pkg/cc_dist_library.bzl

# Rules for distributable C++ libraries

load("@rules_cc//cc:action_names.bzl", cc_action_names = "ACTION_NAMES")
load("@rules_cc//cc:find_cc_toolchain.bzl", "find_cc_toolchain")

################################################################################
# Archive/linking support
################################################################################

def _collect_linker_input_objects(dep_label, cc_info, objs, pic_objs):
    """Accumulate .o and .pic.o files into `objs` and `pic_objs`."""
    link_ctx = cc_info.linking_context
    if link_ctx == None:
        return

    linker_inputs = link_ctx.linker_inputs.to_list()
    for link_input in linker_inputs:
        if link_input.owner != dep_label:
            # This is a transitive dep: skip it.
            continue

        for lib in link_input.libraries:
            objs.extend(lib.objects or [])
            pic_objs.extend(lib.pic_objects or [])

# Creates an action to build the `output_file` static library (archive)
# using `object_files`.
def _create_archive_action(
        ctx,
        feature_configuration,
        cc_toolchain_info,
        output_file,
        object_files):
    # Based on Bazel's src/main/starlark/builtins_bzl/common/cc/cc_import.bzl:

    # Build the command line and add args for all of the input files:
    archiver_variables = cc_common.create_link_variables(
        feature_configuration = feature_configuration,
        cc_toolchain = cc_toolchain_info,
        output_file = output_file.path,
        is_using_linker = False,
    )
    command_line = cc_common.get_memory_inefficient_command_line(
        feature_configuration = feature_configuration,
        action_name = cc_action_names.cpp_link_static_library,
        variables = archiver_variables,
    )
    args = ctx.actions.args()
    args.add_all(command_line)
    args.add_all(object_files)
    args.use_param_file("@%s", use_always = True)

    archiver_path = cc_common.get_tool_for_action(
        feature_configuration = feature_configuration,
        action_name = cc_action_names.cpp_link_static_library,
    )

    env = cc_common.get_environment_variables(
        feature_configuration = feature_configuration,
        action_name = cc_action_names.cpp_link_static_library,
        variables = archiver_variables,
    )

    ctx.actions.run(
        executable = archiver_path,
        arguments = [args],
        env = env,
        inputs = depset(
            direct = object_files,
            transitive = [
                cc_toolchain_info.all_files,
            ],
        ),
        use_default_shell_env = False,
        outputs = [output_file],
        mnemonic = "CppArchiveDist",
    )

def _create_dso_link_action(
        ctx,
        feature_configuration,
        cc_toolchain_info,
        object_files,
        pic_object_files):
    compilation_outputs = cc_common.create_compilation_outputs(
        objects = depset(object_files),
        pic_objects = depset(pic_object_files),
    )
    link_output = cc_common.link(
        actions = ctx.actions,
        feature_configuration = feature_configuration,
        cc_toolchain = cc_toolchain_info,
        compilation_outputs = compilation_outputs,
        name = ctx.label.name,
        output_type = "dynamic_library",
        user_link_flags = ctx.attr.linkopts,
    )
    library_to_link = link_output.library_to_link

    outputs = []

    # Note: library_to_link.dynamic_library and interface_library are often
    # symlinks in the solib directory. For DefaultInfo, prefer reporting
    # the resolved artifact paths.
    if library_to_link.resolved_symlink_dynamic_library != None:
        outputs.append(library_to_link.resolved_symlink_dynamic_library)
    elif library_to_link.dynamic_library != None:
        outputs.append(library_to_link.dynamic_library)

    if library_to_link.resolved_symlink_interface_library != None:
        outputs.append(library_to_link.resolved_symlink_interface_library)
    elif library_to_link.interface_library != None:
        outputs.append(library_to_link.interface_library)

    return outputs

################################################################################
# Source file/header support
################################################################################

CcFileList = provider(
    doc = "List of files to be built into a library.",
    fields = {
        # As a rule of thumb, `hdrs` and `textual_hdrs` are the files that
        # would be installed along with a prebuilt library.
        "hdrs": "public header files, including those used by generated code",
        "textual_hdrs": "files which are included but are not self-contained",

        # The `internal_hdrs` are header files which appear in `srcs`.
        # These are only used when compiling the library.
        "internal_hdrs": "internal header files (only used to build .cc files)",
        "srcs": "source files",
    },
)

def _flatten_target_files(targets):
    return depset(transitive = [
        target.files
        for target in targets
        # Filter out targets from external workspaces. We also filter out
        # utf8_range since that has a separate CMake build for now.
        if (target.label.workspace_name == "" or
            target.label.workspace_name == "com_google_protobuf") and
           not target.label.package.startswith("third_party/utf8_range")
    ])

def _get_transitive_sources(targets, attr, deps):
    return depset(targets, transitive = [getattr(dep[CcFileList], attr) for dep in deps if CcFileList in dep])

def _cc_file_list_aspect_impl(target, ctx):
    # Extract sources from a `cc_library` (or similar):
    if CcInfo not in target:
        return []

    # We're going to reach directly into the attrs on the traversed rule.
    rule_attr = ctx.rule.attr

    # CcInfo is a proxy for what we expect this rule to look like.
    # However, some deps may expose `CcInfo` without having `srcs`,
    # `hdrs`, etc., so we use `getattr` to handle that gracefully.

    internal_hdrs = []
    srcs = []

    # Filter `srcs` so it only contains source files. Headers will go
    # into `internal_headers`.
    for src in _flatten_target_files(getattr(rule_attr, "srcs", [])).to_list():
        if src.extension.lower() in ["c", "cc", "cpp", "cxx"]:
            srcs.append(src)
        else:
            internal_hdrs.append(src)

    return [CcFileList(
        hdrs = _get_transitive_sources(
            _flatten_target_files(getattr(rule_attr, "hdrs", [])).to_list(),
            "hdrs",
            rule_attr.deps,
        ),
        textual_hdrs = _get_transitive_sources(
            _flatten_target_files(getattr(rule_attr, "textual_hdrs", [])).to_list(),
            "textual_hdrs",
            rule_attr.deps,
        ),
        internal_hdrs = _get_transitive_sources(
            internal_hdrs,
            "internal_hdrs",
            rule_attr.deps,
        ),
        srcs = _get_transitive_sources(srcs, "srcs", rule_attr.deps),
    )]

cc_file_list_aspect = aspect(
    doc = """
Aspect to provide the list of sources and headers from a rule.

Output is CcFileList. Example:

  cc_library(
      name = "foo",
      srcs = [
          "foo.cc",
          "foo_internal.h",
      ],
      hdrs = ["foo.h"],
      textual_hdrs = ["foo_inl.inc"],
  )
  # produces:
  # CcFileList(
  #     hdrs = depset([File("foo.h")]),
  #     textual_hdrs = depset([File("foo_inl.inc")]),
  #     internal_hdrs = depset([File("foo_internal.h")]),
  #     srcs = depset([File("foo.cc")]),
  # )
""",
    required_providers = [CcInfo],
    implementation = _cc_file_list_aspect_impl,
    attr_aspects = ["deps"],
)

################################################################################
# Rule impl
################################################################################

def _collect_inputs(deps):
    """Collects files from a list of deps.

    This rule collects source files and linker inputs transitively for C++
    deps.

    The return value is a struct with object files (linker inputs),
    partitioned by PIC and non-pic, and the rules' source and header files:

        struct(
            objects = ...,       # non-PIC object files
            pic_objects = ...,   # PIC objects
            cc_file_list = ...,  # a CcFileList
        )

    Args:
      deps: Iterable of immediate deps, which will be treated as roots to
            recurse transitively.
    Returns:
      A struct with linker inputs, source files, and header files.
    """

    objs = []
    pic_objs = []

    # The returned CcFileList will contain depsets of the deps' file lists.
    # These lists hold `depset()`s from each of `deps`.
    srcs = []
    hdrs = []
    internal_hdrs = []
    textual_hdrs = []

    for dep in deps:
        if CcInfo in dep:
            _collect_linker_input_objects(
                dep.label,
                dep[CcInfo],
                objs,
                pic_objs,
            )

        if CcFileList in dep:
            cfl = dep[CcFileList]
            srcs.append(cfl.srcs)
            hdrs.append(cfl.hdrs)
            internal_hdrs.append(cfl.internal_hdrs)
            textual_hdrs.append(cfl.textual_hdrs)

    return struct(
        objects = objs,
        pic_objects = pic_objs,
        cc_file_list = CcFileList(
            srcs = depset(transitive = srcs),
            hdrs = depset(transitive = hdrs),
            internal_hdrs = depset(transitive = internal_hdrs),
            textual_hdrs = depset(transitive = textual_hdrs),
        ),
    )

# Given structs a and b returned from _collect_inputs(), returns a copy of a
# but with all files from b subtracted out.
def _subtract_files(a, b):
    result_args = {}

    top_level_fields = ["objects", "pic_objects"]
    for field in top_level_fields:
        to_remove = {e: None for e in getattr(b, field)}
        result_args[field] = [e for e in getattr(a, field) if not e in to_remove]

    cc_file_list_args = {}
    file_list_fields = ["srcs", "hdrs", "internal_hdrs", "textual_hdrs"]
    for field in file_list_fields:
        to_remove = {e: None for e in getattr(b.cc_file_list, field).to_list()}
        cc_file_list_args[field] = depset(
            [e for e in getattr(a.cc_file_list, field).to_list() if not e in to_remove],
        )
    result_args["cc_file_list"] = CcFileList(**cc_file_list_args)

    return struct(**result_args)

# Implementation for cc_dist_library rule.
def _cc_dist_library_impl(ctx):
    cc_toolchain_info = find_cc_toolchain(ctx)

    feature_configuration = cc_common.configure_features(
        ctx = ctx,
        cc_toolchain = cc_toolchain_info,
    )

    inputs = _subtract_files(_collect_inputs(ctx.attr.deps), _collect_inputs(ctx.attr.dist_deps))

    # For static libraries, build separately with and without pic.

    stemname = "lib" + ctx.label.name
    outputs = []

    if len(inputs.objects) > 0:
        archive_out = ctx.actions.declare_file(stemname + ".a")
        _create_archive_action(
            ctx,
            feature_configuration,
            cc_toolchain_info,
            archive_out,
            inputs.objects,
        )
        outputs.append(archive_out)

    if len(inputs.pic_objects) > 0:
        pic_archive_out = ctx.actions.declare_file(stemname + ".pic.a")
        _create_archive_action(
            ctx,
            feature_configuration,
            cc_toolchain_info,
            pic_archive_out,
            inputs.pic_objects,
        )
        outputs.append(pic_archive_out)

    # For dynamic libraries, use the `cc_common.link` command to ensure
    # everything gets built correctly according to toolchain definitions.
    outputs.extend(_create_dso_link_action(
        ctx,
        feature_configuration,
        cc_toolchain_info,
        inputs.objects,
        inputs.pic_objects,
    ))

    # We could expose the libraries for use from cc rules:
    #
    # linking_context = cc_common.create_linking_context(
    #     linker_inputs = depset([
    #         cc_common.create_linker_input(
    #             owner = ctx.label,
    #             libraries = depset([library_to_link]),
    #         ),
    #     ]),
    # )
    # cc_info = CcInfo(linking_context = linking_context)  # and return this
    #
    # However, if the goal is to force a protobuf dependency to use the
    # DSO, then `cc_import` is a better-supported way to do so.
    #
    # If we wanted to expose CcInfo from this rule (and make it usable as a
    # C++ dependency), then we would probably want to include the static
    # archive and headers as well. exposing headers would probably require
    # an additional aspect to extract CcInfos with just the deps' headers.

    return [
        DefaultInfo(files = depset(outputs)),
        inputs.cc_file_list,
    ]

cc_dist_library = rule(
    implementation = _cc_dist_library_impl,
    doc = """
Create libraries suitable for distribution.

This rule creates static and dynamic libraries from the libraries listed in
'deps'. The resulting libraries are suitable for distributing all of 'deps'
in a single logical library, for example, in an installable binary package.
The result includes all transitive dependencies, excluding those reachable
from 'dist_deps' or defined in a separate repository (e.g. Abseil).

The outputs of this rule are a dynamic library and a static library. (If
the build produces both PIC and non-PIC object files, then there is also a
second static library.) The example below illustrates additional details.

This rule is different from Bazel's experimental `shared_cc_library` in two
ways. First, this rule produces a static archive library in addition to the
dynamic shared library. Second, this rule is not directly usable as a C++
dependency (although the outputs could be used, e.g., by `cc_import`).

Example:

    cc_library(name = "a", srcs = ["a.cc"], hdrs = ["a.h"])
    cc_library(name = "b", srcs = ["b.cc"], hdrs = ["b.h"], deps = [":a"])
    cc_library(name = "c", srcs = ["c.cc"], hdrs = ["c.h"], deps = [":b"])

    # Creates libdist.so and (typically) libdist.pic.a:
    # (This may also produce libdist.a if the build produces non-PIC objects.)
    cc_dist_library(
        name = "dist",
        linkopts = ["-la"],   # libdist.so dynamically links against liba.so.
        deps = [":b", ":c"],  # Output contains a.o, b.o, and c.o.
    )
""",
    attrs = {
        "deps": attr.label_list(
            doc = ("The list of libraries to be included in the outputs, " +
                   "along with their transitive dependencies."),
            aspects = [cc_file_list_aspect],
        ),
        "dist_deps": attr.label_list(
            doc = ("The list of cc_dist_library dependencies that " +
                   "should be excluded."),
            aspects = [cc_file_list_aspect],
        ),
        "linkopts": attr.string_list(
            doc = ("Add these flags to the C++ linker command when creating " +
                   "the dynamic library."),
        ),
        # C++ toolchain before https://github.com/bazelbuild/bazel/issues/7260:
        "_cc_toolchain": attr.label(
            default = Label("@rules_cc//cc:current_cc_toolchain"),
        ),
    },
    toolchains = [
        # C++ toolchain after https://github.com/bazelbuild/bazel/issues/7260:
        "@bazel_tools//tools/cpp:toolchain_type",
    ],
    fragments = ["cpp"],
)