monado/doc/ipc.md

IPC Design

• Last updated: 24-February-2021

When the service starts, an xrt_instance is created and selected, a native system compositor is initialized, a shared memory segment for device data is initialized, and other internal state is set up. (See ipc_server_process.c.)

There are three main communication needs:

• The client shared library needs to be able to locate a running service, if any, to start communication. (Auto-starting, where available, is handled by platform-specific mechanisms: the client currently has no code to explicitly start up the service.)
• The client and service must share a dedicated channel for IPC calls (aka RPC - remote procedure call), typically a socket.
• The service must share access to device data updating at various rates, shared by all clients. This is typically done with a form of shared memory.

Each platform's implementation has a way of meeting each of these needs. The specific way each need is met is highlighted below.

Linux Platform Details

In an typical Linux environment, the Monado service can be launched one of two ways: manually, or by socket activation (e.g. from systemd). In either case, there is a Unix domain socket with a well-known name (known at compile time, and built-in to both the service executable and the client shared library) used by clients to connect to the service: this provides the locating function. This socket is polled in the service mainloop, using epoll, to detect any new client connections.

Upon a client connection to this "locating" socket, the service will accept the connection, returning an FD, which is passed to start_client_listener_thread() to start a thread specific to that client. The FD produced this way is now also used for the IPC calls - the RPC function - since it is specific to that client-server communication channel. One of the first calls made transports a duplicate of the shared memory segment file descriptor to the client, so it has (read) access to this data.

Android Platform Details

On Android, in order to pass platform objects, allow for service activation, and fit better within the idioms of the platform, Monado provides a Binder/AIDL service instead of a named socket. (The named sockets we typically use are not permitted by the platform, and "abstract" named sockets are currently available, but are not idiomatic for the platform and lack other useful capabilities.) Specifically, we provide a foreground and started (to be able to display), bound service with an interface defined using AIDL. (See also this third-party guide about such AIDL services) This is not like the system services which provide hardware data or system framework data from native code. this has a Java (JVM/Dalvik/ART) component provided by code in an APK, exposed by properties in the package manifest.

NdkBinder is not used because it is mainly suitable for the system type of binder services. An APK-based service would still require some JVM code to expose it, and since the AIDL service is used for so little, mixing languages did not make sense.

The service we expose provides an implementation of our AIDL-described interface, org.freedesktop.monado.ipc.IMonado. This can be modified freely, as both the client and server are built at the same time and packaged in the same APK, even though they get loaded in different processes.

The first main purpose of this service is for automatic startup and the locating function: helping establish communication between the client and the service. The Android framework takes care of launching the service process when the client requests to start and bind our service by name and package. The framework also provides us with method calls when we're started/bound. In this way, the "entry point" of the Monado service on Android is the org.freedesktop.monado.ipc.MonadoService class, which exposes the implementation of our AIDL interface, org.freedesktop.monado.ipc.MonadoImpl.

From there, the native-code mainloop starts when this service is started. By default, the JVM code will signal the mainloop to shut down a short time after the last client disconnects, to work best within the platform.

At startup, just as on Linux, the shared memory segment is created. The ashmem API is used to create/destroy an anonymous shared memory segment on Android, instead of standard POSIX shared memory, but is otherwise treated and used exactly the same as on standard Linux: file descriptors are duplicated and passed through IPC calls, etc.

When the client side starts up, it creates an anonymous socket pair to use for IPC calls (the RPC function) later. It then passes one of the two file descriptors into the AIDL method we defined named "connect". This transports the FD to the service process, which uses it as the unique communication channel for that client in its own thread. This replaces the socket pair produced by connecting/accepting the named socket as used in standard Linux.

The AIDL interface is also used for transporting some platform objects. At this time, the only one transported in this way is the Surface injected into the client activity which is used for displaying rendered output.

Synchronization

Synchronization of new client connections is a special challenge on the Android platform, since new clients arrive via calls into JVM code while the mainloop is native code. Unlike Linux, we cannot simply use epoll to check if there are new connections to our locating socket.

We have the following design goals/constraints:

• All we need to communicate is an integer (file descriptor) within a process.
• Make it fast in the server mainloop in the most common case that there are no new clients.
  • This suggests that we should be able to check if there may be a waiting client in purely native code, without JNI.
• Make it relatively fast in the server mainloop even when there is a client, since it's the compositor thread.
  • This might mean we want to do it all without JNI on the main thread.
• The client should know (and be unblocked) when the server has accepted its connection.
  • This suggests that the method called in MonadoImpl should block until the server consumes/accepts the connection.
  • Not 100% sure this is required, but maybe.
• Resources (file descriptors, etc) should not be leaked.
  • Each should have a well-known owner at each point in time.
• It is OK if only one new client is accepted per mainloop.
  • The mainloop is high rate (compositor rate) and new client connections are relatively infrequent.

The IPC service creates a pipe as well as some state variables, two mutexes, and a condition variable.

When the JVM Service code has a new client, it calls ipc_server_mainloop_add_fd() to pass the FD in. It takes two mutexes, in order: ipc_server_mainloop::client_push_mutex and ipc_server_mainloop::accept_mutex. The purpose of ipc_server_mainloop::client_push_mutex is to allow only one client into the client-acceptance handshake at a time, so that no acknowledgement of client accept is lost. Once those two mutexes are locked, ipc_server_mainloop_add_fd() writes the FD number to the pipe. Then, it waits on the condition variable (releasing accept_mutex) to see either that FD number or the special "shutting down" sentinel value in the last_accepted_fd variable. If it sees the FD number, that indicates that the other side of the communication (the mainloop) has taken ownership of the FD and will handle closing it. If it sees the sentinel value, or has an error at some point, it assumes that ownership is retained and it should close the FD itself.

The other side of the communication works as follows: epoll is used to check if there is new data waiting on the pipe. If so, the ipc_server_mainloop::accept_mutex lock is taken, and an FD number is read from the pipe. A client thread is launched for that FD, then the last_accepted_fd variable is updated and the ipc_server_mainloop::accept_cond condition variable signalled.

The initial plan required that the server also wait on ipc_server_mainloop::accept_cond for the last_accepted_fd to be reset back to 0 by the acknowledged client, thus preventing losing acknowledgements. However, it is undesirable for the clients to be able to block the compositor/server, so this wait was considered not acceptable. Instead, the ipc_server_mainloop::client_push_mutex is used so that at most one un-acknowledged client may have written to the pipe at any given time.