# Copyright 2016 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.

# The GN files in //third_party/flutter all use $flutter_root/
# in place of // to refer to the root of the flutter source tree.
flutter_root = "//third_party/flutter"

declare_args() {
  # Debug build.
  is_debug = true
}

if (target_os == "") {
  target_os = "fuchsia"
}
if (target_cpu == "") {
  target_cpu = host_cpu
}
if (current_cpu == "") {
  current_cpu = target_cpu
}
if (current_os == "") {
  current_os = target_os
}
host_platform = "${host_os}-${host_cpu}"

if (target_os == "fuchsia") {
  target_toolchain = "//build/toolchain/fuchsia:${target_cpu}"
} else {
  assert(false, "Target OS not supported")
}

if (host_os == "linux" || host_os == "mac") {
  host_toolchain = "//build/toolchain:host_${host_cpu}"
} else {
  assert(false, "Host OS not supported")
}

set_default_toolchain(target_toolchain)

# Some projects expect a default value for sources_assignment_filter.
sources_assignment_filter = []

declare_args() {
  # *This should never be set as a build argument.*
  # It exists only to be set in `toolchain_args`.
  # See //build/toolchain/clang_toolchain.gni for details.
  # This variable is a scope giving details about the current toolchain:
  #     `toolchain_variant.base`
  #         [label] The "base" toolchain for this variant, *often the
  #         right thing to use in comparisons, not `current_toolchain`.*
  #         This is the toolchain actually referenced directly in GN
  #         source code.  If the current toolchain is not
  #         `shlib_toolchain` or a variant toolchain, this is the same
  #         as `current_toolchain`.  In one of those derivative
  #         toolchains, this is the toolchain the GN code probably
  #         thought it was in.  This is the right thing to use in a test
  #         like `toolchain_variant.base == target_toolchain`, rather
  #         rather than comparing against `current_toolchain`.
  #     `toolchain_variant.name`
  #         [string] The name of this variant, as used in `variant` fields
  #         in [`select_variant`](#select_variant) clauses.  In the base
  #         toolchain and its `shlib_toolchain`, this is `""`.
  #     `toolchain_variant.suffix`
  #         [string] This is "-${toolchain_variant.name}", "" if name is empty.
  #     `toolchain_variant.is_pic_default`
  #         [bool] This is true in `shlib_toolchain`.
  # The other fields are the variant's effects as defined in
  # [`known_variants`](#known_variants).
  toolchain_variant = {
    base = target_toolchain  # default toolchain
  }
}

if (!defined(toolchain_variant.name)) {
  # Default values describe the "null variant".
  # All the optional fields (except `toolchain_args`) are canonicalized
  # to their default/empty values so the code below doesn't need to have
  # `defined(toolchain_variant.field)` checks all over.
  toolchain_variant.name = ""
  toolchain_variant.suffix = ""
  toolchain_variant.configs = []
  toolchain_variant.remove_common_configs = []
  toolchain_variant.remove_shared_configs = []
  toolchain_variant.deps = []
  toolchain_variant.is_pic_default = false
}

is_android = false
is_fuchsia = false
is_fuchsia_host = false
is_ios = false
is_linux = false
is_mac = false
is_win = false
is_clang = true
is_component_build = false
is_official_build = false

# This is set to allow third party projects to configure their GN build based
# on the knowledge that they're being built in the Fuchsia tree. In the
# subproject this can be tested with
#   `if (defined(is_fuchsia_tree) && is_fuchsia_tree) { ... }`
# thus allowing configuration without requiring all users of the subproject to
# set this variable.
is_fuchsia_tree = true

if (current_os == "fuchsia") {
  is_fuchsia = true
} else if (current_os == "linux") {
  is_linux = true
} else if (current_os == "mac") {
  is_mac = true
}

# Some library targets may be built as different type depending on the target
# platform. This variable specifies the default library type for each target.
if (is_fuchsia) {
  default_library_type = "shared_library"
} else {
  default_library_type = "static_library"
}

# When we are in a variant of host_toolchain, change the value of
# host_toolchain so that `if (current_toolchain == host_toolchain)` tests
# still match, since that is the conventional way to detect being in host
# context.  This means that any "...($host_toolchain)" label references
# from inside a variant of host_toolchain will refer to the variant
# (current_toolchain rather than host_toolchain).  To handle this, the
# `executable()` template below will define its target in other variant
# toolchains as a copy of the real executable.
if (toolchain_variant.base == host_toolchain) {
  is_fuchsia_host = true
  host_toolchain += toolchain_variant.suffix
}

# References should use `"label($shlib_toolchain)"` rather than
# `"label(${target_toolchain}-shared)"` or anything else.
shlib_toolchain = "${toolchain_variant.base}${toolchain_variant.suffix}-shared"

# All binary targets will get this list of configs by default.
default_common_binary_configs = [
  "//build/config:compiler",
  "//build/config:relative_paths",
  "//build/config:default_frame_pointers",
  "//build/config:default_include_dirs",
  "//build/config:default_symbols",
  "//build/config:default_warnings",
  "//build/config:no_exceptions",
  "//build/config:no_rtti",
  "//build/config:symbol_visibility_hidden",
]

if (is_debug) {
  default_common_binary_configs += [ "//build/config:debug" ]
} else {
  default_common_binary_configs += [ "//build/config:release" ]
}

if (is_fuchsia) {
  default_common_binary_configs += [
    "//build/config/fuchsia:icf",
    "//build/config/fuchsia:thread_safety_annotations",
    "//build/config/fuchsia:werror",

    # TODO(mcgrathr): Perhaps restrict this to only affected code.
    # For now, safest to do it everywhere.
    "//build/config/fuchsia:zircon_asserts",
  ]
}

default_common_binary_configs += [ "//build/config/lto:default" ]

# Add and remove configs specified by the variant.
default_common_binary_configs += toolchain_variant.configs
default_common_binary_configs -= toolchain_variant.remove_common_configs

default_shared_library_configs = default_common_binary_configs + [
                                   "//build/config:shared_library_config",
                                   "//build/config:symbol_no_undefined",
                                 ]
default_shared_library_configs -= toolchain_variant.remove_shared_configs

default_executable_configs = default_common_binary_configs + [
                               "//build/config:executable_config",
                               "//build/config:default_libs",
                             ]
default_executable_deps = [ "//build/config/scudo:default_for_executable" ]

if (toolchain_variant.is_pic_default) {
  default_common_binary_configs += [ "//build/config:shared_library_config" ]
}

# Apply that default list to the binary target types.
set_defaults("source_set") {
  configs = default_common_binary_configs
}
set_defaults("static_library") {
  configs = default_common_binary_configs
}
set_defaults("shared_library") {
  configs = default_shared_library_configs
}
set_defaults("loadable_module") {
  configs = default_shared_library_configs
}
set_defaults("executable") {
  configs = default_executable_configs
}

if (is_fuchsia) {
  if (!toolchain_variant.is_pic_default) {
    # In the main toolchain, shared_library just redirects to the same
    # target in the -shared toolchain.
    template("shared_library") {
      group(target_name) {
        public_deps = [
          ":$target_name(${current_toolchain}-shared)",
        ]
        forward_variables_from(invoker,
                               [
                                 "testonly",
                                 "visibility",
                               ])

        # Mark all variables as not needed to suppress errors for unused
        # variables.  The other variables normally passed to shared_library
        # are actually used by the shared_library instantiation in the
        # -shared toolchain, so any going truly unused will be caught there.
        not_needed(invoker, "*")
      }
    }
  } else {
    # In the -shared toolchain, shared_library is just its normal self,
    # but if the invoker constrained the visibility, we must make sure
    # the dependency from the main toolchain is still allowed.
    template("shared_library") {
      shared_library(target_name) {
        # Explicitly forward visibility, implicitly forward everything
        # else.  Forwarding "*" doesn't recurse into nested scopes (to
        # avoid copying all globals into each template invocation), so
        # won't pick up file-scoped variables.  Normally this isn't too
        # bad, but visibility is commonly defined at the file scope.
        # Explicitly forwarding visibility and then excluding it from the
        # "*" set works around this problem.  See http://crbug.com/594610
        # for rationale on why this GN behavior is not considered a bug.
        forward_variables_from(invoker, [ "visibility" ])
        forward_variables_from(invoker, "*", [ "visibility" ])
        if (defined(visibility)) {
          visibility += [ ":$target_name" ]
        }
      }
    }
  }
}

# This is the basic "asan" variant.  Others start with this and modify.
# See `known_variants` (below) for the meaning of fields in this scope.
_asan_variant = {
  configs = [ "//build/config/sanitizers:asan" ]
  if (host_os != "fuchsia") {
    host_only = {
      # On most systems (not Fuchsia), the sanitizer runtimes are normally
      # linked statically and so `-shared` links do not include them.
      # Using `-shared --no-undefined` with sanitized code will get
      # undefined references for the sanitizer runtime calls generated by
      # the compiler.  It shouldn't do much harm, since the non-variant
      # builds will catch the real undefined reference bugs.
      remove_shared_configs = [ "//build/config:symbol_no_undefined" ]
    }
  }
  toolchain_args = {
    # -fsanitize=scudo is incompatible with -fsanitize=address.
    use_scudo = false
  }
}

declare_args() {
  # List of variants that will form the basis for variant toolchains.
  # To make use of a variant, set [`select_variant`](#select_variant).
  #
  # Normally this is not set as a build argument, but it serves to
  # document the available set of variants.
  # See also [`universal_variants`](#universal_variants).
  # Only set this to remove all the default variants here.
  # To add more, set [`extra_variants`](#extra_variants) instead.
  #
  # Each element of the list is one variant, which is a scope defining:
  #
  #   `configs` (optional)
  #       [list of labels] Each label names a config that will be
  #       automatically used by every target built in this variant.
  #       For each config `${label}`, there must also be a target
  #       `${label}_deps`, which each target built in this variant will
  #       automatically depend on.  The `variant()` template is the
  #       recommended way to define a config and its `_deps` target at
  #       the same time.
  #
  #   `remove_common_configs` (optional)
  #   `remove_shared_configs` (optional)
  #       [list of labels] This list will be removed (with `-=`) from
  #       the `default_common_binary_configs` list (or the
  #       `default_shared_library_configs` list, respectively) after
  #       all other defaults (and this variant's configs) have been
  #       added.
  #
  #   `deps` (optional)
  #       [list of labels] Added to the deps of every target linked in
  #       this variant (as well as the automatic `${label}_deps` for
  #       each label in configs).
  #
  #   `name` (required if configs is omitted)
  #       [string] Name of the variant as used in
  #       [`select_variant`](#select_variant) elements' `variant` fields.
  #       It's a good idea to make it something concise and meaningful when
  #       seen as e.g. part of a directory name under `$root_build_dir`.
  #       If name is omitted, configs must be nonempty and the simple names
  #       (not the full label, just the part after all `/`s and `:`s) of these
  #       configs will be used in toolchain names (each prefixed by a "-"),
  #       so the list of config names forming each variant must be unique
  #       among the lists in `known_variants + extra_variants`.
  #
  #   `toolchain_args` (optional)
  #       [scope] Each variable defined in this scope overrides a
  #       build argument in the toolchain context of this variant.
  #
  #   `host_only` (optional)
  #   `target_only` (optional)
  #       [scope] This scope can contain any of the fields above.
  #       These values are used only for host or target, respectively.
  #       Any fields included here should not also be in the outer scope.
  #
  known_variants = [
    {
      configs = [ "//build/config/lto" ]
    },
    {
      configs = [ "//build/config/lto:thinlto" ]
    },

    {
      configs = [ "//build/config/profile" ]
    },

    {
      configs = [ "//build/config/scudo" ]
    },

    {
      configs = [ "//build/config/sanitizers:ubsan" ]
    },
    {
      configs = [
        "//build/config/sanitizers:ubsan",
        "//build/config/sanitizers:sancov",
      ]
    },

    _asan_variant,
    {
      forward_variables_from(_asan_variant, "*")
      configs += [ "//build/config/sanitizers:sancov" ]
    },
    {
      name = "asan_no_detect_leaks"
      forward_variables_from(_asan_variant, "*", [ "toolchain_args" ])
      toolchain_args = {
        forward_variables_from(_asan_variant.toolchain_args, "*")
        asan_default_options = "detect_leaks=0"
      }
    },

    # Fuzzer variants for various sanitizers,  -fsanitize=fuzzer results
    # in undefined symbols in shared objects that are satisfied in the final,
    # statically linked fuzzer.
    {
      forward_variables_from(_asan_variant, "*")
      configs += [ "//build/config/sanitizers:fuzzer" ]
      remove_shared_configs = [ "//build/config:symbol_no_undefined" ]
    },
    {
      forward_variables_from(_asan_variant, "*")
      configs += [ "//build/config/sanitizers:fuzzer_no_link" ]
      remove_shared_configs = [ "//build/config:symbol_no_undefined" ]
    },
    {
      configs = [
        "//build/config/sanitizers:ubsan",
        "//build/config/sanitizers:fuzzer",
      ]
      remove_shared_configs = [ "//build/config:symbol_no_undefined" ]
    },
  ]

  # Additional variant toolchain configs to support.
  # This is just added to [`known_variants`](#known_variants).
  extra_variants = []

  # List of "universal" variants, in addition to
  # [`known_variants`](#known_variants).  Normally this is not set as a
  # build argument, but it serves to document the available set of
  # variants.  These are treated just like
  # [`known_variants`](#known_variants), but as well as being variants by
  # themselves, these are also combined with each of
  # [`known_variants`](#known_variants) to form additional variants,
  # e.g. "asan-debug" or "ubsan-sancov-release".
  universal_variants = []

  # Only one of "debug" and "release" is really available as a universal
  # variant in any given build (depending on the global setting of
  # `is_debug`).  But this gets evaluated separately in every toolchain, so
  # e.g. in the "release" toolchain the sense of `if (is_debug)` tests is
  # inverted and this would list only "debug" as an available variant.  The
  # selection logic in `variant_target()` can only work if the value of
  # `universal_variants` it sees includes the current variant.
  if (is_debug) {
    universal_variants += [
      {
        name = "release"
        configs = []
        toolchain_args = {
          is_debug = false
        }
      },
    ]
  } else {
    universal_variants += [
      {
        name = "debug"
        configs = []
        toolchain_args = {
          is_debug = true
        }
      },
    ]
  }

  # List of short names for commonly-used variant selectors.  Normally this
  # is not set as a build argument, but it serves to document the available
  # set of short-cut names for variant selectors.  Each element of this list
  # is a scope where `.name` is the short name and `.select_variant` is a
  # a list that can be spliced into [`select_variant`](#select_variant).
  select_variant_shortcuts = [
    {
      name = "host_asan"
      select_variant = []
      select_variant = [
        {
          variant = "asan_no_detect_leaks"
          host = true
          dir = [
            # TODO(TO-565): The yasm host tools have leaks.
            "//third_party/yasm",

            # TODO(TO-666): replace futiltiy & cgpt with 1p tools
            "//third_party/vboot_reference",
            "//garnet/tools/vboot_reference",
          ]
        },
        {
          variant = "asan"
          host = true
        },
      ]
    },
    # TODO(TC-241): Remove this when TC-241 is fixed.  For now, don't
    # apply ASan to drivers because all drivers (and nothing else)
    # use -static-libstdc++ and so hit the TC-241 problem.
    {
      name = "asan"
      select_variant = []
      select_variant = [
        {
          target_type = [ "driver_module" ]
          variant = false
        },
        {
          variant = "asan"
          host = false
        },
      ]
    },
  ]
}

# Now elaborate the fixed shortcuts with implicit shortcuts for
# each known variant.  The shortcut is just the name of the variant
# and selects for `host=false`.
_select_variant_shortcuts = select_variant_shortcuts
foreach(variant, known_variants) {
  if (defined(variant.name)) {
    variant = variant.name
  } else {
    # This is how GN spells "let".
    foreach(configs, [ variant.configs ]) {
      variant = ""
      foreach(config, configs) {
        config = get_label_info(config, "name")
        if (variant == "") {
          variant = config
        } else {
          variant += "-$config"
        }
      }
    }
  }
  _select_variant_shortcuts += [
    {
      name = variant
      select_variant = []
      select_variant = [
        {
          variant = name
          host = false
        },
      ]
    },
  ]
  foreach(universal_variant, universal_variants) {
    _select_variant_shortcuts += [
      {
        name = "${variant}-${universal_variant.name}"
        select_variant = []
        select_variant = [
          {
            variant = name
            host = false
          },
        ]
      },
    ]
  }
}
foreach(variant, universal_variants) {
  variant = variant.name
  _select_variant_shortcuts += [
    {
      name = variant
      select_variant = []
      select_variant = [
        {
          variant = name
          host = false
        },
      ]
    },
  ]
}

declare_args() {
  # List of "selectors" to request variant builds of certain targets.
  # Each selector specifies matching criteria and a chosen variant.
  # The first selector in the list to match a given target determines
  # which variant is used for that target.
  #
  # Each selector is either a string or a scope.  A shortcut selector is
  # a string; it gets expanded to a full selector.  A full selector is a
  # scope, described below.
  #
  # A string selector can match a name in
  # [`select_variant_shortcuts`](#select_variant_shortcuts).  If it's not a
  # specific shortcut listed there, then it can be the name of any variant
  # described in [`known_variants`](#known_variants) and
  # [`universal_variants`](#universal_variants) (and combinations thereof).
  # A `selector` that's a simple variant name selects for every binary
  # built in the target toolchain: `{ host=false variant=selector }`.
  #
  # If a string selector contains a slash, then it's `"shortcut/filename"`
  # and selects only the binary in the target toolchain whose `output_name`
  # matches `"filename"`, i.e. it adds `output_name=["filename"]` to each
  # selector scope that the shortcut's name alone would yield.
  #
  # The scope that forms a full selector defines some of these:
  #
  #     variant (required)
  #         [string or `false`] The variant that applies if this selector
  #         matches.  This can be `false` to choose no variant, or a string
  #         that names the variant.  See
  #         [`known_variants`](#known_variants) and
  #         [`universal_variants`](#universal_variants).
  #
  # The rest below are matching criteria.  All are optional.
  # The selector matches if and only if all of its criteria match.
  # If none of these is defined, then the selector always matches.
  #
  # The first selector in the list to match wins and then the rest of
  # the list is ignored.  So construct more complex rules by using a
  # "blacklist" selector with `variant=false` before a catch-all or
  # "whitelist" selector that names a variant.
  #
  # Each "[strings]" criterion is a list of strings, and the criterion
  # is satisfied if any of the strings matches against the candidate string.
  #
  #     host
  #         [boolean] If true, the selector matches in the host toolchain.
  #         If false, the selector matches in the target toolchain.
  #
  #     testonly
  #         [boolean] If true, the selector matches targets with testonly=true.
  #         If false, the selector matches in targets without testonly=true.
  #
  #     target_type
  #         [strings]: `"executable"`, `"loadable_module"`, or `"driver_module"`
  #
  #     output_name
  #         [strings]: target's `output_name` (default: its `target name`)
  #
  #     label
  #         [strings]: target's full label with `:` (without toolchain suffix)
  #
  #     name
  #         [strings]: target's simple name (label after last `/` or `:`)
  #
  #     dir
  #         [strings]: target's label directory (`//dir` for `//dir:name`).
  select_variant = []

  # *This should never be set as a build argument.*
  # It exists only to be set in `toolchain_args`.
  # See //build/toolchain/clang_toolchain.gni for details.
  select_variant_canonical = []
}

# Do this only once, in the default toolchain context.    Then
# clang_toolchain_suite will just pass the results through to every
# other toolchain via toolchain_args so the work is not repeated.
if (toolchain_variant.base == target_toolchain && current_cpu == target_cpu &&
    current_os == target_os && toolchain_variant.name == "" &&
    !toolchain_variant.is_pic_default) {
  assert(select_variant_canonical == [],
         "`select_variant_canonical` cannot be set as a build argument")

  foreach(selector, select_variant) {
    if (selector != "$selector") {
      # It's a scope, not a string.  Just use it as is.
      select_variant_canonical += [ selector ]
    } else {
      # It's a string, not a scope.  Expand the shortcut.
      # If there is a slash, this is "shortcut/output_name".
      # If not, it's just "shortcut".
      foreach(file, [ get_path_info(selector, "file") ]) {
        if (file == selector) {
          file = ""
        } else {
          selector = get_path_info(selector, "dir")
        }
        foreach(shortcut, _select_variant_shortcuts) {
          # file=true stands in for "break".
          if (file != true && selector == shortcut.name) {
            # Found the matching shortcut.
            if (file == "") {
              # It applies to everything, so just splice it in directly.
              select_variant_canonical += shortcut.select_variant
            } else {
              # Add each of the shortcut's clauses amended with the
              # output_name constraint.
              foreach(clause, shortcut.select_variant) {
                select_variant_canonical += [
                  {
                    forward_variables_from(clause, "*")
                    output_name = [ file ]
                  },
                ]
              }
            }
            file = true
          }
        }
        assert(file == true,
               "unknown shortcut `${selector}` used in `select_variant`")
      }
    }
  }
}

template("variant_target") {
  target_type = target_name
  target_name = invoker.target_name
  target_invoker = {
    # Explicitly forward visibility, implicitly forward everything else.
    # See comment in template("shared_library") above for details.
    forward_variables_from(invoker, [ "visibility" ])
    forward_variables_from(invoker,
                           "*",
                           [
                             "_target_type",
                             "target_name",
                             "visibility",
                           ])

    if (!defined(output_name)) {
      output_name = target_name
    }
  }

  # target_type is the real GN target type that builds the thing.
  # selector_target_type is the name matched against target_type selectors.
  if (defined(invoker._target_type)) {
    selector_target_type = invoker._target_type
  } else {
    selector_target_type = target_type
  }

  target_label = get_label_info(":$target_name", "label_no_toolchain")

  # These are not actually used in all possible if branches below,
  # so defang GN's extremely sensitive "unused variable" errors.
  not_needed([
               "selector_target_type",
               "target_invoker",
               "target_label",
               "target_type",
             ])

  target_variant = false
  if (select_variant_canonical != []) {
    # See if there is a selector that matches this target.
    selected = false
    foreach(selector, select_variant_canonical) {
      # The first match wins.
      # GN's loops don't have "break", so do nothing on later iterations.
      if (!selected) {
        # Expand the selector so we don't have to do a lot of defined(...)
        # tests below.
        select = {
        }
        select = {
          target_type = []
          output_name = []
          label = []
          name = []
          dir = []
          forward_variables_from(selector, "*")
        }

        selected = true
        if (selected && defined(selector.host)) {
          selected = current_toolchain == host_toolchain == selector.host
        }

        if (selected && defined(selector.testonly)) {
          selected = (defined(target_invoker.testonly) &&
                      target_invoker.testonly) == selector.testonly
        }

        if (selected && select.target_type != []) {
          selected = false
          candidate = selector_target_type
          foreach(try, select.target_type) {
            if (try == candidate) {
              selected = true
            }
          }
        }

        if (selected && select.output_name != []) {
          selected = false
          candidate = target_invoker.output_name
          foreach(try, select.output_name) {
            if (try == candidate) {
              selected = true
            }
          }
        }

        if (selected && select.label != []) {
          selected = false
          candidate = target_label
          foreach(try, select.label) {
            if (try == candidate) {
              selected = true
            }
          }
        }

        if (selected && select.name != []) {
          selected = false
          candidate = get_label_info(target_label, "name")
          foreach(try, select.name) {
            if (try == candidate) {
              selected = true
            }
          }
        }

        if (selected && select.dir != []) {
          selected = false
          candidate = get_label_info(target_label, "dir")
          foreach(try, select.dir) {
            if (try == candidate) {
              selected = true
            }
          }
        }

        if (selected && selector.variant != false) {
          target_variant = "-${selector.variant}"
        }
      }
    }
  }
  if (target_variant == false) {
    target_variant = ""
  }

  builder_toolchain = toolchain_variant.base + target_variant
  if (invoker._variant_shared) {
    builder_toolchain += "-shared"
  }

  if (current_toolchain == builder_toolchain) {
    # This is the toolchain selected to actually build this target.
    target(target_type, target_name) {
      deps = []
      forward_variables_from(target_invoker, "*")
      deps += toolchain_variant.deps
      foreach(config, toolchain_variant.configs) {
        # Expand the label so it always has a `:name` part.
        config = get_label_info(config, "label_no_toolchain")
        deps += [ "${config}_deps" ]
      }
      if (defined(visibility)) {
        # Other toolchains will define this target as a group or copy
        # rule that depends on this toolchain's definition.  If the
        # invoker constrained the visibility, make sure those
        # dependencies from other toolchains are still allowed.
        visibility += [ ":${target_name}" ]
      }
    }
  } else if (current_toolchain == shlib_toolchain) {
    # Don't copy from a variant into a -shared toolchain, because nobody
    # looks for an executable or loadable_module there.  Instead, just
    # forward any deps to the real target.
    group(target_name) {
      forward_variables_from(target_invoker,
                             [
                               "testonly",
                               "visibility",
                             ])
      if (defined(visibility)) {
        visibility += [ ":${target_name}" ]
      }
      deps = [
        ":${target_name}(${builder_toolchain})",
      ]
    }
  } else {
    # When some variant was selected, then this target in all other
    # toolchains is actually just this copy rule.  The target is built in
    # the selected variant toolchain, but then copied to its usual name in
    # $root_out_dir so that things can find it there.
    copy_vars = {
      forward_variables_from(target_invoker,
                             [
                               "testonly",
                               "visibility",
                             ])
      if (defined(visibility)) {
        visibility += [ ":${target_name}" ]
      }

      deps = [
        ":${target_name}(${builder_toolchain})",
      ]
      variant_out_dir = get_label_info(deps[0], "root_out_dir")

      full_output_name = target_invoker.output_name
      if (defined(target_invoker.output_extension) &&
          target_invoker.output_extension != "") {
        full_output_name += ".${target_invoker.output_extension}"
      }

      sources = [
        "$variant_out_dir/$full_output_name",
      ]
      outputs = [
        "$root_out_dir/$full_output_name",
      ]
    }

    # In the host toolchain, make a symlink rather than a hard link
    # (which is what "copy" rules really do).  Host tools are built with
    # an embedded shared library lookup path based on $ORIGIN on Linux
    # (//build/config/linux:compiler) and the equivalent @loader_path on
    # macOS (//build/config/mac:mac_dynamic_flags).  The dynamic linker
    # translates this to "the directory containing the executable".
    # With hard links, this gets the directory used to invoke the
    # executable, which is host_toolchain's $root_out_dir.  With
    # symlinks, it instead gets the directory containing the actual
    # executable file, which is builder_toolchain's $root_out_dir.
    # Hence the program uses the variant builds of shared libraries that
    # go with the variant build of the executable, rather using than the
    # vanilla host_toolchain builds with the variant executable.
    if (current_toolchain == host_toolchain) {
      action(target_name) {
        forward_variables_from(copy_vars, "*")
        script = "/bin/ln"
        args = [
          "-snf",
          rebase_path(sources[0], get_path_info(outputs[0], "dir")),
          rebase_path(outputs[0]),
        ]
      }
    } else {
      # For Fuchsia, //build/gn/variant.py depends on hard links to
      # identify the variants.
      copy(target_name) {
        forward_variables_from(copy_vars, "*")
      }
    }
  }
}

template("executable") {
  _executable_name = target_name
  _variant_shared = false
  variant_target("executable") {
    deps = []
    target_name = _executable_name
    if (defined(invoker._target_type)) {
      _target_type = invoker._target_type
    }

    # Explicitly forward visibility, implicitly forward everything else.
    # See comment in template("shared_library") above for details.
    forward_variables_from(invoker, [ "visibility" ])
    forward_variables_from(invoker, "*", [ "visibility" ])

    deps += default_executable_deps
  }
}

template("loadable_module") {
  _module_name = target_name
  _variant_shared = true
  variant_target("loadable_module") {
    target_name = _module_name
    if (defined(invoker._target_type)) {
      _target_type = invoker._target_type
    }

    # Explicitly forward visibility, implicitly forward everything else.
    # See comment in template("shared_library") above for details.
    forward_variables_from(invoker, [ "visibility" ])
    forward_variables_from(invoker, "*", [ "visibility" ])
    if (!defined(output_extension)) {
      output_extension = "so"
    }
  }
}

# Some targets we share with Chromium declare themselves to be components,
# which means they can build either as shared libraries or as static libraries.
# We build them as static libraries.
template("component") {
  if (!defined(invoker.sources)) {
    # When there are no sources defined, use a source set to avoid creating
    # an empty static library (which generally don't work).
    _component_mode = "source_set"
  } else {
    _component_mode = "static_library"
  }

  target(_component_mode, target_name) {
    # Explicitly forward visibility, implicitly forward everything else.
    # See comment in template("shared_library") above for details.
    forward_variables_from(invoker, [ "visibility" ])
    forward_variables_from(invoker, "*", [ "visibility" ])
  }
}

set_defaults("component") {
  configs = default_common_binary_configs
}