After an offline discussion with @wjwwood and @tfoote, I've done some refactoring.
Now, the new base class for BufferCore is named BufferCoreInterface for consistency.
The waitForTransform API has been moved to tf2_ros::BufferInterface, which is the ROS-aware (ie. time aware) interface on top of BufferCore.
I've also done a few other small things to make this PR slightly less intrusive to the existing code.

I still need to implement the timeout for waitForTransform.

In order to implement the asynchronous timeout I've introduced the interface CreateTimerInterface that is registered with tf2_ros::Buffer after construction (@wjwwood as per our offline discussion).

I realized that tf2::TransformListener does not take a tf2::Buffer as an argument so, rather than changing it's constructor, I've left it alone and set the CreateTimerInterface in the test:

I realized that tf2::TransformListener does not take a tf2::Buffer as an argument so, rather than changing it's constructor, I've left it alone and set the CreateTimerInterface in the test:

What? the tf2_ros::TransformListener takes the tf2_ros::Buffer as the first argument in the constructor. I don't entirely follow why the tf2_ros::TransformListener cannot call this method on the buffer? You said specifically tf2 and not tf2_ros in your message, is that the difference?

What? the tf2_ros::TransformListener takes the tf2_ros::Buffer as the first argument in the constructor. I don't entirely follow why the tf2_ros::TransformListener cannot call this method on the buffer? You said specifically tf2 and not tf2_ros in your message, is that the difference?

I meant tf2_ros::Buffer. tf2_ros::TransformerListener accepts a tf2::BufferCore (not a tf2_ros::Buffer), so we can't register the new timer interface from inside the transform listener (unless we change the signature).

Edit: We can pass a tf2_ros::Buffer because it is extending tf2::BufferCore

I see what you mean now. Oof. Does it make sense to move all the stuff (timeout based waitFor and the timer creation interface) into tf2::Buffer? I guess not huh?

I suppose we could add a new signature for tf2_ros::TransformListener that takes a tf2_ros::Buffer instead and only sets up the interface if that signature is used. I think most people will be passing an instance of the tf2_ros::Buffer anyways, and if not the new waitFor will not work unless they do so.

Yeah, I don't think it makes sense to add the new async API to tf2::BufferCore since it is time agnostic.

Unless there's a use-case for passing a tf2::BufferCore, I would change the type passed to tf2_ros::TransformListener to tf2_ros::Buffer. But maybe it's safer to add a new signature as you suggest.

Only if you wanted to backport it and not break API. Otherwise, unless @tfoote sees a different reason, I think changing it would be ok.

I've already broken API by changing the signature of tf2_ros::MessageFilter, which I don't think is as straightforward to make backwards compatible; I'd have to think about it.

I think this is ready for review. Here's a description of the changes:

• Add pure virtual interfacetf2::BufferCoreInterface and have tf2::BufferCore implement it.
  It is required that the object passed to tf2_ros::MessageFilter implements this interface.
  The interface signature is very similar to the methods already in tf2::BufferCore to keep the diff smaller.

• Add pure virtual interface tf2_ros::AsyncBufferInterface and have tf2_ros::Buffer implement it.
  The new interface declares a single method waitForTransform.
  It is required that the object passed to tf2_ros::MessageFilter implements this interface.
  Since tf2 doesn't have a sense of time, tf2_ros seems to be the appropriate place to put the interface. An alternative approach is to add the async method to the existing tf2_ros::BufferInterface, but since tf2_ros::MessageFilter doesn't need any of the other interface methods, I chose to create a new interface. This cuts down on the number of methods that need to be implemented by third-parties (e.g. RViz).

• Add pure virtual interface tf2_ros::CreateTimerInterface and concrete class tf2_ros::CreateTimerROS.
  Added a method tf2_ros::Buffer::setCreateTimerInterface() for registering the interface, which is required for computing timeouts for the new asynchronous method (AsyncBufferInterface::waitForTransform()).

• tf2_ros::MessageFilter signature templated on the Buffer type.
  Static assertions are added to ensure the template type implements tf2::BufferCoreInterface and tf2_ros::AsyncBufferInterface.

Pending review and approval, I'll curate the commits so that they reflect the above description.

See proposed usage in RViz: ros2/rviz#422

Packages above tf2 and tf2_ros:

• Linux
• Linux-aarch64
• macOS
• Windows

Edit: Also need to build up to tf2_ros

• Linux
• Linux-aarch64
• macOS
• Windows

One of the new tests exposed an existing bug. A PR with a fix is open: #127

I also made a couple other small fixes with this PR. @wjwwood PTAL

I've rebased to get the fix from #127. The last three commits are new since the last review.

• Linux
• Linux-aarch64
• macOS
• Windows

@jacobperron this code is not compiling for me and I'm unable to figure out why. The trouble seems to be with the added static_asserts where my compiler can't find the tf2::BufferCoreInterface name, even though there's a clear header chain that includes tf2/buffer_core_interface.h:

[Processing: tf2_ros]
--- stderr: tf2_ros
In file included from /home/jlangsfeld/Workspaces/ros2_ws/src/geometry2/tf2_ros/test/message_filter_test.cpp:39:0:
/home/jlangsfeld/Workspaces/ros2_ws/src/geometry2/tf2_ros/include/tf2_ros/message_filter.h: In constructor ‘tf2_ros::MessageFilter<M, BufferT>::MessageFilter(BufferT&, const string&, uint32_t, const SharedPtr&, std::chrono::duration<_Rep, _Period>)’:
/home/jlangsfeld/Workspaces/ros2_wssrc/geometry2/tf2_ros/include/tf2_ros/message_filter.h:135:40: error: ‘BufferCoreInterface’ is not a member of ‘tf2’
     static_assert(std::is_base_of<tf2::BufferCoreInterface, BufferT>::value,
                                        ^~~~~~~~~~~~~~~~~~~
/home/jlangsfeld/Workspaces/ros2_ws/src/geometry2/tf2_ros/include/tf2_ros/message_filter.h:135:40: note: suggested alternative: ‘TimeCacheInterface’
     static_assert(std::is_base_of<tf2::BufferCoreInterface, BufferT>::value,
                                        ^~~~~~~~~~~~~~~~~~~
                                        TimeCacheInterface

If I explicitly add #include <tf2/buffer_core_interface.h> to message_filter.h, this error goes away, but the static_assert fails even though there's a clear inheritance chain from tf2_ros::Buffer to tf2::BufferCoreInterface:

--- stderr: tf2_ros
In file included from /home/jlangsfeld/Workspaces/ros2_ws/src/geometry2/tf2_ros/test/message_filter_test.cpp:39:0:
/home/jlangsfeld/Workspaces/ros2_ws/src/geometry2/tf2_ros/include/tf2_ros/message_filter.h: In instantiation of ‘tf2_ros::MessageFilter<M, BufferT>::MessageFilter(BufferT&, const string&, uint32_t, const SharedPtr&, std::chrono::duration<_Rep, _Period>) [with TimeRepT = long int; TimeT = std::ratio<1, 1000000000>; M = geometry_msgs::msg::PointStamped_<std::allocator<void> >; BufferT = tf2_ros::Buffer; std::__cxx11::string = std::__cxx11::basic_string<char>; uint32_t = unsigned int; rclcpp::Node::SharedPtr = std::shared_ptr<rclcpp::Node>]’:
/home/jlangsfeld/Workspaces/ros2_ws/src/geometry2/tf2_ros/test/message_filter_test.cpp:59:92:   required from here
/home/jlangsfeld/Workspaces/ros2_ws/src/geometry2/tf2_ros/include/tf2_ros/message_filter.h:136:5: error: static assertion failed: Buffer type must implement tf2::BufferCoreInterface
     static_assert(std::is_base_of<tf2::BufferCoreInterface, BufferT>::value,

Any suggestions?

@jdlangs I can reproduce the error if I source a binary installation of Dashing and then try to compile geometry2. If I build ROS 2 from source, then I do not run into this issue. I suspect there's a mix-up happening with the Dashing installation, which doesn't contain the changes introduced in this PR, where tf2_ros is linking against the binary installation of tf2. Unfortunately, I believe in order to get the tf2_ros library to link against the latest version of tf2, we also need to have source builds of its other direct dependencies (rclcpp, message_filters, and geometry_msgs). Because there have been API/ABI breaking changes to rclcpp and its dependencies since Dashing, I suggest building ROS 2 from source if you want to take advantage of this change.

Edit: fixed in #156

Since the message filter is referencing tf2::BufferCoreInterface, we better include the appropriate header in any case.

@jdlangs Nevermind, this is a bug that can be resolved by changing the order of linked dependencies. See #156 for a fix.