build/icu/third_party_icu_headers.gni

# Copyright 2023 The Fuchsia Authors. All rights reserved.
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.

import("config.gni")

# Generates an include file that includes all needed third_party/icu
# headers, and presents them as a build target.
#
# To use, do the following:
#
# - In your BUILD file:
#
#   import("//build/icu/third_party_icu_headers.gni")
#
#   third_party_icu_headers("some_headers") {
#     headers = [
#       "third_party/icu/foo/bar/baz.h",
#     ]
#   }
#
#   source_set("your_library") {
#     # ...
#     deps = [ ":some_headers" ]
#     # ...
#   }
#
# - In your source file:
#
#   #include "your/source/file/directory/some_headers.h"
#
# This will produce a file `some_headers.h` in
#
#   "${target_gen_dir}/your/source/file/directory/some_headers.h"
#
# which consists of only includes of the named headers, but with the initial
# "third_party/icu" substituted with the content of `$icu_root`. If
# the value was set to `icu_root = "//other_dir/icu", the resulting file
# content would be:
#
#   #include "other_dir/icu/foo/bar/baz.h"
#
# for each header listed in the `headers` parameter.
#
# This allows us to place the library `//third_party/icu` into a different
# directory, in a way that is transparent to the library users, so long as
# the headers are declared using `third_party_icu_headers`.
#
# We use this approach because we must vary the path to the ICU library to
# account for issues in our build process for some products.  The key variable
# is $icu_root, which determines where the build system will look for the ICU
# library. However, since the header files are also included in our source
# files, and their names vary with the choice of the ICU library location, we
# must fix up those include paths as well. Since there is no elegant way to do
# this in source directly, we generate the appropriate headers and include
# those.
#
# Args:
#
#    headers: list(target): a list of headers from "third_party/icu" that you
#             need to include.
#
#    icu_assembly: bool: if set, builds ICU assembly flavors of this target.
#             This is useful for a gradual rollout of flavored builds.
template("third_party_icu_headers") {
  forward_variables_from(invoker,
                         [
                           "headers",
                           "icu_assembly",
                         ])

  icu_target_name = target_name

  _header_name = "${target_gen_dir}/${icu_target_name}.h"

  icu_assembly = false
  if (defined(invoker.icu_assembly)) {
    icu_assembly = invoker.icu_assembly
  }

  # Separate source targets, one for each ICU flavor supported.  All built from
  # the same file, which has different FUCHSIA_ICU_VARIANT env variables defined.
  if (icu_assembly) {
    foreach(icu_flavor, icu_flavors) {
      source_set(
          "${icu_target_name}.icu_${icu_flavor.name}_${icu_flavor.commit_id}") {
        # gn check is wrong about includes here. Don't check.
        # See the same declaration at the very bottom of this file for the
        # reason.
        check_includes = false
        sources = [ _header_name ]
        public_deps = [ "//third_party/icu/${icu_flavor.name}:icu" ]
        public_configs = [
          # defines FUCHSIA_ICU_VARIANT
          "//src/lib/icu:${icu_flavor.name}",
        ]
      }
    }
  }

  # Writes out the header file contents during the analysis phase.
  _contents = []

  # "//third_party/icu" --> "third_party/icu"
  _icu_dir = string_replace(icu_root, "//", "", 1)

  if (icu_assembly) {
    _icu_dirs = {
      default = "third_party/icu/default"
      stable = "third_party/icu/stable"
      latest = "third_party/icu/latest"
    }
  }

  _contents_map = {
    default = []
    latest = []
    stable = []
  }
  foreach(_header, headers) {
    # #include "third_party/icu/blah" --> #include "${icu_root}/blah"
    _fixed_header_path = string_replace(_header, "third_party/icu", _icu_dir, 1)
    _contents += [ "#include \"${_fixed_header_path}\"" ]

    if (icu_assembly) {
      # Same as above, but for flavors.
      _header_path = {
      }
      _header_path = {
        default =
            string_replace(_header, "third_party/icu", _icu_dirs.default, 1)
        stable = string_replace(_header, "third_party/icu", _icu_dirs.stable, 1)
        latest = string_replace(_header, "third_party/icu", _icu_dirs.latest, 1)
      }

      _contents_map.default += [ "#include \"${_header_path.default}\"" ]
      _contents_map.stable += [ "#include \"${_header_path.stable}\"" ]
      _contents_map.latest += [ "#include \"${_header_path.latest}\"" ]
    }
  }

  _all_contents =
      [
        "/* Autogenerated by //build/icu/third_party_icu_headers. */",
        "#ifndef FUCHSIA_ICU_VARIANT /* For non-varianted builds. */",
      ] + _contents +
      [ "#elif FUCHSIA_ICU_VARIANT == 0 /* //src/lib/icu:default */" ] +
      _contents_map.default +
      [ "#elif FUCHSIA_ICU_VARIANT == 1 /* //src/lib/icu:stable */" ] +
      _contents_map.stable +
      [ "#elif FUCHSIA_ICU_VARIANT == 2 /* //src/lib/icu:latest */" ] +
      _contents_map.latest +
      [ "#elif FUCHSIA_ICU_VARIANT == 3 /* //src/lib/icu:icu_root */" ] +
      _contents +
      [
        "#else",
        "/* Refer to //src/lib/icu:{default,stable,latest,icu_root} */",
        "#  error FUCHSIA_ICU_VARIANT needs a value between 0 or 3",
        "#endif",
      ]

  write_file(_header_name, _all_contents, "list lines")

  source_set("${icu_target_name}") {
    # gn check is wrong about includes here. Don't check.
    #
    # gn check does not seem to take the preprocessor results when checking
    # for inclusion. The generated file here has several mutually exclusive
    # ifdefs, each including its appropriate public_deps.  Each such ifdef
    # branch has its includes and deps set correctly, but gn does not know
    # that and objects about missing deps for includes that are effectively
    # ifdef'd out.
    check_includes = false
    sources = [ _header_name ]
    public_deps = [
      "//src/lib/icu:data",
      "//src/lib/icu:lib",
    ]
    public_configs = [
      "${icu_root}:icu_config",

      # defines FUCHSIA_ICU_VARIANT
      "//src/lib/icu:icu_root",
    ]
  }
}