# Copyright 2020 The Pigweed Authors
#
# Licensed under the Apache License, Version 2.0 (the "License"); you may not
# use this file except in compliance with the License. You may obtain a copy of
# the License at
#
#     https://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
# License for the specific language governing permissions and limitations under
# the License.

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

import("$dir_pw_build/error.gni")
import("$dir_pw_build/input_group.gni")
import("$dir_pw_build/mirror_tree.gni")
import("$dir_pw_build/python.gni")
import("$dir_pw_build/python_action.gni")
import("$dir_pw_build/target_types.gni")
import("$dir_pw_third_party/nanopb/nanopb.gni")
import("toolchain.gni")

# Variables forwarded from the public pw_proto_library template to the final
# pw_source_set.
_forwarded_vars = [
  "testonly",
  "visibility",
]

# Internal template that invokes protoc with a pw_python_action. This should not
# be used outside of this file; use pw_proto_library instead.
#
# This creates the internal GN target $target_name.$language._gen that compiles
# proto files with protoc.
template("_pw_invoke_protoc") {
  if (current_toolchain == pw_protobuf_compiler_TOOLCHAIN) {
    if (defined(invoker.out_dir)) {
      _out_dir = invoker.out_dir
    } else {
      _out_dir = "${invoker.base_out_dir}/${invoker.language}"
      if (defined(invoker.module_as_package) &&
          invoker.module_as_package != "") {
        assert(invoker.language == "python")
        _out_dir = "$_out_dir/${invoker.module_as_package}"
      }
    }

    _includes =
        rebase_path(get_target_outputs(":${invoker.base_target}._includes"),
                    root_build_dir)

    pw_python_action("$target_name._gen") {
      script =
          "$dir_pw_protobuf_compiler/py/pw_protobuf_compiler/generate_protos.py"

      python_deps = [ "$dir_pw_protobuf_compiler/py" ]
      if (defined(invoker.python_deps)) {
        python_deps += invoker.python_deps
      }

      deps = [
        ":${invoker.base_target}._includes",
        ":${invoker.base_target}._sources",
      ]

      foreach(dep, invoker.deps) {
        deps += [ get_label_info(dep, "label_no_toolchain") + "._gen" ]
      }

      if (defined(invoker.other_deps)) {
        deps += invoker.other_deps
      }

      args = [
               "--language",
               invoker.language,
               "--include-file",
               _includes[0],
               "--compile-dir",
               rebase_path(invoker.compile_dir, root_build_dir),
               "--out-dir",
               rebase_path(_out_dir, root_build_dir),
               "--sources",
             ] + rebase_path(invoker.sources, root_build_dir)

      if (defined(invoker.plugin)) {
        inputs = [ invoker.plugin ]
        args +=
            [ "--plugin-path=" + rebase_path(invoker.plugin, root_build_dir) ]
      }

      if (defined(invoker.outputs)) {
        outputs = invoker.outputs
      } else {
        stamp = true
      }

      if (defined(invoker.metadata)) {
        metadata = invoker.metadata
      }
    }

    # Output a .json file with information about this proto library.
    _proto_info = {
      label = get_label_info(":${invoker.target_name}", "label_no_toolchain")
      protoc_outputs =
          rebase_path(get_target_outputs(":$target_name._gen"), root_build_dir)
      root = rebase_path(_out_dir, root_build_dir)
      package = invoker.package

      nested_in_python_package = ""
      if (defined(invoker.python_package)) {
        nested_in_python_package =
            get_label_info(invoker.python_package, "label_no_toolchain")
      }

      dependencies = []
      foreach(dep, invoker.deps) {
        dependencies +=
            rebase_path([ get_label_info(dep, "target_gen_dir") + "/" +
                              get_label_info(dep, "name") + ".json" ],
                        root_build_dir)
      }
    }
    write_file("$target_gen_dir/$target_name.json", _proto_info, "json")
  } else {
    # protoc is only ever invoked from pw_protobuf_compiler_TOOLCHAIN.
    not_needed([ "target_name" ])
    not_needed(invoker, "*")
  }
}

# Generates pw_protobuf C++ code for proto files, creating a source_set of the
# generated files. This is internal and should not be used outside of this file.
# Use pw_proto_library instead.
template("_pw_pwpb_rpc_proto_library") {
  # Create a target which runs protoc configured with the pwpb_rpc plugin to
  # generate the C++ proto RPC headers.
  _pw_invoke_protoc(target_name) {
    forward_variables_from(invoker, "*", _forwarded_vars)
    language = "pwpb_rpc"
    plugin = "$dir_pw_rpc/py/pw_rpc/plugin_pwpb.py"
    python_deps = [ "$dir_pw_rpc/py" ]
  }

  # Create a library with the generated source files.
  config("$target_name._include_path") {
    include_dirs = [ "${invoker.base_out_dir}/pwpb_rpc" ]
    visibility = [ ":*" ]
  }

  pw_source_set(target_name) {
    forward_variables_from(invoker, _forwarded_vars)
    public_configs = [ ":$target_name._include_path" ]
    deps = [ ":$target_name._gen($pw_protobuf_compiler_TOOLCHAIN)" ]
    public_deps = [
                    ":${invoker.base_target}.pwpb",
                    "$dir_pw_protobuf",
                    "$dir_pw_rpc:server",
                    "$dir_pw_rpc/pwpb:client_api",
                    "$dir_pw_rpc/pwpb:server_api",
                  ] + invoker.deps
    public = invoker.outputs
    check_includes = false
  }
}

template("_pw_pwpb_proto_library") {
  _pw_invoke_protoc(target_name) {
    forward_variables_from(invoker, "*", _forwarded_vars)
    language = "pwpb"
    plugin = "$dir_pw_protobuf/py/pw_protobuf/plugin.py"
    python_deps = [ "$dir_pw_protobuf/py" ]
  }

  # Create a library with the generated source files.
  config("$target_name._include_path") {
    include_dirs = [ "${invoker.base_out_dir}/pwpb" ]
    visibility = [ ":*" ]
  }

  pw_source_set(target_name) {
    forward_variables_from(invoker, _forwarded_vars)
    public_configs = [ ":$target_name._include_path" ]
    deps = [ ":$target_name._gen($pw_protobuf_compiler_TOOLCHAIN)" ]
    public_deps = [
                    "$dir_pw_containers:vector",
                    dir_pw_assert,
                    dir_pw_function,
                    dir_pw_preprocessor,
                    dir_pw_protobuf,
                    dir_pw_result,
                    dir_pw_status,
                  ] + invoker.deps
    sources = invoker.outputs
    public = filter_include(sources, [ "*.pwpb.h" ])
  }
}

# Generates nanopb RPC code for proto files, creating a source_set of the
# generated files. This is internal and should not be used outside of this file.
# Use pw_proto_library instead.
template("_pw_nanopb_rpc_proto_library") {
  # Create a target which runs protoc configured with the nanopb_rpc plugin to
  # generate the C++ proto RPC headers.
  _pw_invoke_protoc(target_name) {
    forward_variables_from(invoker, "*", _forwarded_vars)
    language = "nanopb_rpc"
    plugin = "$dir_pw_rpc/py/pw_rpc/plugin_nanopb.py"
    python_deps = [ "$dir_pw_rpc/py" ]
  }

  # Create a library with the generated source files.
  config("$target_name._include_path") {
    include_dirs = [ "${invoker.base_out_dir}/nanopb_rpc" ]
    visibility = [ ":*" ]
  }

  pw_source_set(target_name) {
    forward_variables_from(invoker, _forwarded_vars)
    public_configs = [ ":$target_name._include_path" ]
    deps = [ ":$target_name._gen($pw_protobuf_compiler_TOOLCHAIN)" ]
    public_deps = [
                    ":${invoker.base_target}.nanopb",
                    "$dir_pw_rpc:server",
                    "$dir_pw_rpc/nanopb:client_api",
                    "$dir_pw_rpc/nanopb:server_api",
                    "$dir_pw_third_party/nanopb",
                  ] + invoker.deps
    public = invoker.outputs
    check_includes = false
  }
}

# Generates nanopb code for proto files, creating a source_set of the generated
# files. This is internal and should not be used outside of this file. Use
# pw_proto_library instead.
template("_pw_nanopb_proto_library") {
  # When compiling with the Nanopb plugin, the nanopb.proto file is already
  # compiled internally, so skip recompiling it with protoc.
  if (rebase_path(invoker.sources, invoker.compile_dir) == [ "nanopb.proto" ]) {
    group("$target_name._gen") {
      deps = [
        ":${invoker.base_target}._sources($pw_protobuf_compiler_TOOLCHAIN)",
      ]
    }

    group("$target_name") {
      deps = invoker.deps +
             [ ":$target_name._gen($pw_protobuf_compiler_TOOLCHAIN)" ]
    }
  } else {
    # Create a target which runs protoc configured with the nanopb plugin to
    # generate the C proto sources.
    _pw_invoke_protoc(target_name) {
      forward_variables_from(invoker, "*", _forwarded_vars)
      language = "nanopb"
      plugin = "$dir_pw_third_party_nanopb/generator/protoc-gen-nanopb"
      other_deps = [ "$dir_pw_third_party/nanopb:generate_nanopb_proto.action" ]
    }

    # Create a library with the generated source files.
    config("$target_name._include_path") {
      include_dirs = [ "${invoker.base_out_dir}/nanopb" ]
      visibility = [ ":*" ]
    }

    pw_source_set(target_name) {
      forward_variables_from(invoker, _forwarded_vars)
      public_configs = [ ":$target_name._include_path" ]
      deps = [ ":$target_name._gen($pw_protobuf_compiler_TOOLCHAIN)" ]
      public_deps = [ "$dir_pw_third_party/nanopb" ] + invoker.deps
      sources = invoker.outputs
      public = filter_include(sources, [ "*.pb.h" ])
    }
  }
}

# Generates raw RPC code for proto files, creating a source_set of the generated
# files. This is internal and should not be used outside of this file. Use
# pw_proto_library instead.
template("_pw_raw_rpc_proto_library") {
  # Create a target which runs protoc configured with the nanopb_rpc plugin to
  # generate the C++ proto RPC headers.
  _pw_invoke_protoc(target_name) {
    forward_variables_from(invoker, "*", _forwarded_vars)
    language = "raw_rpc"
    plugin = "$dir_pw_rpc/py/pw_rpc/plugin_raw.py"
    python_deps = [ "$dir_pw_rpc/py" ]
  }

  # Create a library with the generated source files.
  config("$target_name._include_path") {
    include_dirs = [ "${invoker.base_out_dir}/raw_rpc" ]
    visibility = [ ":*" ]
  }

  pw_source_set(target_name) {
    forward_variables_from(invoker, _forwarded_vars)
    public_configs = [ ":$target_name._include_path" ]
    deps = [ ":$target_name._gen($pw_protobuf_compiler_TOOLCHAIN)" ]
    public_deps = [
                    "$dir_pw_rpc:server",
                    "$dir_pw_rpc/raw:client_api",
                    "$dir_pw_rpc/raw:server_api",
                  ] + invoker.deps
    public = invoker.outputs
    check_includes = false
  }
}

# Generates Go code for proto files, listing the proto output directory in the
# metadata variable GOPATH. Internal use only.
template("_pw_go_proto_library") {
  _proto_gopath = "$root_gen_dir/go"

  _pw_invoke_protoc(target_name) {
    forward_variables_from(invoker, "*")
    language = "go"
    metadata = {
      gopath = [ "GOPATH+=" + rebase_path(_proto_gopath) ]
      external_deps = [
        "github.com/golang/protobuf/proto",
        "google.golang.org/grpc",
      ]
    }

    # Override the default "$base_out_dir/$language" output path.
    out_dir = "$_proto_gopath/src"
  }

  group(target_name) {
    deps =
        invoker.deps + [ ":$target_name._gen($pw_protobuf_compiler_TOOLCHAIN)" ]
  }
}

# Generates Python code for proto files, creating a pw_python_package containing
# the generated files. This is internal and should not be used outside of this
# file. Use pw_proto_library instead.
template("_pw_python_proto_library") {
  _pw_invoke_protoc(target_name) {
    forward_variables_from(invoker, "*", _forwarded_vars + [ "python_package" ])
    language = "python"
    python_deps = [ "$dir_pw_protobuf_compiler:protobuf_requirements" ]

    if (defined(invoker.python_package)) {
      python_package = invoker.python_package
    }
  }

  if (defined(invoker.python_package) && invoker.python_package != "") {
    # This package is nested in another Python package. Depending on this
    # its python subtarget is equivalent to depending on the Python package it
    # is nested in.
    pw_python_group(target_name) {
      python_deps = [ invoker.python_package ]
    }

    # This proto library is merged into another package, but create a target to
    # collect its dependencies that the other package can depend on.
    pw_python_group(target_name + "._deps") {
      python_deps = invoker.deps
      other_deps =
          [ ":${invoker.target_name}._gen($pw_protobuf_compiler_TOOLCHAIN)" ]
    }
  } else {
    # Create a Python package with the generated source files.
    pw_python_package(target_name) {
      forward_variables_from(invoker, _forwarded_vars)
      generate_setup = {
        metadata = {
          name = invoker._package_dir
          version =
              "0.0.1"  # TODO(hepler): Need to be able to set this verison.
        }
      }
      sources = invoker.outputs
      strip_prefix = "${invoker.base_out_dir}/python"
      python_deps = invoker.deps
      other_deps = [ ":$target_name._gen($pw_protobuf_compiler_TOOLCHAIN)" ]
      static_analysis = []

      _pw_module_as_package = invoker.module_as_package != ""
    }
  }
}

# Generates protobuf code from .proto definitions for various languages.
# For each supported generator, creates a sub-target named:
#
#   <target_name>.<generator>
#
# GN permits using abbreviated labels when the target name matches the directory
# name (e.g. //foo for //foo:foo). For consistency with this, the sub-targets
# for each generator are aliased to the directory when the target name is the
# same. For example, these two labels are equivalent:
#
#   //path/to/my_protos:my_protos.pwpb
#   //path/to/my_protos:pwpb
#
# pw_protobuf_library targets generate Python packages. As such, they must have
# globally unique package names. The first directory of the prefix or the first
# common directory of the sources is used as the Python package.
#
# Args:
#   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.
#   python_package: Label of Python package to which to add the proto modules.
#       The .python subtarget will redirect to this package.
#
template("pw_proto_library") {
  assert(defined(invoker.sources) && invoker.sources != [],
         "pw_proto_library requires .proto source files")

  if (defined(invoker.python_module_as_package)) {
    _module_as_package = invoker.python_module_as_package

    _must_be_one_source = invoker.sources
    assert([ _must_be_one_source[0] ] == _must_be_one_source,
           "'python_module_as_package' requires exactly one source file")
    assert(_module_as_package != "",
           "'python_module_as_package' cannot be be empty")
    assert(string_split(_module_as_package, "/") == [ _module_as_package ],
           "'python_module_as_package' cannot contain slashes")
    assert(!defined(invoker.prefix),
           "'prefix' cannot be provided with 'python_module_as_package'")
  } else {
    _module_as_package = ""
  }

  if (defined(invoker.strip_prefix)) {
    _source_root = get_path_info(invoker.strip_prefix, "abspath")
  } else {
    _source_root = get_path_info(".", "abspath")
  }

  if (defined(invoker.prefix)) {
    _prefix = invoker.prefix
  } else {
    _prefix = ""
  }

  _package_dir = ""
  _source_names = []

  # Determine the Python package name to use for these protos. If there is no
  # prefix, the first directory the sources are nested under is used.
  foreach(source, rebase_path(invoker.sources, _source_root)) {
    _path_components = []
    _path_components = string_split(source, "/")

    if (_package_dir == "") {
      _package_dir = _path_components[0]
    } else {
      assert(_prefix != "" || _path_components[0] == _package_dir,
             "Unless 'prefix' is supplied, all .proto sources in a " +
                 "pw_proto_library must be in the same directory tree")
    }

    _source_names +=
        [ get_path_info(source, "dir") + "/" + get_path_info(source, "name") ]
  }

  # If the 'prefix' was supplied, use that for the package directory.
  if (_prefix != "") {
    _prefix_path_components = string_split(_prefix, "/")
    _package_dir = _prefix_path_components[0]
  }

  assert(_package_dir != "" && _package_dir != "." && _package_dir != "..",
         "Either a 'prefix' must be specified or all sources must be nested " +
             "under a common directory")

  # Define an action that is never executed to prevent duplicate proto packages
  # from being declared. The target name and the output file include only the
  # package directory, so different targets that use the same proto package name
  # will conflict.
  action("pw_proto_library.$_package_dir") {
    script = "$dir_pw_build/py/pw_build/nop.py"
    visibility = []

    # Place an error message in the output path (which is never created). If the
    # package name conflicts occur in different BUILD.gn files, this results in
    # an otherwise cryptic Ninja error, rather than a GN error.
    outputs = [ "$root_out_dir/ " +
                "ERROR - Multiple pw_proto_library targets create the " +
                "'$_package_dir' package. Change the package name by setting " +
                "the \"prefix\" arg or move the protos to a different " +
                "directory, then re-run gn gen." ]
  }

  if (defined(invoker.deps)) {
    _deps = invoker.deps
  } else {
    _deps = []
  }

  _common = {
    base_target = target_name

    # This is the output directory for all files related to this proto library.
    # Sources are mirrored to "$base_out_dir/sources" and protoc puts outputs in
    # "$base_out_dir/$language" by default.
    base_out_dir =
        get_label_info(":$target_name($pw_protobuf_compiler_TOOLCHAIN)",
                       "target_gen_dir") + "/$target_name.proto_library"

    compile_dir = "$base_out_dir/sources"

    # Refer to the source files as the are mirrored to the output directory.
    sources = []
    foreach(file, rebase_path(invoker.sources, _source_root)) {
      sources += [ "$compile_dir/$_prefix/$file" ]
    }

    package = _package_dir
  }

  # For each proto target, create a file which collects the base directories of
  # all of its dependencies to list as include paths to protoc.
  generated_file("$target_name._includes") {
    # Collect metadata from the include path files of each dependency.

    deps = []
    foreach(dep, _deps) {
      _base = get_label_info(dep, "label_no_toolchain")
      deps += [ "$_base._includes(" + get_label_info(dep, "toolchain") + ")" ]
    }

    data_keys = [ "protoc_includes" ]
    outputs = [ "${_common.base_out_dir}/includes.txt" ]

    # Indicate this library's base directory for its dependents.
    metadata = {
      protoc_includes = [ rebase_path(_common.compile_dir, root_build_dir) ]
    }
  }

  # Mirror the proto sources to the output directory with the prefix added.
  if (current_toolchain == pw_protobuf_compiler_TOOLCHAIN) {
    pw_mirror_tree("$target_name._sources") {
      source_root = _source_root
      sources = invoker.sources

      if (defined(invoker.inputs)) {
        sources += invoker.inputs
      }

      directory = "${_common.compile_dir}/$_prefix"
    }
  } else {
    not_needed(invoker, [ "inputs" ])
  }

  # Enumerate all of the protobuf generator targets.

  _pw_pwpb_rpc_proto_library("$target_name.pwpb_rpc") {
    forward_variables_from(invoker, _forwarded_vars)
    forward_variables_from(_common, "*")

    deps = []
    foreach(dep, _deps) {
      _base = get_label_info(dep, "label_no_toolchain")
      deps += [ "$_base.pwpb_rpc(" + get_label_info(dep, "toolchain") + ")" ]
    }

    outputs = []
    foreach(name, _source_names) {
      outputs += [ "$base_out_dir/pwpb_rpc/$_prefix/${name}.rpc.pwpb.h" ]
    }
  }

  _pw_pwpb_proto_library("$target_name.pwpb") {
    forward_variables_from(invoker, _forwarded_vars)
    forward_variables_from(_common, "*")

    deps = []
    foreach(dep, _deps) {
      _base = get_label_info(dep, "label_no_toolchain")
      deps += [ "$_base.pwpb(" + get_label_info(dep, "toolchain") + ")" ]
    }

    outputs = []
    foreach(name, _source_names) {
      outputs += [ "$base_out_dir/pwpb/$_prefix/${name}.pwpb.h" ]
    }
  }

  if (dir_pw_third_party_nanopb != "") {
    _pw_nanopb_rpc_proto_library("$target_name.nanopb_rpc") {
      forward_variables_from(invoker, _forwarded_vars)
      forward_variables_from(_common, "*")

      deps = []
      foreach(dep, _deps) {
        _lbl = get_label_info(dep, "label_no_toolchain")
        deps += [ "$_lbl.nanopb_rpc(" + get_label_info(dep, "toolchain") + ")" ]
      }

      outputs = []
      foreach(name, _source_names) {
        outputs += [ "$base_out_dir/nanopb_rpc/$_prefix/${name}.rpc.pb.h" ]
      }
    }

    _pw_nanopb_proto_library("$target_name.nanopb") {
      forward_variables_from(invoker, _forwarded_vars)
      forward_variables_from(_common, "*")

      deps = []
      foreach(dep, _deps) {
        _base = get_label_info(dep, "label_no_toolchain")
        deps += [ "$_base.nanopb(" + get_label_info(dep, "toolchain") + ")" ]
      }

      outputs = []
      foreach(name, _source_names) {
        outputs += [
          "$base_out_dir/nanopb/$_prefix/${name}.pb.h",
          "$base_out_dir/nanopb/$_prefix/${name}.pb.c",
        ]
      }
    }
  } else {
    pw_error("$target_name.nanopb_rpc") {
      message =
          "\$dir_pw_third_party_nanopb must be set to generate nanopb RPC code."
    }

    pw_error("$target_name.nanopb") {
      message =
          "\$dir_pw_third_party_nanopb must be set to compile nanopb protobufs."
    }
  }

  _pw_raw_rpc_proto_library("$target_name.raw_rpc") {
    forward_variables_from(invoker, _forwarded_vars)
    forward_variables_from(_common, "*")

    deps = []
    foreach(dep, _deps) {
      _base = get_label_info(dep, "label_no_toolchain")
      deps += [ "$_base.raw_rpc(" + get_label_info(dep, "toolchain") + ")" ]
    }

    outputs = []
    foreach(name, _source_names) {
      outputs += [ "$base_out_dir/raw_rpc/$_prefix/${name}.raw_rpc.pb.h" ]
    }
  }

  _pw_go_proto_library("$target_name.go") {
    sources = _common.sources

    deps = []
    foreach(dep, _deps) {
      _base = get_label_info(dep, "label_no_toolchain")
      deps += [ "$_base.go(" + get_label_info(dep, "toolchain") + ")" ]
    }

    forward_variables_from(_common, "*")
  }

  _pw_python_proto_library("$target_name.python") {
    forward_variables_from(_common, "*")
    forward_variables_from(invoker, [ "python_package" ])
    module_as_package = _module_as_package

    deps = []
    foreach(dep, _deps) {
      _base = get_label_info(dep, "label_no_toolchain")
      deps += [ "$_base.python(" + get_label_info(dep, "toolchain") + ")" ]
    }

    if (module_as_package == "") {
      _python_prefix = "$base_out_dir/python/$_prefix"
    } else {
      _python_prefix = "$base_out_dir/python/$module_as_package"
    }

    outputs = []
    foreach(name, _source_names) {
      outputs += [
        "$_python_prefix/${name}_pb2.py",
        "$_python_prefix/${name}_pb2.pyi",
      ]
    }
  }

  # All supported pw_protobuf generators.
  _protobuf_generators = [
    "pwpb",
    "pwpb_rpc",
    "nanopb",
    "nanopb_rpc",
    "raw_rpc",
    "go",
    "python",
  ]

  # If the label matches the directory name, alias the subtargets to the
  # directory (e.g. //foo:nanopb is an alias for //foo:foo.nanopb).
  if (get_label_info(":$target_name", "name") ==
      get_path_info(get_label_info(":$target_name", "dir"), "name")) {
    foreach(_generator, _protobuf_generators - [ "python" ]) {
      group(_generator) {
        public_deps = [ ":${invoker.target_name}.$_generator" ]
      }
    }

    pw_python_group("python") {
      python_deps = [ ":${invoker.target_name}.python" ]
    }
  }

  # If the user attempts to use the target directly instead of one of the
  # generator targets, run a script which prints a nice error message.
  pw_python_action(target_name) {
    script = string_join("/",
                         [
                           dir_pw_protobuf_compiler,
                           "py",
                           "pw_protobuf_compiler",
                           "proto_target_invalid.py",
                         ])
    args = [
             "--target",
             target_name,
             "--dir",
             get_path_info(".", "abspath"),
             "--root",
             "//",
           ] + _protobuf_generators
    stamp = true
  }
}