.. _module-pw_protobuf_compiler:

--------------------
pw_protobuf_compiler
--------------------
The Protobuf compiler module provides build system integration and wrapper
scripts for generating source code for Protobuf definitions.

Generator support
=================
Protobuf code generation is currently supported for the following generators:

+-------------+----------------+-----------------------------------------------+
| Generator   | Code           | Notes                                         |
+-------------+----------------+-----------------------------------------------+
| pw_protobuf | ``pwpb``       | Compiles using ``pw_protobuf``.               |
+-------------+----------------+-----------------------------------------------+
| Nanopb      | ``nanopb``     | Compiles using Nanopb. The build argument     |
|             |                | ``dir_pw_third_party_nanopb`` must be set to  |
|             |                | point to a local nanopb installation.         |
+-------------+----------------+-----------------------------------------------+
| Nanopb RPC  | ``nanopb_rpc`` | Compiles pw_rpc service and client code for   |
|             |                | nanopb. Requires a nanopb installation.       |
+-------------+----------------+-----------------------------------------------+
| Raw RPC     | ``raw_rpc``    | Compiles raw binary pw_rpc service code.      |
+-------------+----------------+-----------------------------------------------+
| Go          | ``go``         | Compiles using the standard Go protobuf       |
|             |                | plugin with gRPC service support.             |
+-------------+----------------+-----------------------------------------------+
| Python      | ``python``     | Compiles using the standard Python protobuf   |
|             |                | plugin, creating a ``pw_python_package``.     |
+-------------+----------------+-----------------------------------------------+

GN template
===========
The ``pw_proto_library`` GN template is provided by the module.

It defines a collection of protobuf files that should be compiled together. The
template creates a sub-target for each supported generator, named
``<target_name>.<generator>``. These sub-targets generate their respective
protobuf code, and expose it to the build system appropriately (e.g. a
``pw_source_set`` for C/C++).

For example, given the following target:

.. code-block::

  pw_proto_library("test_protos") {
    sources = [ "my_test_protos/test.proto" ]
  }

``test_protos.pwpb`` compiles code for pw_protobuf, and ``test_protos.nanopb``
compiles using Nanopb (if it's installed).

Protobuf code is only generated when a generator sub-target is listed as a
dependency of another GN target.

**Arguments**

* ``sources``: List of input .proto files.
* ``deps``: List of other pw_proto_library dependencies.
* ``inputs``: Other files on which the protos depend (e.g. nanopb ``.options``
  files).
* ``prefix``: A prefix to add to the source protos prior to compilation. For
  example, a source called ``"foo.proto"`` with ``prefix = "nested"`` will be
  compiled with protoc as ``"nested/foo.proto"``.
* ``strip_prefix``: Remove this prefix from the source protos. All source and
  input files must be nested under this path.

**Example**

.. code-block::

  import("$dir_pw_protobuf_compiler/proto.gni")

  pw_proto_library("my_protos") {
    sources = [
      "my_protos/foo.proto",
      "my_protos/bar.proto",
    ]
  }

  pw_proto_library("my_other_protos") {
    sources = [ "some/other/path/baz.proto" ]  # imports foo.proto

    # This removes the "some/other/path" prefix from the proto files.
    strip_prefix = "some/other/path"

    # This adds the "my_other_protos/" prefix to the proto files.
    prefix = "my_other_protos"

    # Proto libraries depend on other proto libraries directly.
    deps = [ ":my_protos" ]
  }

  source_set("my_cc_code") {
    sources = [
      "foo.cc",
      "bar.cc",
      "baz.cc",
    ]

    # When depending on protos in a source_set, specify the generator suffix.
    deps = [ ":my_other_protos.pwpb" ]
  }

From C++, ``baz.proto`` included as follows:

.. code-block:: cpp

  #include "my_other_protos/baz.pwpb.h"

From Python, ``baz.proto`` is imported as follows:

.. code-block:: python

  from my_other_protos import baz_pb2

Proto file structure
--------------------
Protobuf source files must be nested under another directory when they are
compiled. This ensures that they can be packaged properly in Python. The first
directory is used as the Python package name, so must be unique across the
build. The ``prefix`` option may be used to set this directory.

Using ``prefix`` and ``strip_prefix`` together allows remapping proto files to
a completely different path. This can be useful when working with protos defined
in external libraries. For example, consider this proto library:

.. code-block::

  pw_proto_library("external_protos") {
    sources = [
      "//other/external/some_library/src/protos/alpha.proto",
      "//other/external/some_library/src/protos/beta.proto,
      "//other/external/some_library/src/protos/internal/gamma.proto",
    ]
    strip_prefix = "//other/external/some_library/src/protos"
    prefix = "some_library"
  }

These protos will be compiled by protoc as if they were in this file structure:

.. code-block::

  some_library/
  ├── alpha.proto
  ├── beta.proto
  └── internal
      └── gamma.proto

Working with externally defined protos
--------------------------------------
``pw_proto_library`` targets may be used to build ``.proto`` sources from
existing projects. In these cases, it may be necessary to supply the
``strip_prefix`` argument, which specifies the protobuf include path to use for
``protoc``. If only a single external protobuf is being compiled, the
``python_module_as_package`` option can be used to override the requirement that
the protobuf be nested under a directory. This option generates a Python package
with the same name as the proto file, so that the generated proto can be
imported as if it were a standalone Python module.

For example, the ``pw_proto_library`` target for Nanopb sets
``python_module_as_package`` to ``nanopb_pb2``.

.. code-block::

  pw_proto_library("proto") {
    strip_prefix = "$dir_pw_third_party_nanopb/generator/proto"
    sources = [ "$dir_pw_third_party_nanopb/generator/proto/nanopb.proto" ]
    python_module_as_package = "nanopb_pb2"
  }

In Python, this makes ``nanopb.proto`` available as ``import nanopb_pb2`` via
the ``nanopb_pb2`` Python package. In C++, ``nanopb.proto`` is accessed as
``#include "nanopb.pwpb.h"``.

The ``python_module_as_package`` feature should only be used when absolutely
necessary --- for example, to support proto files that include
``import "nanopb.proto"``.

CMake
=====
CMake provides a ``pw_proto_library`` function with similar features as the
GN template. The CMake build only supports building firmware code, so
``pw_proto_library`` does not generate a Python package.

**Arguments**

* ``NAME``: the base name of the libraries to create
* ``SOURCES``: .proto source files
* ``DEPS``: dependencies on other ``pw_proto_library`` targets
* ``PREFIX``: prefix add to the proto files
* ``STRIP_PREFIX``: prefix to remove from the proto files
* ``INPUTS``: files to include along with the .proto files (such as Nanopb
  .options files)

**Example**

 .. code-block:: cmake

  include($ENV{PW_ROOT}/pw_build/pigweed.cmake)
  include($ENV{PW_ROOT}/pw_protobuf_compiler/proto.cmake)

  pw_proto_library(my_module.my_protos
    SOURCES
      my_protos/foo.proto
      my_protos/bar.proto
  )

  pw_proto_library(my_module.my_protos
    SOURCES
      my_protos/foo.proto
      my_protos/bar.proto
  )

  pw_proto_library(my_module.my_other_protos
    SOURCES
      some/other/path/baz.proto  # imports foo.proto

    # This removes the "some/other/path" prefix from the proto files.
    STRIP_PREFIX
      some/other/path

    # This adds the "my_other_protos/" prefix to the proto files.
    PREFIX
      my_other_protos

    # Proto libraries depend on other proto libraries directly.
    DEPS
      my_module.my_protos
  )

  add_library(my_module.my_cc_code
      foo.cc
      bar.cc
      baz.cc
  )

  # When depending on protos in a source_set, specify the generator suffix.
  target_link_libraries(my_module.my_cc_code PUBLIC
    my_module.my_other_protos.pwpb
  )

These proto files are accessed in C++ the same as in the GN build:

.. code-block:: cpp

  #include "my_other_protos/baz.pwpb"