.. _module-pw_rpc:

======
pw_rpc
======
The ``pw_rpc`` module provides a system for defining and invoking remote
procedure calls (RPCs) on a device.

This document discusses the ``pw_rpc`` protocol and its C++ implementation.
``pw_rpc`` implementations for other languages are described in their own
documents:

.. toctree::
   :maxdepth: 1

   py/docs
   ts/docs

.. admonition:: Try it out!

   For a quick intro to ``pw_rpc``, see the
   :ref:`module-pw_hdlc-rpc-example` in the :ref:`module-pw_hdlc` module.

.. warning::

   This documentation is under construction. Many sections are outdated or
   incomplete. The content needs to be reorgnanized.

---------------
Implementations
---------------
Pigweed provides several client and server implementations of ``pw_rpc``.

.. list-table::
  :header-rows: 1

  * - Language
    - Server
    - Client
  * - C++ (raw)
    - ✅
    - ✅
  * - C++ (Nanopb)
    - ✅
    - ✅
  * - C++ (pw_protobuf)
    - ✅
    - ✅
  * - Java
    -
    - ✅
  * - Python
    -
    - ✅
  * - TypeScript
    -
    - in development

.. warning::

  ``pw_protobuf`` and ``nanopb`` RPC services cannot currently coexist within the
  same RPC server. Unless you are running multiple RPC servers, you cannot
  incrementally migrate services from one protobuf implementation to another,
  or otherwise mix and match. See
  `Issue 234874320 <https://issues.pigweed.dev/issues/234874320>`_.

-------------
RPC semantics
-------------
The semantics of ``pw_rpc`` are similar to `gRPC
<https://grpc.io/docs/what-is-grpc/core-concepts/>`_.

RPC call lifecycle
==================
In ``pw_rpc``, an RPC begins when the client sends an initial packet. The server
receives the packet, looks up the relevant service method, then calls into the
RPC function. The RPC is considered active until the server sends a status to
finish the RPC. The client may terminate an ongoing RPC by cancelling it.
Multiple concurrent RPC requests to the same method may be made simultaneously
(Note: Concurrent requests are not yet possible using the Java client. See
`Issue 237418397 <https://issues.pigweed.dev/issues/237418397>`_).

Depending the type of RPC, the client and server exchange zero or more protobuf
request or response payloads. There are four RPC types:

* **Unary**. The client sends one request and the server sends one
  response with a status.
* **Server streaming**. The client sends one request and the server sends zero
  or more responses followed by a status.
* **Client streaming**. The client sends zero or more requests and the server
  sends one response with a status.
* **Bidirectional streaming**. The client sends zero or more requests and the
  server sends zero or more responses followed by a status.

Events
------
The key events in the RPC lifecycle are:

* **Start**. The client initiates the RPC. The server's RPC body executes.
* **Finish**. The server sends a status and completes the RPC. The client calls
  a callback.
* **Request**. The client sends a request protobuf. The server calls a callback
  when it receives it. In unary and server streaming RPCs, there is only one
  request and it is handled when the RPC starts.
* **Response**. The server sends a response protobuf. The client calls a
  callback when it receives it. In unary and client streaming RPCs, there is
  only one response and it is handled when the RPC completes.
* **Error**. The server or client terminates the RPC abnormally with a status.
  The receiving endpoint calls a callback.
* **Request Completion**. The client sends a message that it would like to
  request call completion. The server calls a callback when it receives it. Some
  servers may ignore the request completion message. In client and bidirectional
  streaming RPCs, this also indicates that client has finished sending requests.

Status codes
============
``pw_rpc`` call objects (``ClientReaderWriter``, ``ServerReaderWriter``, etc.)
use certain status codes to indicate what occurred. These codes are returned
from functions like ``Write()`` or ``Finish()``.

* ``OK`` -- The operation succeeded.
* ``UNAVAILABLE`` -- The channel is not currently registered with the server or
  client.
* ``UNKNOWN`` -- Sending a packet failed due to an unrecoverable
  :cpp:func:`pw::rpc::ChannelOutput::Send` error.

Unrequested responses
=====================
``pw_rpc`` supports sending responses to RPCs that have not yet been invoked by
a client. This is useful in testing and in situations like an RPC that triggers
reboot. After the reboot, the device opens the writer object and sends its
response to the client.

The C++ API for opening a server reader/writer takes the generated RPC function
as a template parameter. The server to use, channel ID, and service instance are
passed as arguments. The API is the same for all RPC types, except the
appropriate reader/writer class must be used.

.. code-block:: c++

   // Open a ServerWriter for a server streaming RPC.
   auto writer = RawServerWriter::Open<pw_rpc::raw::ServiceName::MethodName>(
       server, channel_id, service_instance);

   // Send some responses, even though the client has not yet called this RPC.
   CHECK_OK(writer.Write(encoded_response_1));
   CHECK_OK(writer.Write(encoded_response_2));

   // Finish the RPC.
   CHECK_OK(writer.Finish(OkStatus()));

Errata
------
Prior to support for concurrent requests to a single method, no identifier was
present to distinguish different calls to the same method. When a "call ID"
feature was first introduced to solve this issue, existing clients and servers
(1) set this value to zero and (2) ignored this value.

When initial support for concurrent methods was added, a separate
"open call ID" was introduced to distinguish unrequested responses. However,
legacy servers built prior to this change continue to send unrequested
responses with call ID zero. Prior to
`this fix <https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/192311>`,
clients which used "open call ID" would not accept unrequested responses from
legacy servers. Clients built after that change will accept unrequested
responses which use both "open call ID" and call ID zero.

See
`Issue 237418397 <https://issues.pigweed.dev/issues/237418397>`_
for more details and discussion.

---------------
Creating an RPC
---------------

1. RPC service declaration
==========================
Pigweed RPCs are declared in a protocol buffer service definition.

* `Protocol Buffer service documentation
  <https://developers.google.com/protocol-buffers/docs/proto3#services>`_
* `gRPC service definition documentation
  <https://grpc.io/docs/what-is-grpc/core-concepts/#service-definition>`_

.. code-block:: protobuf

   syntax = "proto3";

   package foo.bar;

   message Request {}

   message Response {
     int32 number = 1;
   }

   service TheService {
     rpc MethodOne(Request) returns (Response) {}
     rpc MethodTwo(Request) returns (stream Response) {}
   }

This protocol buffer is declared in a ``BUILD.gn`` file as follows:

.. code-block:: python

   import("//build_overrides/pigweed.gni")
   import("$dir_pw_protobuf_compiler/proto.gni")

   pw_proto_library("the_service_proto") {
     sources = [ "foo_bar/the_service.proto" ]
   }

.. admonition:: proto2 or proto3 syntax?

   Always use proto3 syntax rather than proto2 for new protocol buffers. Proto2
   protobufs can be compiled for ``pw_rpc``, but they are not as well supported
   as proto3. Specifically, ``pw_rpc`` lacks support for non-zero default values
   in proto2. When using Nanopb with ``pw_rpc``, proto2 response protobufs with
   non-zero field defaults should be manually initialized to the default struct.

   In the past, proto3 was sometimes avoided because it lacked support for field
   presence detection. Fortunately, this has been fixed: proto3 now supports
   ``optional`` fields, which are equivalent to proto2 ``optional`` fields.

   If you need to distinguish between a default-valued field and a missing field,
   mark the field as ``optional``. The presence of the field can be detected
   with ``std::optional``, a ``HasField(name)``, or ``has_<field>`` member,
   depending on the library.

   Optional fields have some overhead --- if using Nanopb, default-valued fields
   are included in the encoded proto, and the proto structs have a
   ``has_<field>`` flag for each optional field. Use plain fields if field
   presence detection is not needed.

   .. code-block:: protobuf

      syntax = "proto3";

      message MyMessage {
        // Leaving this field unset is equivalent to setting it to 0.
        int32 number = 1;

        // Setting this field to 0 is different from leaving it unset.
        optional int32 other_number = 2;
      }

2. RPC code generation
======================
``pw_rpc`` generates a C++ header file for each ``.proto`` file. This header is
generated in the build output directory. Its exact location varies by build
system and toolchain, but the C++ include path always matches the sources
declaration in the ``pw_proto_library``. The ``.proto`` extension is replaced
with an extension corresponding to the protobuf library in use.

================== =============== =============== =============
Protobuf libraries Build subtarget Protobuf header pw_rpc header
================== =============== =============== =============
Raw only           .raw_rpc        (none)          .raw_rpc.pb.h
Nanopb or raw      .nanopb_rpc     .pb.h           .rpc.pb.h
pw_protobuf or raw .pwpb_rpc       .pwpb.h         .rpc.pwpb.h
================== =============== =============== =============

For example, the generated RPC header for ``"foo_bar/the_service.proto"`` is
``"foo_bar/the_service.rpc.pb.h"`` for Nanopb or
``"foo_bar/the_service.raw_rpc.pb.h"`` for raw RPCs.

The generated header defines a base class for each RPC service declared in the
``.proto`` file. A service named ``TheService`` in package ``foo.bar`` would
generate the following base class for pw_protobuf:

.. cpp:class:: template <typename Implementation> foo::bar::pw_rpc::pwpb::TheService::Service

3. RPC service definition
=========================
The serivce class is implemented by inheriting from the generated RPC service
base class and defining a method for each RPC. The methods must match the name
and function signature for one of the supported protobuf implementations.
Services may mix and match protobuf implementations within one service.

.. tip::

   The generated code includes RPC service implementation stubs. You can
   reference or copy and paste these to get started with implementing a service.
   These stub classes are generated at the bottom of the pw_rpc proto header.

   To use the stubs, do the following:

   #. Locate the generated RPC header in the build directory. For example:

      .. code-block:: sh

         find out/ -name <proto_name>.rpc.pwpb.h

   #. Scroll to the bottom of the generated RPC header.
   #. Copy the stub class declaration to a header file.
   #. Copy the member function definitions to a source file.
   #. Rename the class or change the namespace, if desired.
   #. List these files in a build target with a dependency on the
      ``pw_proto_library``.

A pw_protobuf implementation of this service would be as follows:

.. code-block:: cpp

   #include "foo_bar/the_service.rpc.pwpb.h"

   namespace foo::bar {

   class TheService : public pw_rpc::pwpb::TheService::Service<TheService> {
    public:
     pw::Status MethodOne(const Request::Message& request,
                          Response::Message& response) {
       // implementation
       response.number = 123;
       return pw::OkStatus();
     }

     void MethodTwo(const Request::Message& request,
                    ServerWriter<Response::Message>& response) {
       // implementation
       response.Write({.number = 123});
     }
   };

   }  // namespace foo::bar

The pw_protobuf implementation would be declared in a ``BUILD.gn``:

.. code-block:: python

   import("//build_overrides/pigweed.gni")

   import("$dir_pw_build/target_types.gni")

   pw_source_set("the_service") {
     public_configs = [ ":public" ]
     public = [ "public/foo_bar/service.h" ]
     public_deps = [ ":the_service_proto.pwpb_rpc" ]
   }

4. Register the service with a server
=====================================
This example code sets up an RPC server with an :ref:`HDLC<module-pw_hdlc>`
channel output and the example service.

.. code-block:: cpp

   // Set up the output channel for the pw_rpc server to use. This configures the
   // pw_rpc server to use HDLC over UART; projects not using UART and HDLC must
   // adapt this as necessary.
   pw::stream::SysIoWriter writer;
   pw::rpc::FixedMtuChannelOutput<kMaxTransmissionUnit> hdlc_channel_output(
       writer, pw::hdlc::kDefaultRpcAddress, "HDLC output");

   // Allocate an array of channels for the server to use. If dynamic allocation
   // is enabled (PW_RPC_DYNAMIC_ALLOCATION=1), the server can be initialized
   // without any channels, and they can be added later.
   pw::rpc::Channel channels[] = {
       pw::rpc::Channel::Create<1>(&hdlc_channel_output)};

   // Declare the pw_rpc server with the HDLC channel.
   pw::rpc::Server server(channels);

   foo::bar::TheService the_service;
   pw::rpc::SomeOtherService some_other_service;

   void RegisterServices() {
     // Register the foo.bar.TheService example service and another service.
     server.RegisterService(the_service, some_other_service);
   }

   int main() {
     // Set up the server.
     RegisterServices();

     // Declare a buffer for decoding incoming HDLC frames.
     constexpr size_t kDecoderBufferSize =
       pw::hdlc::Decoder::RequiredBufferSizeForFrameSize(kMaxTransmissionUnit);

     std::array<std::byte, kDecoderBufferSize> input_buffer;

     PW_LOG_INFO("Starting pw_rpc server");
     pw::hdlc::Decoder decoder(input_buffer);

     while (true) {
       std::byte byte;
       pw::Status ret_val = pw::sys_io::ReadByte(&byte);
       if (!ret_val.ok()) {
         return ret_val;
       }
       if (auto result = decoder.Process(byte); result.ok()) {
         pw::hdlc::Frame& frame = result.value();
         if (frame.address() == pw::hdlc::kDefaultRpcAddress) {
           server.ProcessPacket(frame.data());
         }
       }
     }
   }

--------
Channels
--------
``pw_rpc`` sends all of its packets over channels. These are logical,
application-layer routes used to tell the RPC system where a packet should go.

Channels over a client-server connection must all have a unique ID, which can be
assigned statically at compile time or dynamically.

.. code-block:: cpp

   // Creating a channel with the static ID 3.
   pw::rpc::Channel static_channel = pw::rpc::Channel::Create<3>(&output);

   // Grouping channel IDs within an enum can lead to clearer code.
   enum ChannelId {
     kUartChannel = 1,
     kSpiChannel = 2,
   };

   // Creating a channel with a static ID defined within an enum.
   pw::rpc::Channel another_static_channel =
       pw::rpc::Channel::Create<ChannelId::kUartChannel>(&output);

   // Creating a channel with a dynamic ID (note that no output is provided; it
   // will be set when the channel is used.
   pw::rpc::Channel dynamic_channel;

Sometimes, the ID and output of a channel are not known at compile time as they
depend on information stored on the physical device. To support this use case, a
dynamically-assignable channel can be configured once at runtime with an ID and
output.

.. code-block:: cpp

   // Create a dynamic channel without a compile-time ID or output.
   pw::rpc::Channel dynamic_channel;

   void Init() {
     // Called during boot to pull the channel configuration from the system.
     dynamic_channel.Configure(GetChannelId(), some_output);
   }

Adding and removing channels
============================
New channels may be registered with the ``OpenChannel`` function. If dynamic
allocation is enabled (:c:macro:`PW_RPC_DYNAMIC_ALLOCATION` is 1), any number of
channels may be registered. If dynamic allocation is disabled, new channels may
only be registered if there are availale channel slots in the span provided to
the RPC endpoint at construction.

A channel may be closed and unregistered with an endpoint by calling
``ChannelClose`` on the endpoint with the corresponding channel ID.  This
will terminate any pending calls and call their ``on_error`` callback
with the ``ABORTED`` status.

.. code-block:: cpp

   // When a channel is closed, any pending calls will receive
   // on_error callbacks with ABORTED status.
   client->CloseChannel(1);

.. _module-pw_rpc-remap:

Remapping channels
==================
Some pw_rpc deployments may find it helpful to remap channel IDs in RPC packets.
This can remove the need for globally known channel IDs. Clients can use a
generic channel ID. The server remaps the generic channel ID to an ID associated
with the transport the client is using.

.. doxygengroup:: pw_rpc_channel_functions
   :content-only:

A future revision of the pw_rpc protocol will remove the need for global channel
IDs without requiring remapping.

--------
Services
--------
A service is a logical grouping of RPCs defined within a .proto file. ``pw_rpc``
uses these .proto definitions to generate code for a base service, from which
user-defined RPCs are implemented.

``pw_rpc`` supports multiple protobuf libraries, and the generated code API
depends on which is used.

Services must be registered with a server in order to call their methods.
Services may later be unregistered, which aborts calls for methods in that
service and prevents future calls to them, until the service is re-registered.

.. _module-pw_rpc-protobuf-library-apis:

---------------------
Protobuf library APIs
---------------------

.. toctree::
   :maxdepth: 1

   pwpb/docs
   nanopb/docs

----------------------------
Testing a pw_rpc integration
----------------------------
After setting up a ``pw_rpc`` server in your project, you can test that it is
working as intended by registering the provided ``EchoService``, defined in
``echo.proto``, which echoes back a message that it receives.

.. literalinclude:: echo.proto
   :language: protobuf
   :lines: 14-

For example, in C++ with pw_protobuf:

.. code-block:: c++

   #include "pw_rpc/server.h"

   // Include the apporpriate header for your protobuf library.
   #include "pw_rpc/echo_service_pwpb.h"

   constexpr pw::rpc::Channel kChannels[] = { /* ... */ };
   static pw::rpc::Server server(kChannels);

   static pw::rpc::EchoService echo_service;

   void Init() {
     server.RegisterService(echo_service);
   }

Benchmarking and stress testing
===============================

.. toctree::
   :maxdepth: 1
   :hidden:

   benchmark

``pw_rpc`` provides an RPC service and Python module for stress testing and
benchmarking a ``pw_rpc`` deployment. See :ref:`module-pw_rpc-benchmark`.

------
Naming
------

Reserved names
==============
``pw_rpc`` reserves a few service method names so they can be used for generated
classes. The following names cannnot be used for service methods:

- ``Client``
- ``Service``
- Any reserved words in the languages ``pw_rpc`` supports (e.g. ``class``).

``pw_rpc`` does not reserve any service names, but the restriction of avoiding
reserved words in supported languages applies.

Service naming style
====================
``pw_rpc`` service names should use capitalized camel case and should not use
the term "Service". Appending "Service" to a service name is redundant, similar
to appending "Class" or "Function" to a class or function name. The
C++ implementation class may use "Service" in its name, however.

For example, a service for accessing a file system should simply be named
``service FileSystem``, rather than ``service FileSystemService``, in the
``.proto`` file.

.. code-block:: protobuf

   // file.proto
   package pw.file;

   service FileSystem {
       rpc List(ListRequest) returns (stream ListResponse);
   }

The C++ service implementation class may append "Service" to the name.

.. code-block:: cpp

   // file_system_service.h
   #include "pw_file/file.raw_rpc.pb.h"

   namespace pw::file {

   class FileSystemService : public pw_rpc::raw::FileSystem::Service<FileSystemService> {
     void List(ConstByteSpan request, RawServerWriter& writer);
   };

   }  // namespace pw::file

For upstream Pigweed services, this naming style is a requirement. Note that
some services created before this was established may use non-compliant
names. For Pigweed users, this naming style is a suggestion.

------------------------------
C++ payload sizing limitations
------------------------------
The individual size of each sent RPC request or response is limited by
``pw_rpc``'s ``PW_RPC_ENCODING_BUFFER_SIZE_BYTES`` configuration option when
using Pigweed's C++ implementation. While multiple RPC messages can be enqueued
(as permitted by the underlying transport), if a single individual sent message
exceeds the limitations of the statically allocated encode buffer, the packet
will fail to encode and be dropped.

This applies to all C++ RPC service implementations (nanopb, raw, and pwpb),
so it's important to ensure request and response message sizes do not exceed
this limitation.

As ``pw_rpc`` has some additional encoding overhead, a helper,
``pw::rpc::MaxSafePayloadSize()`` is provided to expose the practical max RPC
message payload size.

.. code-block:: cpp

   #include "pw_file/file.raw_rpc.pb.h"
   #include "pw_rpc/channel.h"

   namespace pw::file {

   class FileSystemService : public pw_rpc::raw::FileSystem::Service<FileSystemService> {
    public:
     void List(ConstByteSpan request, RawServerWriter& writer);

    private:
     // Allocate a buffer for building proto responses.
     static constexpr size_t kEncodeBufferSize = pw::rpc::MaxSafePayloadSize();
     std::array<std::byte, kEncodeBufferSize> encode_buffer_;
   };

   }  // namespace pw::file

--------------------
Protocol description
--------------------
Pigweed RPC servers and clients communicate using ``pw_rpc`` packets. These
packets are used to send requests and responses, control streams, cancel ongoing
RPCs, and report errors.

Packet format
=============
Pigweed RPC packets consist of a type and a set of fields. The packets are
encoded as protocol buffers. The full packet format is described in
``pw_rpc/pw_rpc/internal/packet.proto``.

.. literalinclude:: internal/packet.proto
   :language: protobuf
   :lines: 14-

The packet type and RPC type determine which fields are present in a Pigweed RPC
packet. Each packet type is only sent by either the client or the server.
These tables describe the meaning of and fields included with each packet type.

Client-to-server packets
------------------------
+---------------------------+-------------------------------------+
| packet type               | description                         |
+===========================+=====================================+
| REQUEST                   | Invoke an RPC                       |
|                           |                                     |
|                           | .. code-block:: text                |
|                           |                                     |
|                           |   - channel_id                      |
|                           |   - service_id                      |
|                           |   - method_id                       |
|                           |   - payload                         |
|                           |     (unary & server streaming only) |
|                           |   - call_id (optional)              |
|                           |                                     |
+---------------------------+-------------------------------------+
| CLIENT_STREAM             | Message in a client stream          |
|                           |                                     |
|                           | .. code-block:: text                |
|                           |                                     |
|                           |   - channel_id                      |
|                           |   - service_id                      |
|                           |   - method_id                       |
|                           |   - payload                         |
|                           |   - call_id (if set in REQUEST)     |
|                           |                                     |
+---------------------------+-------------------------------------+
| CLIENT_REQUEST_COMPLETION | Client requested stream completion  |
|                           |                                     |
|                           | .. code-block:: text                |
|                           |                                     |
|                           |   - channel_id                      |
|                           |   - service_id                      |
|                           |   - method_id                       |
|                           |   - call_id (if set in REQUEST)     |
|                           |                                     |
+---------------------------+-------------------------------------+
| CLIENT_ERROR              | Abort an ongoing RPC                |
|                           |                                     |
|                           | .. code-block:: text                |
|                           |                                     |
|                           |   - channel_id                      |
|                           |   - service_id                      |
|                           |   - method_id                       |
|                           |   - status                          |
|                           |   - call_id (if set in REQUEST)     |
|                           |                                     |
+---------------------------+-------------------------------------+

**Client errors**

The client sends ``CLIENT_ERROR`` packets to a server when it receives a packet
it did not request. If possible, the server should abort it.

The status code indicates the type of error. The status code is logged, but all
status codes result in the same action by the server: aborting the RPC.

* ``CANCELLED`` -- The client requested that the RPC be cancelled.
* ``ABORTED`` -- The RPC was aborted due its channel being closed.
* ``NOT_FOUND`` -- Received a packet for a service method the client does not
  recognize.
* ``FAILED_PRECONDITION`` -- Received a packet for a service method that the
  client did not invoke.
* ``DATA_LOSS`` -- Received a corrupt packet for a pending service method.
* ``INVALID_ARGUMENT`` -- The server sent a packet type to an RPC that does not
  support it (a ``SERVER_STREAM`` was sent to an RPC with no server stream).
* ``UNAVAILABLE`` -- Received a packet for an unknown channel.

Server-to-client packets
------------------------
+-------------------+-------------------------------------+
| packet type       | description                         |
+===================+=====================================+
| RESPONSE          | The RPC is complete                 |
|                   |                                     |
|                   | .. code-block:: text                |
|                   |                                     |
|                   |   - channel_id                      |
|                   |   - service_id                      |
|                   |   - method_id                       |
|                   |   - status                          |
|                   |   - payload                         |
|                   |     (unary & client streaming only) |
|                   |   - call_id (if set in REQUEST)     |
|                   |                                     |
+-------------------+-------------------------------------+
| SERVER_STREAM     | Message in a server stream          |
|                   |                                     |
|                   | .. code-block:: text                |
|                   |                                     |
|                   |   - channel_id                      |
|                   |   - service_id                      |
|                   |   - method_id                       |
|                   |   - payload                         |
|                   |   - call_id (if set in REQUEST)     |
|                   |                                     |
+-------------------+-------------------------------------+
| SERVER_ERROR      | Received unexpected packet          |
|                   |                                     |
|                   | .. code-block:: text                |
|                   |                                     |
|                   |   - channel_id                      |
|                   |   - service_id (if relevant)        |
|                   |   - method_id (if relevant)         |
|                   |   - status                          |
|                   |   - call_id (if set in REQUEST)     |
|                   |                                     |
+-------------------+-------------------------------------+

All server packets contain the same ``call_id`` that was set in the initial
request made by the client, if any.

**Server errors**

The server sends ``SERVER_ERROR`` packets when it receives a packet it cannot
process. The client should abort any RPC for which it receives an error. The
status field indicates the type of error.

* ``NOT_FOUND`` -- The requested service or method does not exist.
* ``FAILED_PRECONDITION`` -- A client stream or cancel packet was sent for an
  RPC that is not pending.
* ``INVALID_ARGUMENT`` -- The client sent a packet type to an RPC that does not
  support it (a ``CLIENT_STREAM`` was sent to an RPC with no client stream).
* ``RESOURCE_EXHAUSTED`` -- The request came on a new channel, but a channel
  could not be allocated for it.
* ``ABORTED`` -- The RPC was aborted due its channel being closed.
* ``INTERNAL`` -- The server was unable to respond to an RPC due to an
  unrecoverable internal error.
* ``UNAVAILABLE`` -- Received a packet for an unknown channel.

Invoking a service method
=========================
Calling an RPC requires a specific sequence of packets. This section describes
the protocol for calling service methods of each type: unary, server streaming,
client streaming, and bidirectional streaming.

The basic flow for all RPC invocations is as follows:

* Client sends a ``REQUEST`` packet. Includes a payload for unary & server
  streaming RPCs.
* For client and bidirectional streaming RPCs, the client may send any number of
  ``CLIENT_STREAM`` packets with payloads.
* For server and bidirectional streaming RPCs, the server may send any number of
  ``SERVER_STREAM`` packets.
* The server sends a ``RESPONSE`` packet. Includes a payload for unary & client
  streaming RPCs. The RPC is complete.

The client may cancel an ongoing RPC at any time by sending a ``CLIENT_ERROR``
packet with status ``CANCELLED``. The server may finish an ongoing RPC at any
time by sending the ``RESPONSE`` packet.

Unary RPC
---------
In a unary RPC, the client sends a single request and the server sends a single
response.

.. mermaid::
   :alt: Unary RPC
   :align: center

   sequenceDiagram
       participant C as Client
       participant S as Server
       C->>+S: request
       Note left of C: PacketType.REQUEST<br>channel ID<br>service ID<br>method ID<br>payload

       S->>-C: response
       Note right of S: PacketType.RESPONSE<br>channel ID<br>service ID<br>method ID<br>payload<br>status

The client may attempt to cancel a unary RPC by sending a ``CLIENT_ERROR``
packet with status ``CANCELLED``. The server sends no response to a cancelled
RPC. If the server processes the unary RPC synchronously (the handling thread
sends the response), it may not be possible to cancel the RPC.

.. mermaid::
   :alt: Cancelled Unary RPC
   :align: center

   sequenceDiagram
       participant C as Client
       participant S as Server
       C->>+S: request
       Note left of C: PacketType.REQUEST<br>channel ID<br>service ID<br>method ID<br>payload

       C->>S: cancel
       Note left of C: PacketType.CLIENT_ERROR<br>channel ID<br>service ID<br>method ID<br>status=CANCELLED


Server streaming RPC
--------------------
In a server streaming RPC, the client sends a single request and the server
sends any number of ``SERVER_STREAM`` packets followed by a ``RESPONSE`` packet.

.. mermaid::
   :alt: Server Streaming RPC
   :align: center

   sequenceDiagram
       participant C as Client
       participant S as Server
       C->>+S: request
       Note left of C: PacketType.REQUEST<br>channel ID<br>service ID<br>method ID<br>payload

       S-->>C: messages (zero or more)
       Note right of S: PacketType.SERVER_STREAM<br>channel ID<br>service ID<br>method ID<br>payload

       S->>-C: done
       Note right of S: PacketType.RESPONSE<br>channel ID<br>service ID<br>method ID<br>status


The client may terminate a server streaming RPC by sending a ``CLIENT_STREAM``
packet with status ``CANCELLED``. The server sends no response.

.. mermaid::
   :alt: Cancelled Server Streaming RPC
   :align: center

   sequenceDiagram
       participant C as Client
       participant S as Server
       C->>S: request
       Note left of C: PacketType.REQUEST<br>channel ID<br>service ID<br>method ID<br>payload

       S-->>C: messages (zero or more)
       Note right of S: PacketType.SERVER_STREAM<br>channel ID<br>service ID<br>method ID<br>payload

       C->>S: cancel
       Note left of C: PacketType.CLIENT_ERROR<br>channel ID<br>service ID<br>method ID<br>status=CANCELLED


Client streaming RPC
--------------------
In a client streaming RPC, the client starts the RPC by sending a ``REQUEST``
packet with no payload. It then sends any number of messages in
``CLIENT_STREAM`` packets, followed by a ``CLIENT_REQUEST_COMPLETION``. The server sends
a single ``RESPONSE`` to finish the RPC.

.. mermaid::
   :alt: Client Streaming RPC
   :align: center

   sequenceDiagram
       participant C as Client
       participant S as Server
       C->>S: start
       Note left of C: PacketType.REQUEST<br>channel ID<br>service ID<br>method ID<br>payload

       C-->>S: messages (zero or more)
       Note left of C: PacketType.CLIENT_STREAM<br>channel ID<br>service ID<br>method ID<br>payload

       C->>S: done
       Note left of C: PacketType.CLIENT_REQUEST_COMPLETION<br>channel ID<br>service ID<br>method ID

       S->>C: response
       Note right of S: PacketType.RESPONSE<br>channel ID<br>service ID<br>method ID<br>payload<br>status

The server may finish the RPC at any time by sending its ``RESPONSE`` packet,
even if it has not yet received the ``CLIENT_REQUEST_COMPLETION`` packet. The client may
terminate the RPC at any time by sending a ``CLIENT_ERROR`` packet with status
``CANCELLED``.

.. mermaid::
   :alt: Cancelled Client Streaming RPC
   :align: center

   sequenceDiagram
       participant C as Client
       participant S as Server
       C->>S: start
       Note left of C: PacketType.REQUEST<br>channel ID<br>service ID<br>method ID

       C-->>S: messages (zero or more)
       Note left of C: PacketType.CLIENT_STREAM<br>channel ID<br>service ID<br>method ID<br>payload

       C->>S: cancel
       Note right of S: PacketType.CLIENT_ERROR<br>channel ID<br>service ID<br>method ID<br>status=CANCELLED

Bidirectional streaming RPC
---------------------------
In a bidirectional streaming RPC, the client sends any number of requests and
the server sends any number of responses. The client invokes the RPC by sending
a ``REQUEST`` with no payload. It sends a ``CLIENT_REQUEST_COMPLETION`` packet when it
has finished sending requests. The server sends a ``RESPONSE`` packet to finish
the RPC.

.. mermaid::
   :alt: Bidirectional Streaming RPC
   :align: center

   sequenceDiagram
       participant C as Client
       participant S as Server
       C->>S: start
       Note left of C: PacketType.REQUEST<br>channel ID<br>service ID<br>method ID<br>payload

       C-->>S: messages (zero or more)
       Note left of C: PacketType.CLIENT_STREAM<br>channel ID<br>service ID<br>method ID<br>payload

       C-->S: (messages in any order)

       S-->>C: messages (zero or more)
       Note right of S: PacketType.SERVER_STREAM<br>channel ID<br>service ID<br>method ID<br>payload

       C->>S: done
       Note left of C: PacketType.CLIENT_REQUEST_COMPLETION<br>channel ID<br>service ID<br>method ID

       S->>C: done
       Note right of S: PacketType.RESPONSE<br>channel ID<br>service ID<br>method ID<br>status


The server may finish the RPC at any time by sending the ``RESPONSE`` packet,
even if it has not received the ``CLIENT_REQUEST_COMPLETION`` packet. The client may
terminate the RPC at any time by sending a ``CLIENT_ERROR`` packet with status
``CANCELLED``.

.. mermaid::
   :alt: Client Streaming RPC
   :align: center

   sequenceDiagram
       participant C as Client
       participant S as Server
       C->>S: start
       Note left of C: PacketType.REQUEST<br>channel ID<br>service ID<br>method ID<br>payload

       C-->>S: messages (zero or more)
       Note left of C: PacketType.CLIENT_STREAM<br>channel ID<br>service ID<br>method ID<br>payload

       C->>S: done
       Note left of C: PacketType.CLIENT_REQUEST_COMPLETION<br>channel ID<br>service ID<br>method ID

       S->>C: response
       Note right of S: PacketType.RESPONSE<br>channel ID<br>service ID<br>method ID<br>payload<br>status

-------
C++ API
-------

RPC server
==========
Declare an instance of ``rpc::Server`` and register services with it.

.. admonition:: TODO

   Document the public interface

Size report
-----------
The following size report showcases the memory usage of the core RPC server. It
is configured with a single channel using a basic transport interface that
directly reads from and writes to ``pw_sys_io``. The transport has a 128-byte
packet buffer, which comprises the plurality of the example's RAM usage. This is
not a suitable transport for an actual product; a real implementation would have
additional overhead proportional to the complexity of the transport.

.. include:: server_size

RPC server implementation
-------------------------

The Method class
^^^^^^^^^^^^^^^^
The RPC Server depends on the ``pw::rpc::internal::Method`` class. ``Method``
serves as the bridge between the ``pw_rpc`` server library and the user-defined
RPC functions. Each supported protobuf implementation extends ``Method`` to
implement its request and response proto handling. The ``pw_rpc`` server
calls into the ``Method`` implementation through the base class's ``Invoke``
function.

``Method`` implementations store metadata about each method, including a
function pointer to the user-defined method implementation. They also provide
``static constexpr`` functions for creating each type of method. ``Method``
implementations must satisfy the ``MethodImplTester`` test class in
``pw_rpc/internal/method_impl_tester.h``.

See ``pw_rpc/internal/method.h`` for more details about ``Method``.

Packet flow
^^^^^^^^^^^

Requests
........

.. mermaid::
   :alt: Request Packet Flow

   flowchart LR
       packets[Packets]

       subgraph pw_rpc [pw_rpc Library]
           direction TB
           internalMethod[[internal::Method]]
           Server --> Service --> internalMethod
       end

       packets --> Server

       generatedServices{{generated services}}
       userDefinedRPCs(user-defined RPCs)

       generatedServices --> userDefinedRPCs
       internalMethod --> generatedServices

Responses
.........

.. mermaid::
   :alt: Request Packet Flow

   flowchart LR
       generatedServices{{generated services}}
       userDefinedRPCs(user-defined RPCs)

       subgraph pw_rpc [pw_rpc Library]
           direction TB
           internalMethod[[internal::Method]]
           internalMethod --> Server --> Channel
       end

       packets[Packets]
       Channel --> packets

       userDefinedRPCs --> generatedServices
       generatedServices --> internalMethod

RPC client
==========
The RPC client is used to send requests to a server and manages the contexts of
ongoing RPCs.

Setting up a client
-------------------
The ``pw::rpc::Client`` class is instantiated with a list of channels that it
uses to communicate. These channels can be shared with a server, but multiple
clients cannot use the same channels.

To send incoming RPC packets from the transport layer to be processed by a
client, the client's ``ProcessPacket`` function is called with the packet data.

.. code-block:: c++

   #include "pw_rpc/client.h"

   namespace {

   pw::rpc::Channel my_channels[] = {
       pw::rpc::Channel::Create<1>(&my_channel_output)};
   pw::rpc::Client my_client(my_channels);

   }  // namespace

   // Called when the transport layer receives an RPC packet.
   void ProcessRpcPacket(ConstByteSpan packet) {
     my_client.ProcessPacket(packet);
   }

Note that client processing such as callbacks will be invoked within
the body of ``ProcessPacket``.

If certain packets need to be filtered out, or if certain client processing
needs to be invoked from a specific thread or context, the ``PacketMeta`` class
can be used to determine which service or channel a packet is targeting. After
filtering, ``ProcessPacket`` can be called from the appropriate environment.

.. _module-pw_rpc-making-calls:

Making RPC calls
----------------
RPC calls are not made directly through the client, but using one of its
registered channels instead. A service client class is generated from a .proto
file for each selected protobuf library, which is then used to send RPC requests
through a given channel. The API for this depends on the protobuf library;
please refer to the
:ref:`appropriate documentation<module-pw_rpc-protobuf-library-apis>`. Multiple
service client implementations can exist simulatenously and share the same
``Client`` class.

When a call is made, a call object is returned to the caller. This object tracks
the ongoing RPC call, and can be used to manage it. An RPC call is only active
as long as its call object is alive.

.. tip::

   Use ``std::move`` when passing around call objects to keep RPCs alive.

Example
^^^^^^^
.. code-block:: c++

   #include "pw_rpc/echo_service_nanopb.h"

   namespace {
   // Generated clients are namespaced with their proto library.
   using EchoClient = pw_rpc::nanopb::EchoService::Client;

   // RPC channel ID on which to make client calls. RPC calls cannot be made on
   // channel 0 (Channel::kUnassignedChannelId).
   constexpr uint32_t kDefaultChannelId = 1;

   pw::rpc::NanopbUnaryReceiver<pw_rpc_EchoMessage> echo_call;

   // Callback invoked when a response is received. This is called synchronously
   // from Client::ProcessPacket.
   void EchoResponse(const pw_rpc_EchoMessage& response,
                     pw::Status status) {
     if (status.ok()) {
       PW_LOG_INFO("Received echo response: %s", response.msg);
     } else {
       PW_LOG_ERROR("Echo failed with status %d",
                    static_cast<int>(status.code()));
     }
   }

   }  // namespace

   void CallEcho(const char* message) {
     // Create a client to call the EchoService.
     EchoClient echo_client(my_rpc_client, kDefaultChannelId);

     pw_rpc_EchoMessage request{};
     pw::string::Copy(message, request.msg);

     // By assigning the returned call to the global echo_call, the RPC
     // call is kept alive until it completes. When a response is received, it
     // will be logged by the handler function and the call will complete.
     echo_call = echo_client.Echo(request, EchoResponse);
     if (!echo_call.active()) {
       // The RPC call was not sent. This could occur due to, for example, an
       // invalid channel ID. Handle if necessary.
     }
   }

Call objects
============
An RPC call is represented by a call object. Server and client calls use the
same base call class in C++, but the public API is different depending on the
type of call (see `RPC call lifecycle`_) and whether it is being used by the
server or client.

The public call types are as follows:

.. list-table::
   :header-rows: 1

   * - RPC Type
     - Server call
     - Client call
   * - Unary
     - ``(Raw|Nanopb|Pwpb)UnaryResponder``
     - ``(Raw|Nanopb|Pwpb)UnaryReceiver``
   * - Server streaming
     - ``(Raw|Nanopb|Pwpb)ServerWriter``
     - ``(Raw|Nanopb|Pwpb)ClientReader``
   * - Client streaming
     - ``(Raw|Nanopb|Pwpb)ServerReader``
     - ``(Raw|Nanopb|Pwpb)ClientWriter``
   * - Bidirectional streaming
     - ``(Raw|Nanopb|Pwpb)ServerReaderWriter``
     - ``(Raw|Nanopb|Pwpb)ClientReaderWriter``

Client call API
---------------
Client call objects provide a few common methods.

.. cpp:class:: pw::rpc::ClientCallType

   The ``ClientCallType`` will be one of the following types:

   - ``(Raw|Nanopb|Pwpb)UnaryReceiver`` for unary
   - ``(Raw|Nanopb|Pwpb)ClientReader`` for server streaming
   - ``(Raw|Nanopb|Pwpb)ClientWriter`` for client streaming
   - ``(Raw|Nanopb|Pwpb)ClientReaderWriter`` for bidirectional streaming

   .. cpp:function:: bool active() const

      Returns true if the call is active.

   .. cpp:function:: uint32_t channel_id() const

      Returns the channel ID of this call, which is 0 if the call is inactive.

   .. cpp:function:: uint32_t id() const

      Returns the call ID, a unique identifier for this call.

   .. cpp:function:: void Write(RequestType)

      Only available on client and bidirectional streaming calls. Sends a stream
      request. Returns:

      - ``OK`` - the request was successfully sent
      - ``FAILED_PRECONDITION`` - the writer is closed
      - ``INTERNAL`` - pw_rpc was unable to encode message; does not apply to
        raw calls
      - other errors - the :cpp:class:`ChannelOutput` failed to send the packet;
        the error codes are determined by the :cpp:class:`ChannelOutput`
        implementation

   .. cpp:function:: pw::Status RequestCompletion()

      Notifies the server that client has requested for call completion. On
      client and bidirectional streaming calls no further client stream messages
      will be sent.

   .. cpp:function:: pw::Status Cancel()

      Cancels this RPC. Closes the call and sends a ``CANCELLED`` error to the
      server. Return statuses are the same as :cpp:func:`Write`.

   .. cpp:function:: void Abandon()

      Closes this RPC locally. Sends a ``CLIENT_REQUEST_COMPLETION``, but no cancellation
      packet. Future packets for this RPC are dropped, and the client sends a
      ``FAILED_PRECONDITION`` error in response because the call is not active.

   .. cpp:function:: void CloseAndWaitForCallbacks()

      Abandons this RPC and additionally blocks on completion of any running callbacks.

   .. cpp:function:: void set_on_completed(pw::Function<void(ResponseTypeIfUnaryOnly, pw::Status)>)

      Sets the callback that is called when the RPC completes normally. The
      signature depends on whether the call has a unary or stream response.

   .. cpp:function:: void set_on_error(pw::Function<void(pw::Status)>)

      Sets the callback that is called when the RPC is terminated due to an error.

   .. cpp:function:: void set_on_next(pw::Function<void(ResponseType)>)

      Only available on server and bidirectional streaming calls. Sets the callback
      that is called for each stream response.

Callbacks
---------
The C++ call objects allow users to set callbacks that are invoked when RPC
`events`_ occur.

.. list-table::
   :header-rows: 1

   * - Name
     - Stream signature
     - Non-stream signature
     - Server
     - Client
   * - ``on_error``
     - ``void(pw::Status)``
     - ``void(pw::Status)``
     - ✅
     - ✅
   * - ``on_next``
     - n/a
     - ``void(const PayloadType&)``
     - ✅
     - ✅
   * - ``on_completed``
     - ``void(pw::Status)``
     - ``void(const PayloadType&, pw::Status)``
     -
     - ✅
   * - ``on_client_requested_completion``
     - ``void()``
     - n/a
     - ✅ (:c:macro:`optional <PW_RPC_COMPLETION_REQUEST_CALLBACK>`)
     -

Limitations and restrictions
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
RPC callbacks are free to perform most actions, including invoking new RPCs or
cancelling pending calls. However, the C++ implementation imposes some
limitations and restrictions that must be observed.

Destructors & moves wait for callbacks to complete
...................................................
* Callbacks must not destroy their call object. Attempting to do so will result
  in deadlock.
* Other threads may destroy a call while its callback is running, but that
  thread will block until all callbacks complete.
* Callbacks must not move their call object if it the call is still active. They
  may move their call object after it has terminated. Callbacks may move a
  different call into their call object, since moving closes the destination
  call.
* Other threads may move a call object while it has a callback running, but they
  will block until the callback completes if the call is still active.

.. warning::

   Deadlocks or crashes occur if a callback:

   - attempts to destroy its call object
   - attempts to move its call object while the call is still active
   - never returns

   If ``pw_rpc`` a callback violates these restrictions, a crash may occur,
   depending on the value of :c:macro:`PW_RPC_CALLBACK_TIMEOUT_TICKS`. These
   crashes have a message like the following:

   .. code-block:: text

      A callback for RPC 1:cc0f6de0/31e616ce has not finished after 10000 ticks.
      This may indicate that an RPC callback attempted to destroy or move its own
      call object, which is not permitted. Fix this condition or change the value of
      PW_RPC_CALLBACK_TIMEOUT_TICKS to avoid this crash.

      See https://pigweed.dev/pw_rpc#destructors-moves-wait-for-callbacks-to-complete
      for details.

Only one thread at a time may execute ``on_next``
.................................................
Only one thread may execute the ``on_next`` callback for a specific service
method at a time. If a second thread calls ``ProcessPacket()`` with a stream
packet before the ``on_next`` callback for the previous packet completes, the
second packet will be dropped. The RPC endpoint logs a warning when this occurs.

Example warning for a dropped stream packet:

.. code-block:: text

   WRN  Received stream packet for 1:cc0f6de0/31e616ce before the callback for
        a previous packet completed! This packet will be dropped. This can be
        avoided by handling packets for a particular RPC on only one thread.

RPC calls introspection
=======================
``pw_rpc`` provides ``pw_rpc/method_info.h`` header that allows to obtain
information about the generated RPC method in compile time.

For now it provides only two types: ``MethodRequestType<RpcMethod>`` and
``MethodResponseType<RpcMethod>``. They are aliases to the types that are used
as a request and response respectively for the given RpcMethod.

Example
-------
We have an RPC service ``SpecialService`` with ``MyMethod`` method:

.. code-block:: protobuf

   package some.package;
   service SpecialService {
     rpc MyMethod(MyMethodRequest) returns (MyMethodResponse) {}
   }

We also have a templated Storage type alias:

.. code-block:: c++

   template <auto kMethod>
   using Storage =
      std::pair<MethodRequestType<kMethod>, MethodResponseType<kMethod>>;

``Storage<some::package::pw_rpc::pwpb::SpecialService::MyMethod>`` will
instantiate as:

.. code-block:: c++

   std::pair<some::package::MyMethodRequest::Message,
             some::package::MyMethodResponse::Message>;

.. note::

   Only nanopb and pw_protobuf have real types as
   ``MethodRequestType<RpcMethod>``/``MethodResponseType<RpcMethod>``. Raw has
   them both set as ``void``. In reality, they are ``pw::ConstByteSpan``. Any
   helper/trait that wants to use this types for raw methods should do a custom
   implementation that copies the bytes under the span instead of copying just
   the span.

.. _module-pw_rpc-client-sync-call-wrappers:

Client synchronous call wrappers
================================
.. doxygenfile:: pw_rpc/synchronous_call.h
   :sections: detaileddescription

Example
-------
.. code-block:: c++

   #include "pw_rpc/synchronous_call.h"

   void InvokeUnaryRpc() {
     pw::rpc::Client client;
     pw::rpc::Channel channel;

     RoomInfoRequest request;
     SynchronousCallResult<RoomInfoResponse> result =
       SynchronousCall<Chat::GetRoomInformation>(client, channel.id(), request);

     if (result.is_rpc_error()) {
       ShutdownClient(client);
     } else if (result.is_server_error()) {
       HandleServerError(result.status());
     } else if (result.is_timeout()) {
       // SynchronousCall will block indefinitely, so we should never get here.
       PW_UNREACHABLE();
     }
     HandleRoomInformation(std::move(result).response());
   }

   void AnotherExample() {
     pw_rpc::nanopb::Chat::Client chat_client(client, channel);
     constexpr auto kTimeout = pw::chrono::SystemClock::for_at_least(500ms);

     RoomInfoRequest request;
     auto result = SynchronousCallFor<Chat::GetRoomInformation>(
         chat_client, request, kTimeout);

     if (result.is_timeout()) {
       RetryRoomRequest();
     } else {
     ...
     }
   }

The ``SynchronousCallResult<Response>`` is also compatible with the
:c:macro:`PW_TRY` family of macros, but users should be aware that their use
will lose information about the type of error. This should only be used if the
caller will handle all error scenarios the same.

.. code-block:: c++

   pw::Status SyncRpc() {
     const RoomInfoRequest request;
     PW_TRY_ASSIGN(const RoomInfoResponse& response,
                   SynchronousCall<Chat::GetRoomInformation>(client, request));
     HandleRoomInformation(response);
     return pw::OkStatus();
   }

ClientServer
============
Sometimes, a device needs to both process RPCs as a server, as well as making
calls to another device as a client. To do this, both a client and server must
be set up, and incoming packets must be sent to both of them.

Pigweed simplifies this setup by providing a ``ClientServer`` class which wraps
an RPC client and server with the same set of channels.

.. code-block:: cpp

   pw::rpc::Channel channels[] = {
       pw::rpc::Channel::Create<1>(&channel_output)};

   // Creates both a client and a server.
   pw::rpc::ClientServer client_server(channels);

   void ProcessRpcData(pw::ConstByteSpan packet) {
     // Calls into both the client and the server, sending the packet to the
     // appropriate one.
     client_server.ProcessPacket(packet);
   }

Testing
=======
``pw_rpc`` provides utilities for unit testing RPC services and client calls.

Client unit testing in C++
--------------------------
``pw_rpc`` supports invoking RPCs, simulating server responses, and checking
what packets are sent by an RPC client in tests. Raw, Nanopb and Pwpb interfaces
are supported. Code that uses the raw API may be tested with the raw test
helpers, and vice versa. The Nanopb and Pwpb APIs also provides a test helper
with a real client-server pair that supports testing of asynchronous messaging.

To test synchronous code that invokes RPCs, declare a ``RawClientTestContext``,
``PwpbClientTestContext``,  or ``NanopbClientTestContext``. These test context
objects provide a preconfigured RPC client, channel, server fake, and buffer for
encoding packets.

These test classes are defined in ``pw_rpc/raw/client_testing.h``,
``pw_rpc/pwpb/client_testing.h``, or ``pw_rpc/nanopb/client_testing.h``.

Use the context's ``client()`` and ``channel()`` to invoke RPCs. Use the
context's ``server()`` to simulate responses. To verify that the client sent the
expected data, use the context's ``output()``, which is a ``FakeChannelOutput``.

For example, the following tests a class that invokes an RPC. It checks that
the expected data was sent and then simulates a response from the server.

.. code-block:: cpp

   #include "pw_rpc/raw/client_testing.h"

   class ClientUnderTest {
    public:
     // To support injecting an RPC client for testing, classes that make RPC
     // calls should take an RPC client and channel ID or an RPC service client
     // (e.g. pw_rpc::raw::MyService::Client).
     ClientUnderTest(pw::rpc::Client& client, uint32_t channel_id);

     void DoSomethingThatInvokesAnRpc();

     bool SetToTrueWhenRpcCompletes();
   };

   TEST(TestAThing, InvokesRpcAndHandlesResponse) {
     RawClientTestContext context;
     ClientUnderTest thing(context.client(), context.channel().id());

     // Execute the code that invokes the MyService.TheMethod RPC.
     things.DoSomethingThatInvokesAnRpc();

     // Find and verify the payloads sent for the MyService.TheMethod RPC.
     auto msgs = context.output().payloads<pw_rpc::raw::MyService::TheMethod>();
     ASSERT_EQ(msgs.size(), 1u);

     VerifyThatTheExpectedMessageWasSent(msgs.back());

     // Send the response packet from the server and verify that the class reacts
     // accordingly.
     EXPECT_FALSE(thing.SetToTrueWhenRpcCompletes());

     context_.server().SendResponse<pw_rpc::raw::MyService::TheMethod>(
         final_message, OkStatus());

     EXPECT_TRUE(thing.SetToTrueWhenRpcCompletes());
   }

To test client code that uses asynchronous responses, encapsulates multiple
rpc calls to one or more services, or uses a custom service implemenation,
declare a ``NanopbClientServerTestContextThreaded`` or
``PwpbClientServerTestContextThreaded``. These test object are defined in
``pw_rpc/nanopb/client_server_testing_threaded.h`` and
``pw_rpc/pwpb/client_server_testing_threaded.h``.

Use the context's ``server()`` to register a ``Service`` implementation, and
``client()`` and ``channel()`` to invoke RPCs. Create a ``Thread`` using the
context as a ``ThreadCore`` to have it asycronously forward request/responses or
call ``ForwardNewPackets`` to synchronously process all messages. To verify that
the client/server sent the expected data, use the context's
``request(uint32_t index)`` and ``response(uint32_t index)`` to retrieve the
ordered messages.

For example, the following tests a class that invokes an RPC and blocks till a
response is received. It verifies that expected data was both sent and received.

.. code-block:: cpp

   #include "my_library_protos/my_service.rpc.pb.h"
   #include "pw_rpc/nanopb/client_server_testing_threaded.h"
   #include "pw_thread_stl/options.h"

   class ClientUnderTest {
    public:
     // To support injecting an RPC client for testing, classes that make RPC
     // calls should take an RPC client and channel ID or an RPC service client
     // (e.g. pw_rpc::raw::MyService::Client).
     ClientUnderTest(pw::rpc::Client& client, uint32_t channel_id);

     Status BlockOnResponse(uint32_t value);
   };


   class TestService final : public MyService<TestService> {
    public:
     Status TheMethod(const pw_rpc_test_TheMethod& request,
                         pw_rpc_test_TheMethod& response) {
       response.value = request.integer + 1;
       return pw::OkStatus();
     }
   };

   TEST(TestServiceTest, ReceivesUnaryRpcReponse) {
     NanopbClientServerTestContextThreaded<> ctx(pw::thread::stl::Options{});
     TestService service;
     ctx.server().RegisterService(service);
     ClientUnderTest client(ctx.client(), ctx.channel().id());

     // Execute the code that invokes the MyService.TheMethod RPC.
     constexpr uint32_t value = 1;
     const auto result = client.BlockOnResponse(value);
     const auto request = ctx.request<MyService::TheMethod>(0);
     const auto response = ctx.resonse<MyService::TheMethod>(0);

     // Verify content of messages
     EXPECT_EQ(result, pw::OkStatus());
     EXPECT_EQ(request.value, value);
     EXPECT_EQ(response.value, value + 1);
   }

Use the context's
``response(uint32_t index, Response<kMethod>& response)`` to decode messages
into a provided response object. You would use this version if decoder callbacks
are needed to fully decode a message. For instance if it uses ``repeated``
fields.

.. code-block:: cpp

   TestResponse::Message response{};
   response.repeated_field.SetDecoder(
       [&values](TestResponse::StreamDecoder& decoder) {
         return decoder.ReadRepeatedField(values);
       });
   ctx.response<test::GeneratedService::TestAnotherUnaryRpc>(0, response);

Synchronous versions of these test contexts also exist that may be used on
non-threaded systems ``NanopbClientServerTestContext`` and
``PwpbClientServerTestContext``. While these do not allow for asynchronous
messaging they support the use of service implemenations and use a similar
syntax. When these are used ``.ForwardNewPackets()`` should be called after each
rpc call to trigger sending of queued messages.

For example, the following tests a class that invokes an RPC that is responded
to with a test service implemenation.

.. code-block:: cpp

   #include "my_library_protos/my_service.rpc.pb.h"
   #include "pw_rpc/nanopb/client_server_testing.h"

   class ClientUnderTest {
    public:
     ClientUnderTest(pw::rpc::Client& client, uint32_t channel_id);

     Status SendRpcCall(uint32_t value);
   };


   class TestService final : public MyService<TestService> {
    public:
     Status TheMethod(const pw_rpc_test_TheMethod& request,
                         pw_rpc_test_TheMethod& response) {
       response.value = request.integer + 1;
       return pw::OkStatus();
     }
   };

   TEST(TestServiceTest, ReceivesUnaryRpcResponse) {
     NanopbClientServerTestContext<> ctx();
     TestService service;
     ctx.server().RegisterService(service);
     ClientUnderTest client(ctx.client(), ctx.channel().id());

     // Execute the code that invokes the MyService.TheMethod RPC.
     constexpr uint32_t value = 1;
     const auto result = client.SendRpcCall(value);
     // Needed after ever RPC call to trigger forward of packets
     ctx.ForwardNewPackets();
     const auto request = ctx.request<MyService::TheMethod>(0);
     const auto response = ctx.response<MyService::TheMethod>(0);

     // Verify content of messages
     EXPECT_EQ(result, pw::OkStatus());
     EXPECT_EQ(request.value, value);
     EXPECT_EQ(response.value, value + 1);
   }

Custom packet processing for ClientServerTestContext
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Optional constructor arguments for nanopb/pwpb ``*ClientServerTestContext`` and
``*ClientServerTestContextThreaded`` allow allow customized packet processing.
By default the only thing is done is ``ProcessPacket()`` call on the
``ClientServer`` instance.

For cases when additional instrumentation or offloading to separate thread is
needed, separate client and server processors can be passed to context
constructors. A packet processor is a function that returns ``pw::Status`` and
accepts two arguments: ``pw::rpc::ClientServer&`` and ``pw::ConstByteSpan``.
Default packet processing is equivalent to the next processor:

.. code-block:: cpp

   [](ClientServer& client_server, pw::ConstByteSpan packet) -> pw::Status {
     return client_server.ProcessPacket(packet);
   };

The Server processor will be applied to all packets sent to the server (i.e.
requests) and client processor will be applied to all packets sent to the client
(i.e. responses).

.. note::

  The packet processor MUST call ``ClientServer::ProcessPacket()`` method.
  Otherwise the packet won't be processed.

.. note::

  If the packet processor offloads processing to the separate thread, it MUST
  copy the ``packet``. After the packet processor returns, the underlying array
  can go out of scope or be reused for other purposes.

SendResponseIfCalled() helper
-----------------------------
``SendResponseIfCalled()`` function waits on ``*ClientTestContext*`` output to
have a call for the specified method and then responses to it. It supports
timeout for the waiting part (default timeout is 100ms).

.. code-block:: c++

   #include "pw_rpc/test_helpers.h"

   pw::rpc::PwpbClientTestContext client_context;
   other::pw_rpc::pwpb::OtherService::Client other_service_client(
       client_context.client(), client_context.channel().id());

   PW_PWPB_TEST_METHOD_CONTEXT(MyService, GetData)
   context(other_service_client);
   context.call({});

   ASSERT_OK(pw::rpc::test::SendResponseIfCalled<
             other::pw_rpc::pwpb::OtherService::GetPart>(
       client_context, {.value = 42}));

   // At this point MyService::GetData handler received the GetPartResponse.

Integration testing with ``pw_rpc``
-----------------------------------
``pw_rpc`` provides utilities to simplify writing integration tests for systems
that communicate with ``pw_rpc``. The integration test utitilies set up a socket
to use for IPC between an RPC server and client process.

The server binary uses the system RPC server facade defined
``pw_rpc_system_server/rpc_server.h``. The client binary uses the functions
defined in ``pw_rpc/integration_testing.h``:

.. cpp:var:: constexpr uint32_t kChannelId

   The RPC channel for integration test RPCs.

.. cpp:function:: pw::rpc::Client& pw::rpc::integration_test::Client()

   Returns the global RPC client for integration test use.

.. cpp:function:: pw::Status pw::rpc::integration_test::InitializeClient(int argc, char* argv[], const char* usage_args = "PORT")

   Initializes logging and the global RPC client for integration testing. Starts
   a background thread that processes incoming.

Module Configuration Options
============================
The following configurations can be adjusted via compile-time configuration of
this module, see the
:ref:`module documentation <module-structure-compile-time-configuration>` for
more details.

.. doxygenfile:: pw_rpc/public/pw_rpc/internal/config.h
   :sections: define

Sharing server and client code
==============================
Streaming RPCs support writing multiple requests or responses. To facilitate
sharing code between servers and clients, ``pw_rpc`` provides the
``pw::rpc::Writer`` interface. On the client side, a client or bidirectional
streaming RPC call object (``ClientWriter`` or ``ClientReaderWriter``) can be
used as a ``pw::rpc::Writer&``. On the server side, a server or bidirectional
streaming RPC call object (``ServerWriter`` or ``ServerReaderWriter``) can be
used as a ``pw::rpc::Writer&``. Call ``as_writer()`` to get a ``Writer&`` of the
client or server call object.

Zephyr
======
To enable ``pw_rpc.*`` for Zephyr add ``CONFIG_PIGWEED_RPC=y`` to the project's
configuration. This will enable the Kconfig menu for the following:

* ``pw_rpc.server`` which can be enabled via ``CONFIG_PIGWEED_RPC_SERVER=y``.
* ``pw_rpc.client`` which can be enabled via ``CONFIG_PIGWEED_RPC_CLIENT=y``.
* ``pw_rpc.client_server`` which can be enabled via
  ``CONFIG_PIGWEED_RPC_CLIENT_SERVER=y``.
* ``pw_rpc.common` which can be enabled via ``CONFIG_PIGWEED_RPC_COMMON=y``.

Encoding and sending packets
============================
``pw_rpc`` has to manage interactions among multiple RPC clients, servers,
client calls, and server calls. To safely synchronize these interactions with
minimal overhead, ``pw_rpc`` uses a single, global mutex (when
``PW_RPC_USE_GLOBAL_MUTEX`` is enabled).

Because ``pw_rpc`` uses a global mutex, it also uses a global buffer to encode
outgoing packets. The size of the buffer is set with
``PW_RPC_ENCODING_BUFFER_SIZE_BYTES``, which defaults to 512 B. If dynamic
allocation is enabled, this size does not affect how large RPC messages can be,
but it is still used for sizing buffers in test utilities.

Users of ``pw_rpc`` must implement the :cpp:class:`pw::rpc::ChannelOutput`
interface.

.. _module-pw_rpc-ChannelOutput:
.. cpp:class:: pw::rpc::ChannelOutput

   ``pw_rpc`` endpoints use :cpp:class:`ChannelOutput` instances to send
   packets.  Systems that integrate pw_rpc must use one or more
   :cpp:class:`ChannelOutput` instances.

   .. cpp:member:: static constexpr size_t kUnlimited = std::numeric_limits<size_t>::max()

      Value returned from :cpp:func:`MaximumTransmissionUnit` to indicate an
      unlimited MTU.

   .. cpp:function:: virtual size_t MaximumTransmissionUnit()

      Returns the size of the largest packet the :cpp:class:`ChannelOutput` can
      send. :cpp:class:`ChannelOutput` implementations should only override this
      function if they impose a limit on the MTU. The default implementation
      returns :cpp:member:`kUnlimited`, which indicates that there is no MTU
      limit.

   .. cpp:function:: virtual pw::Status Send(span<std::byte> packet)

      Sends an encoded RPC packet. Returns OK if further packets may be sent,
      even if the current packet could not be sent. Returns any other status if
      the Channel is no longer able to send packets.

      The RPC system's internal lock is held while this function is
      called. Avoid long-running operations, since these will delay any other
      users of the RPC system.

      .. danger::

         No ``pw_rpc`` APIs may be accessed in this function! Implementations
         MUST NOT access any RPC endpoints (:cpp:class:`pw::rpc::Client`,
         :cpp:class:`pw::rpc::Server`) or call objects
         (:cpp:class:`pw::rpc::ServerReaderWriter`
         :cpp:class:`pw::rpc::ClientReaderWriter`, etc.) inside the
         :cpp:func:`Send` function or any descendent calls. Doing so will result
         in deadlock! RPC APIs may be used by other threads, just not within
         :cpp:func:`Send`.

         The buffer provided in ``packet`` must NOT be accessed outside of this
         function. It must be sent immediately or copied elsewhere before the
         function returns.

Evolution
=========
Concurrent requests were not initially supported in pw_rpc (added in
`C++ <https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/109077>`,
`Python <https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/139610>`,
and
`TypeScript <https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/160792>`).
As a result, some user-written service implementations may not expect or
correctly support concurrent requests.