpp

@@ -0,0 +1,92 @@
+// Copyright 2019 Nobleo Technology
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef RCLCPP__EXECUTABLE_LIST_HPP_
+#define RCLCPP__EXECUTABLE_LIST_HPP_
+
+#include <memory>
+#include <vector>
+
+#include "rclcpp/client.hpp"
+#include "rclcpp/service.hpp"
+#include "rclcpp/subscription_base.hpp"
+#include "rclcpp/timer.hpp"
+#include "rclcpp/visibility_control.hpp"
+#include "rclcpp/waitable.hpp"
+
+namespace rclcpp
+{
+namespace executor
+{
+
+/// This class contains subscriptionbase, timerbase, etc. which can be used to run callbacks.
+class ExecutableList final
+{
+public:
+  RCLCPP_PUBLIC
+  ExecutableList();
+
+  RCLCPP_PUBLIC
+  ~ExecutableList();
+
+  RCLCPP_PUBLIC
+  void
+  clear();
+
+  RCLCPP_PUBLIC
+  void
+  add_subscription(rclcpp::SubscriptionBase::SharedPtr subscription);
+
+  RCLCPP_PUBLIC
+  void
+  add_timer(rclcpp::TimerBase::SharedPtr timer);
+
+  RCLCPP_PUBLIC
+  void
+  add_service(rclcpp::ServiceBase::SharedPtr service);
+
+  RCLCPP_PUBLIC
+  void
+  add_client(rclcpp::ClientBase::SharedPtr client);
+
+  RCLCPP_PUBLIC
+  void
+  add_waitable(rclcpp::Waitable::SharedPtr waitable);
+
+  // Vector containing the SubscriptionBase of all the subscriptions added to the executor.
+  std::vector<rclcpp::SubscriptionBase::SharedPtr> subscription;
+  // Contains the count of added subscriptions
+  size_t number_of_subscriptions;
+  // Vector containing the TimerBase of all the timers added to the executor.
+  std::vector<rclcpp::TimerBase::SharedPtr> timer;
+  // Contains the count of added timers
+  size_t number_of_timers;
+  // Vector containing the ServiceBase of all the services added to the executor.
+  std::vector<rclcpp::ServiceBase::SharedPtr> service;
+  // Contains the count of added services
+  size_t number_of_services;
+  // Vector containing the ClientBase of all the clients added to the executor.
+  std::vector<rclcpp::ClientBase::SharedPtr> client;
+  // Contains the count of added clients
+  size_t number_of_clients;
+  // Vector containing all the waitables added to the executor.
+  std::vector<rclcpp::Waitable::SharedPtr> waitable;
+  // Contains the count of added waitables
+  size_t number_of_waitables;
+};
+
+}  // namespace executor
+}  // namespace rclcpp
+
+#endif  // RCLCPP__EXECUTABLE_LIST_HPP_

rclcpp/include/rclcpp/executor.hpp

@@ -357,7 +357,6 @@ protected:
   /// The context associated with this executor.
   std::shared_ptr<rclcpp::Context> context_;
 
-private:
   RCLCPP_DISABLE_COPY(Executor)
 
   std::list<rclcpp::node_interfaces::NodeBaseInterface::WeakPtr> weak_nodes_;

rclcpp/include/rclcpp/executors.hpp

@@ -20,6 +20,7 @@
 
 #include "rclcpp/executors/multi_threaded_executor.hpp"
 #include "rclcpp/executors/single_threaded_executor.hpp"
+#include "rclcpp/executors/static_single_threaded_executor.hpp"
 #include "rclcpp/node.hpp"
 #include "rclcpp/utilities.hpp"
 #include "rclcpp/visibility_control.hpp"

rclcpp/include/rclcpp/executors/static_executor_entities_collector.hpp

@@ -0,0 +1,167 @@
+// Copyright 2020 Open Source Robotics Foundation, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef RCLCPP__EXECUTORS__STATIC_EXECUTOR_ENTITIES_COLLECTOR_HPP_
+#define RCLCPP__EXECUTORS__STATIC_EXECUTOR_ENTITIES_COLLECTOR_HPP_
+
+#include <list>
+#include <memory>
+
+#include "rcl/guard_condition.h"
+#include "rcl/wait.h"
+
+#include "rclcpp/executable_list.hpp"
+#include "rclcpp/macros.hpp"
+#include "rclcpp/memory_strategy.hpp"
+#include "rclcpp/visibility_control.hpp"
+#include "rclcpp/waitable.hpp"
+
+namespace rclcpp
+{
+namespace executors
+{
+
+class StaticExecutorEntitiesCollector final
+  : public rclcpp::Waitable,
+  public std::enable_shared_from_this<StaticExecutorEntitiesCollector>
+{
+public:
+  RCLCPP_SMART_PTR_DEFINITIONS(StaticExecutorEntitiesCollector)
+
+  // Constructor
+  RCLCPP_PUBLIC
+  StaticExecutorEntitiesCollector() = default;
+
+  // Destructor
+  ~StaticExecutorEntitiesCollector();
+
+  RCLCPP_PUBLIC
+  void
+  init(
+    rcl_wait_set_t * p_wait_set,
+    rclcpp::memory_strategy::MemoryStrategy::SharedPtr & memory_strategy,
+    rcl_guard_condition_t * executor_guard_condition);
+
+  RCLCPP_PUBLIC
+  void
+  execute() override;
+
+  RCLCPP_PUBLIC
+  void
+  fill_memory_strategy();
+
+  RCLCPP_PUBLIC
+  void
+  fill_executable_list();
+
+  /// Function to reallocate space for entities in the wait set.
+  RCLCPP_PUBLIC
+  void
+  prepare_wait_set();
+
+  /// Function to add_handles_to_wait_set and wait for work and
+  // block until the wait set is ready or until the timeout has been exceeded.
+  RCLCPP_PUBLIC
+  void
+  refresh_wait_set(std::chrono::nanoseconds timeout = std::chrono::nanoseconds(-1));
+
+  RCLCPP_PUBLIC
+  bool
+  add_to_wait_set(rcl_wait_set_t * wait_set) override;
+
+  RCLCPP_PUBLIC
+  size_t
+  get_number_of_ready_guard_conditions() override;
+
+  RCLCPP_PUBLIC
+  void
+  add_node(
+    rclcpp::node_interfaces::NodeBaseInterface::SharedPtr node_ptr);
+
+  RCLCPP_PUBLIC
+  bool
+  remove_node(
+    rclcpp::node_interfaces::NodeBaseInterface::SharedPtr node_ptr);
+
+  /// Complete all available queued work without blocking.
+  /**
+   * This function checks if after the guard condition was triggered
+   * (or a spurious wakeup happened) we are really ready to execute
+   * i.e. re-collect entities
+   */
+  RCLCPP_PUBLIC
+  bool
+  is_ready(rcl_wait_set_t * wait_set) override;
+
+  RCLCPP_PUBLIC
+  size_t
+  get_number_of_timers() {return exec_list_.number_of_timers;}
+
+  RCLCPP_PUBLIC
+  size_t
+  get_number_of_subscriptions() {return exec_list_.number_of_subscriptions;}
+
+  RCLCPP_PUBLIC
+  size_t
+  get_number_of_services() {return exec_list_.number_of_services;}
+
+  RCLCPP_PUBLIC
+  size_t
+  get_number_of_clients() {return exec_list_.number_of_clients;}
+
+  RCLCPP_PUBLIC
+  size_t
+  get_number_of_waitables() {return exec_list_.number_of_waitables;}
+
+  RCLCPP_PUBLIC
+  rclcpp::SubscriptionBase::SharedPtr
+  get_subscription(size_t i) {return exec_list_.subscription[i];}
+
+  RCLCPP_PUBLIC
+  rclcpp::TimerBase::SharedPtr
+  get_timer(size_t i) {return exec_list_.timer[i];}
+
+  RCLCPP_PUBLIC
+  rclcpp::ServiceBase::SharedPtr
+  get_service(size_t i) {return exec_list_.service[i];}
+
+  RCLCPP_PUBLIC
+  rclcpp::ClientBase::SharedPtr
+  get_client(size_t i) {return exec_list_.client[i];}
+
+  RCLCPP_PUBLIC
+  rclcpp::Waitable::SharedPtr
+  get_waitable(size_t i) {return exec_list_.waitable[i];}
+
+private:
+  /// Nodes guard conditions which trigger this waitable
+  std::list<const rcl_guard_condition_t *> guard_conditions_;
+
+  /// Memory strategy: an interface for handling user-defined memory allocation strategies.
+  rclcpp::memory_strategy::MemoryStrategy::SharedPtr memory_strategy_;
+
+  /// List of weak nodes registered in the static executor
+  std::list<rclcpp::node_interfaces::NodeBaseInterface::WeakPtr> weak_nodes_;
+
+  /// Wait set for managing entities that the rmw layer waits on.
+  rcl_wait_set_t * p_wait_set_ = nullptr;
+
+  /// Executable list: timers, subscribers, clients, services and waitables
+  rclcpp::executor::ExecutableList exec_list_;
+};
+
+}  // namespace executors
+}  // namespace rclcpp
+
+#endif  // RCLCPP__EXECUTORS__STATIC_EXECUTOR_ENTITIES_COLLECTOR_HPP_

rclcpp/include/rclcpp/executors/static_single_threaded_executor.hpp

@@ -0,0 +1,200 @@
+// Copyright 2019 Nobleo Technology
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef RCLCPP__EXECUTORS__STATIC_SINGLE_THREADED_EXECUTOR_HPP_
+#define RCLCPP__EXECUTORS__STATIC_SINGLE_THREADED_EXECUTOR_HPP_
+
+#include <cassert>
+#include <cstdlib>
+#include <memory>
+#include <vector>
+#include <string>
+
+#include "rmw/rmw.h"
+
+#include "rclcpp/executable_list.hpp"
+#include "rclcpp/executor.hpp"
+#include "rclcpp/executors/static_executor_entities_collector.hpp"
+#include "rclcpp/macros.hpp"
+#include "rclcpp/memory_strategies.hpp"
+#include "rclcpp/node.hpp"
+#include "rclcpp/rate.hpp"
+#include "rclcpp/utilities.hpp"
+#include "rclcpp/visibility_control.hpp"
+
+namespace rclcpp
+{
+namespace executors
+{
+
+/// Static executor implementation
+/**
+ * This executor is a static version of the original single threaded executor.
+ * It's static because it doesn't reconstruct the executable list for every iteration.
+ * All nodes, callbackgroups, timers, subscriptions etc. are created before
+ * spin() is called, and modified only when an entity is added/removed to/from a node.
+ *
+ * To run this executor instead of SingleThreadedExecutor replace:
+ * rclcpp::executors::SingleThreadedExecutor exec;
+ * by
+ * rclcpp::executors::StaticSingleThreadedExecutor exec;
+ * in your source code and spin node(s) in the following way:
+ * exec.add_node(node);
+ * exec.spin();
+ * exec.remove_node(node);
+ */
+class StaticSingleThreadedExecutor : public executor::Executor
+{
+public:
+  RCLCPP_SMART_PTR_DEFINITIONS(StaticSingleThreadedExecutor)
+
+  /// Default constructor. See the default constructor for Executor.
+  RCLCPP_PUBLIC
+  explicit StaticSingleThreadedExecutor(
+    const executor::ExecutorArgs & args = executor::ExecutorArgs());
+
+  /// Default destrcutor.
+  RCLCPP_PUBLIC
+  virtual ~StaticSingleThreadedExecutor();
+
+  /// Static executor implementation of spin.
+  // This function will block until work comes in, execute it, and keep blocking.
+  // It will only be interrupt by a CTRL-C (managed by the global signal handler).
+  RCLCPP_PUBLIC
+  void
+  spin() override;
+
+  /// Add a node to the executor.
+  /**
+   * An executor can have zero or more nodes which provide work during `spin` functions.
+   * \param[in] node_ptr Shared pointer to the node to be added.
+   * \param[in] notify True to trigger the interrupt guard condition during this function. If
+   * the executor is blocked at the rmw layer while waiting for work and it is notified that a new
+   * node was added, it will wake up.
+   */
+  RCLCPP_PUBLIC
+  void
+  add_node(
+    rclcpp::node_interfaces::NodeBaseInterface::SharedPtr node_ptr,
+    bool notify = true) override;
+
+  /// Convenience function which takes Node and forwards NodeBaseInterface.
+  RCLCPP_PUBLIC
+  void
+  add_node(std::shared_ptr<rclcpp::Node> node_ptr, bool notify = true) override;
+
+  /// Remove a node from the executor.
+  /**
+   * \param[in] node_ptr Shared pointer to the node to remove.
+   * \param[in] notify True to trigger the interrupt guard condition and wake up the executor.
+   * This is useful if the last node was removed from the executor while the executor was blocked
+   * waiting for work in another thread, because otherwise the executor would never be notified.
+   */
+  RCLCPP_PUBLIC
+  void
+  remove_node(
+    rclcpp::node_interfaces::NodeBaseInterface::SharedPtr node_ptr,
+    bool notify = true) override;
+
+  /// Convenience function which takes Node and forwards NodeBaseInterface.
+  RCLCPP_PUBLIC
+  void
+  remove_node(std::shared_ptr<rclcpp::Node> node_ptr, bool notify = true) override;
+
+  /// Spin (blocking) until the future is complete, it times out waiting, or rclcpp is interrupted.
+  /**
+   * \param[in] future The future to wait on. If this function returns SUCCESS, the future can be
+   *   accessed without blocking (though it may still throw an exception).
+   * \param[in] timeout Optional timeout parameter, which gets passed to
+   *    Executor::execute_ready_executables.
+   *   `-1` is block forever, `0` is non-blocking.
+   *   If the time spent inside the blocking loop exceeds this timeout, return a TIMEOUT return
+   *   code.
+   * \return The return code, one of `SUCCESS`, `INTERRUPTED`, or `TIMEOUT`.
+   *
+   *  Example usage:
+   *  rclcpp::executors::StaticSingleThreadedExecutor exec;
+   *  // ... other part of code like creating node
+   *  // define future
+   *  exec.add_node(node);
+   *  exec.spin_until_future_complete(future);
+   */
+  template<typename ResponseT, typename TimeRepT = int64_t, typename TimeT = std::milli>
+  rclcpp::executor::FutureReturnCode
+  spin_until_future_complete(
+    std::shared_future<ResponseT> & future,
+    std::chrono::duration<TimeRepT, TimeT> timeout = std::chrono::duration<TimeRepT, TimeT>(-1))
+  {
+    std::future_status status = future.wait_for(std::chrono::seconds(0));
+    if (status == std::future_status::ready) {
+      return rclcpp::executor::FutureReturnCode::SUCCESS;
+    }
+
+    auto end_time = std::chrono::steady_clock::now();
+    std::chrono::nanoseconds timeout_ns = std::chrono::duration_cast<std::chrono::nanoseconds>(
+      timeout);
+    if (timeout_ns > std::chrono::nanoseconds::zero()) {
+      end_time += timeout_ns;
+    }
+    std::chrono::nanoseconds timeout_left = timeout_ns;
+
+    entities_collector_ = std::make_shared<StaticExecutorEntitiesCollector>();
+    entities_collector_->init(&wait_set_, memory_strategy_, &interrupt_guard_condition_);
+
+    while (rclcpp::ok(this->context_)) {
+      // Do one set of work.
+      entities_collector_->refresh_wait_set(timeout_left);
+      execute_ready_executables();
+      // Check if the future is set, return SUCCESS if it is.
+      status = future.wait_for(std::chrono::seconds(0));
+      if (status == std::future_status::ready) {
+        return rclcpp::executor::FutureReturnCode::SUCCESS;
+      }
+      // If the original timeout is < 0, then this is blocking, never TIMEOUT.
+      if (timeout_ns < std::chrono::nanoseconds::zero()) {
+        continue;
+      }
+      // Otherwise check if we still have time to wait, return TIMEOUT if not.
+      auto now = std::chrono::steady_clock::now();
+      if (now >= end_time) {
+        return rclcpp::executor::FutureReturnCode::TIMEOUT;
+      }
+      // Subtract the elapsed time from the original timeout.
+      timeout_left = std::chrono::duration_cast<std::chrono::nanoseconds>(end_time - now);
+    }
+
+    // The future did not complete before ok() returned false, return INTERRUPTED.
+    return rclcpp::executor::FutureReturnCode::INTERRUPTED;
+  }
+
+protected:
+  /// Check which executables in ExecutableList struct are ready from wait_set and execute them.
+  /**
+   * \param[in] exec_list Structure that can hold subscriptionbases, timerbases, etc
+   * \param[in] timeout Optional timeout parameter.
+   */
+  RCLCPP_PUBLIC
+  void
+  execute_ready_executables();
+
+private:
+  RCLCPP_DISABLE_COPY(StaticSingleThreadedExecutor)
+
+  StaticExecutorEntitiesCollector::SharedPtr entities_collector_;
+};
+
+}  // namespace executors
+}  // namespace rclcpp
+
+#endif  // RCLCPP__EXECUTORS__STATIC_SINGLE_THREADED_EXECUTOR_HPP_

rclcpp/include/rclcpp/memory_strategy.hpp

@@ -56,6 +56,7 @@ public:
   virtual size_t number_of_guard_conditions() const = 0;
   virtual size_t number_of_waitables() const = 0;
 
+  virtual void add_waitable_handle(const rclcpp::Waitable::SharedPtr & waitable) = 0;
   virtual bool add_handles_to_wait_set(rcl_wait_set_t * wait_set) = 0;
   virtual void clear_handles() = 0;
   virtual void remove_null_handles(rcl_wait_set_t * wait_set) = 0;

rclcpp/include/rclcpp/strategies/allocator_memory_strategy.hpp

@@ -169,7 +169,6 @@ public:
             subscription_handles_.push_back(subscription->get_subscription_handle());
             return false;
           });
-
         group->find_service_ptrs_if(
           [this](const rclcpp::ServiceBase::SharedPtr & service) {
             service_handles_.push_back(service->get_service_handle());
@@ -195,6 +194,14 @@ public:
     return has_invalid_weak_nodes;
   }
 
+  void add_waitable_handle(const rclcpp::Waitable::SharedPtr & waitable)
+  {
+    if (nullptr == waitable) {
+      throw std::runtime_error("waitable object unexpectedly nullptr");
+    }
+    waitable_handles_.push_back(waitable);
+  }
+
   bool add_handles_to_wait_set(rcl_wait_set_t * wait_set) override
   {
     for (auto subscription : subscription_handles_) {

rclcpp/src/rclcpp/executable_list.cpp

@@ -0,0 +1,84 @@
+// Copyright 2019 Nobleo Technology
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <utility>
+
+#include "rclcpp/executable_list.hpp"
+
+using rclcpp::executor::ExecutableList;
+
+ExecutableList::ExecutableList()
+: number_of_subscriptions(0),
+  number_of_timers(0),
+  number_of_services(0),
+  number_of_clients(0),
+  number_of_waitables(0)
+{}
+
+ExecutableList::~ExecutableList()
+{}
+
+void
+ExecutableList::clear()
+{
+  this->timer.clear();
+  this->number_of_timers = 0;
+
+  this->subscription.clear();
+  this->number_of_subscriptions = 0;
+
+  this->service.clear();
+  this->number_of_services = 0;
+
+  this->client.clear();
+  this->number_of_clients = 0;
+
+  this->waitable.clear();
+  this->number_of_waitables = 0;
+}
+
+void
+ExecutableList::add_subscription(rclcpp::SubscriptionBase::SharedPtr subscription)
+{
+  this->subscription.push_back(std::move(subscription));
+  this->number_of_subscriptions++;
+}
+
+void
+ExecutableList::add_timer(rclcpp::TimerBase::SharedPtr timer)
+{
+  this->timer.push_back(std::move(timer));
+  this->number_of_timers++;
+}
+
+void
+ExecutableList::add_service(rclcpp::ServiceBase::SharedPtr service)
+{
+  this->service.push_back(std::move(service));
+  this->number_of_services++;
+}
+
+void
+ExecutableList::add_client(rclcpp::ClientBase::SharedPtr client)
+{
+  this->client.push_back(std::move(client));
+  this->number_of_clients++;
+}
+
+void
+ExecutableList::add_waitable(rclcpp::Waitable::SharedPtr waitable)
+{
+  this->waitable.push_back(std::move(waitable));
+  this->number_of_waitables++;
+}

rclcpp/src/rclcpp/executors/static_executor_entities_collector.cpp

@@ -0,0 +1,286 @@
+// Copyright 2020 Open Source Robotics Foundation, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "rclcpp/executors/static_executor_entities_collector.hpp"
+
+#include <string>
+#include <memory>
+
+#include "rclcpp/executors/static_single_threaded_executor.hpp"
+
+using rclcpp::executors::StaticExecutorEntitiesCollector;
+
+StaticExecutorEntitiesCollector::~StaticExecutorEntitiesCollector()
+{
+  // Disassociate all nodes
+  for (auto & weak_node : weak_nodes_) {
+    auto node = weak_node.lock();
+    if (node) {
+      std::atomic_bool & has_executor = node->get_associated_with_executor_atomic();
+      has_executor.store(false);
+    }
+  }
+  exec_list_.clear();
+  weak_nodes_.clear();
+  guard_conditions_.clear();
+}
+
+void
+StaticExecutorEntitiesCollector::init(
+  rcl_wait_set_t * p_wait_set,
+  memory_strategy::MemoryStrategy::SharedPtr & memory_strategy,
+  rcl_guard_condition_t * executor_guard_condition)
+{
+  // Empty initialize executable list
+  exec_list_ = executor::ExecutableList();
+  // Get executor's wait_set_ pointer
+  p_wait_set_ = p_wait_set;
+  // Get executor's memory strategy ptr
+  if (memory_strategy == nullptr) {
+    throw std::runtime_error("Received NULL memory strategy in executor waitable.");
+  }
+  memory_strategy_ = memory_strategy;
+
+  // Add executor's guard condition
+  guard_conditions_.push_back(executor_guard_condition);
+
+  // Get memory strategy and executable list. Prepare wait_set_
+  execute();
+}
+
+void
+StaticExecutorEntitiesCollector::execute()
+{
+  // Fill memory strategy with entities coming from weak_nodes_
+  fill_memory_strategy();
+  // Fill exec_list_ with entities coming from weak_nodes_ (same as memory strategy)
+  fill_executable_list();
+  // Resize the wait_set_ based on memory_strategy handles (rcl_wait_set_resize)
+  prepare_wait_set();
+}
+
+void
+StaticExecutorEntitiesCollector::fill_memory_strategy()
+{
+  memory_strategy_->clear_handles();
+  bool has_invalid_weak_nodes = memory_strategy_->collect_entities(weak_nodes_);
+
+  // Clean up any invalid nodes, if they were detected
+  if (has_invalid_weak_nodes) {
+    auto node_it = weak_nodes_.begin();
+    while (node_it != weak_nodes_.end()) {
+      if (node_it->expired()) {
+        node_it = weak_nodes_.erase(node_it);
+      } else {
+        ++node_it;
+      }
+    }
+  }
+
+  // Add the static executor waitable to the memory strategy
+  memory_strategy_->add_waitable_handle(this->shared_from_this());
+}
+
+void
+StaticExecutorEntitiesCollector::fill_executable_list()
+{
+  exec_list_.clear();
+
+  for (auto & weak_node : weak_nodes_) {
+    auto node = weak_node.lock();
+    if (!node) {
+      continue;
+    }
+    // Check in all the callback groups
+    for (auto & weak_group : node->get_callback_groups()) {
+      auto group = weak_group.lock();
+      if (!group || !group->can_be_taken_from().load()) {
+        continue;
+      }
+
+      group->find_timer_ptrs_if(
+        [this](const rclcpp::TimerBase::SharedPtr & timer) {
+          if (timer) {
+            exec_list_.add_timer(timer);
+          }
+          return false;
+        });
+      group->find_subscription_ptrs_if(
+        [this](const rclcpp::SubscriptionBase::SharedPtr & subscription) {
+          if (subscription) {
+            exec_list_.add_subscription(subscription);
+          }
+          return false;
+        });
+      group->find_service_ptrs_if(
+        [this](const rclcpp::ServiceBase::SharedPtr & service) {
+          if (service) {
+            exec_list_.add_service(service);
+          }
+          return false;
+        });
+      group->find_client_ptrs_if(
+        [this](const rclcpp::ClientBase::SharedPtr & client) {
+          if (client) {
+            exec_list_.add_client(client);
+          }
+          return false;
+        });
+      group->find_waitable_ptrs_if(
+        [this](const rclcpp::Waitable::SharedPtr & waitable) {
+          if (waitable) {
+            exec_list_.add_waitable(waitable);
+          }
+          return false;
+        });
+    }
+  }
+
+  // Add the executor's waitable to the executable list
+  exec_list_.add_waitable(shared_from_this());
+}
+
+void
+StaticExecutorEntitiesCollector::prepare_wait_set()
+{
+  // clear wait set
+  if (rcl_wait_set_clear(p_wait_set_) != RCL_RET_OK) {
+    throw std::runtime_error("Couldn't clear wait set");
+  }
+
+  // The size of waitables are accounted for in size of the other entities
+  rcl_ret_t ret = rcl_wait_set_resize(
+    p_wait_set_, memory_strategy_->number_of_ready_subscriptions(),
+    memory_strategy_->number_of_guard_conditions(), memory_strategy_->number_of_ready_timers(),
+    memory_strategy_->number_of_ready_clients(), memory_strategy_->number_of_ready_services(),
+    memory_strategy_->number_of_ready_events());
+
+  if (RCL_RET_OK != ret) {
+    throw std::runtime_error(
+            std::string("Couldn't resize the wait set : ") + rcl_get_error_string().str);
+  }
+}
+
+void
+StaticExecutorEntitiesCollector::refresh_wait_set(std::chrono::nanoseconds timeout)
+{
+  // clear wait set (memeset to '0' all wait_set_ entities
+  // but keeps the wait_set_ number of entities)
+  if (rcl_wait_set_clear(p_wait_set_) != RCL_RET_OK) {
+    throw std::runtime_error("Couldn't clear wait set");
+  }
+
+  if (!memory_strategy_->add_handles_to_wait_set(p_wait_set_)) {
+    throw std::runtime_error("Couldn't fill wait set");
+  }
+
+  rcl_ret_t status =
+    rcl_wait(p_wait_set_, std::chrono::duration_cast<std::chrono::nanoseconds>(timeout).count());
+
+  if (status == RCL_RET_WAIT_SET_EMPTY) {
+    RCUTILS_LOG_WARN_NAMED(
+      "rclcpp",
+      "empty wait set received in rcl_wait(). This should never happen.");
+  } else if (status != RCL_RET_OK && status != RCL_RET_TIMEOUT) {
+    using rclcpp::exceptions::throw_from_rcl_error;
+    throw_from_rcl_error(status, "rcl_wait() failed");
+  }
+}
+
+bool
+StaticExecutorEntitiesCollector::add_to_wait_set(rcl_wait_set_t * wait_set)
+{
+  // Add waitable guard conditions (one for each registered node) into the wait set.
+  for (const auto & gc : guard_conditions_) {
+    rcl_ret_t ret = rcl_wait_set_add_guard_condition(wait_set, gc, NULL);
+    if (ret != RCL_RET_OK) {
+      throw std::runtime_error("Executor waitable: couldn't add guard condition to wait set");
+    }
+  }
+  return true;
+}
+
+size_t StaticExecutorEntitiesCollector::get_number_of_ready_guard_conditions()
+{
+  return guard_conditions_.size();
+}
+
+void
+StaticExecutorEntitiesCollector::add_node(
+  rclcpp::node_interfaces::NodeBaseInterface::SharedPtr node_ptr)
+{
+  // Check to ensure node not already added
+  for (auto & weak_node : weak_nodes_) {
+    auto node = weak_node.lock();
+    if (node == node_ptr) {
+      // TODO(jacquelinekay): Use a different error here?
+      throw std::runtime_error("Cannot add node to executor, node already added.");
+    }
+  }
+
+  weak_nodes_.push_back(node_ptr);
+  guard_conditions_.push_back(node_ptr->get_notify_guard_condition());
+}
+
+bool
+StaticExecutorEntitiesCollector::remove_node(
+  rclcpp::node_interfaces::NodeBaseInterface::SharedPtr node_ptr)
+{
+  auto node_it = weak_nodes_.begin();
+
+  while (node_it != weak_nodes_.end()) {
+    bool matched = (node_it->lock() == node_ptr);
+    if (matched) {
+      // Find and remove node and its guard condition
+      auto gc_it = std::find(
+        guard_conditions_.begin(),
+        guard_conditions_.end(),
+        node_ptr->get_notify_guard_condition());
+
+      if (gc_it != guard_conditions_.end()) {
+        guard_conditions_.erase(gc_it);
+        weak_nodes_.erase(node_it);
+        return true;
+      }
+
+      throw std::runtime_error("Didn't find guard condition associated with node.");
+
+    } else {
+      ++node_it;
+    }
+  }
+
+  return false;
+}
+
+bool
+StaticExecutorEntitiesCollector::is_ready(rcl_wait_set_t * p_wait_set)
+{
+  // Check wait_set guard_conditions for added/removed entities to/from a node
+  for (size_t i = 0; i < p_wait_set->size_of_guard_conditions; ++i) {
+    if (p_wait_set->guard_conditions[i] != NULL) {
+      // Check if the guard condition triggered belongs to a node
+      auto it = std::find(
+        guard_conditions_.begin(), guard_conditions_.end(),
+        p_wait_set->guard_conditions[i]);
+
+      // If it does, we are ready to re-collect entities
+      if (it != guard_conditions_.end()) {
+        return true;
+      }
+    }
+  }
+  // None of the guard conditions triggered belong to a registered node
+  return false;
+}

rclcpp/src/rclcpp/executors/static_single_threaded_executor.cpp

@@ -0,0 +1,144 @@
+// Copyright 2019 Nobleo Technology
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "rclcpp/executors/static_single_threaded_executor.hpp"
+
+#include <memory>
+
+#include "rclcpp/scope_exit.hpp"
+
+using rclcpp::executors::StaticSingleThreadedExecutor;
+using rclcpp::executor::ExecutableList;
+
+StaticSingleThreadedExecutor::StaticSingleThreadedExecutor(
+  const rclcpp::executor::ExecutorArgs & args)
+: executor::Executor(args)
+{
+  entities_collector_ = std::make_shared<StaticExecutorEntitiesCollector>();
+}
+
+StaticSingleThreadedExecutor::~StaticSingleThreadedExecutor() {}
+
+void
+StaticSingleThreadedExecutor::spin()
+{
+  if (spinning.exchange(true)) {
+    throw std::runtime_error("spin() called while already spinning");
+  }
+  RCLCPP_SCOPE_EXIT(this->spinning.store(false); );
+
+  // Set memory_strategy_ and exec_list_ based on weak_nodes_
+  // Prepare wait_set_ based on memory_strategy_
+  entities_collector_->init(&wait_set_, memory_strategy_, &interrupt_guard_condition_);
+
+  while (rclcpp::ok(this->context_) && spinning.load()) {
+    // Refresh wait set and wait for work
+    entities_collector_->refresh_wait_set();
+    execute_ready_executables();
+  }
+}
+
+void
+StaticSingleThreadedExecutor::add_node(
+  rclcpp::node_interfaces::NodeBaseInterface::SharedPtr node_ptr, bool notify)
+{
+  // If the node already has an executor
+  std::atomic_bool & has_executor = node_ptr->get_associated_with_executor_atomic();
+  if (has_executor.exchange(true)) {
+    throw std::runtime_error("Node has already been added to an executor.");
+  }
+
+  if (notify) {
+    // Interrupt waiting to handle new node
+    if (rcl_trigger_guard_condition(&interrupt_guard_condition_) != RCL_RET_OK) {
+      throw std::runtime_error(rcl_get_error_string().str);
+    }
+  }
+
+  entities_collector_->add_node(node_ptr);
+}
+
+void
+StaticSingleThreadedExecutor::add_node(std::shared_ptr<rclcpp::Node> node_ptr, bool notify)
+{
+  this->add_node(node_ptr->get_node_base_interface(), notify);
+}
+
+void
+StaticSingleThreadedExecutor::remove_node(
+  rclcpp::node_interfaces::NodeBaseInterface::SharedPtr node_ptr, bool notify)
+{
+  bool node_removed = entities_collector_->remove_node(node_ptr);
+
+  if (notify) {
+    // If the node was matched and removed, interrupt waiting
+    if (node_removed) {
+      if (rcl_trigger_guard_condition(&interrupt_guard_condition_) != RCL_RET_OK) {
+        throw std::runtime_error(rcl_get_error_string().str);
+      }
+    }
+  }
+
+  std::atomic_bool & has_executor = node_ptr->get_associated_with_executor_atomic();
+  has_executor.store(false);
+}
+
+void
+StaticSingleThreadedExecutor::remove_node(std::shared_ptr<rclcpp::Node> node_ptr, bool notify)
+{
+  this->remove_node(node_ptr->get_node_base_interface(), notify);
+}
+
+void
+StaticSingleThreadedExecutor::execute_ready_executables()
+{
+  // Execute all the ready subscriptions
+  for (size_t i = 0; i < wait_set_.size_of_subscriptions; ++i) {
+    if (i < entities_collector_->get_number_of_subscriptions()) {
+      if (wait_set_.subscriptions[i]) {
+        execute_subscription(entities_collector_->get_subscription(i));
+      }
+    }
+  }
+  // Execute all the ready timers
+  for (size_t i = 0; i < wait_set_.size_of_timers; ++i) {
+    if (i < entities_collector_->get_number_of_timers()) {
+      if (wait_set_.timers[i] && entities_collector_->get_timer(i)->is_ready()) {
+        execute_timer(entities_collector_->get_timer(i));
+      }
+    }
+  }
+  // Execute all the ready services
+  for (size_t i = 0; i < wait_set_.size_of_services; ++i) {
+    if (i < entities_collector_->get_number_of_services()) {
+      if (wait_set_.services[i]) {
+        execute_service(entities_collector_->get_service(i));
+      }
+    }
+  }
+  // Execute all the ready clients
+  for (size_t i = 0; i < wait_set_.size_of_clients; ++i) {
+    if (i < entities_collector_->get_number_of_clients()) {
+      if (wait_set_.clients[i]) {
+        execute_client(entities_collector_->get_client(i));
+      }
+    }
+  }
+  // Execute all the ready waitables
+  for (size_t i = 0; i < entities_collector_->get_number_of_waitables(); ++i) {
+    if (entities_collector_->get_waitable(i)->is_ready(&wait_set_)) {
+      entities_collector_->get_waitable(i)->execute();
+    }
+  }
+}