..bazel-rules_cc_extensions

...

.bazelrc

# https://github.com/bazelbuild/bazel/commit/f417aa985790a36c56a7da82ef46f76aba4c984e
common --experimental_rule_extension_api=true

.bazelversion

8.0.0-pre.20240618.2
d73e9bf9be20cdaefd5c1b265999af1ed9d0c6df34c7eae6135d377fe51e2433
# ^ sha256 for `.dist.zip`

# https://releases.bazel.build/8.0.0/rolling/8.0.0-pre.20240618.2/index.html
# rev: 0c2ed165335327a2c5b534312324baa9ae6b6ebd

.envrc

use flake

.gitignore

/bazel-*
/.direnv
result*

a.cc

#include <dlfcn.h>

#include "common.h"
#include "a.h"

using Func = void(void);

void A::run() noexcept(true) {
    auto lib = CHECK(dlopen, ("libruntime-a.so", RTLD_NOW | RTLD_DEEPBIND));
    auto func = reinterpret_cast<Func*>(CHECK(dlsym, (lib, "print_a")));
    func();
}

a.h

#pragma once

struct A {
    static void run() noexcept(true);
};

b.cc

#include <dlfcn.h>

#include "common.h"
#include "b.h"
#include "a.h"

void B::run() noexcept(true) {
    A::run();

    auto lib = CHECK(dlopen, ("libruntime-b.so", RTLD_NOW | RTLD_DEEPBIND));
    auto func = reinterpret_cast<Func*>(CHECK(dlsym, (lib, "print_b")));
    func();
}

b.h

#pragma once

struct B {
    static void run() noexcept(true);
};

BUILD.bazel

load(":defs.bzl", "cc_library", "cc_binary")

cc_binary(
    name = "libruntime-a.so",
    srcs = [":runtime-lib-a.cc"],
    linkstatic = True,
    linkshared = True,
)

cc_binary(
    name = "libruntime-b.so",
    srcs = [":runtime-lib-b.cc"],
    linkstatic = True,
    linkshared = True,
)

#-------------------------------------------------------------------------------

cc_library(
    name = "a",
    deps = [":libruntime-a.so"],
    srcs = [":a.cc", "common.h"],
    hdrs = [":a.h"],
    data = [":libruntime-a.so"],
    # copts = ["-Wl,-rpath='$ORIGIN/liba.so.runfiles/_main'"],

    # TODO: this is propagated to rdeps; we don't want this to be
    #
    # NOTE: in rdeps, $ORIGIN is a symlink path; `./<name>.runfiles` is not
    # available in these contexts..
    #
    # the symlink path is something like
    # `<rdep's name>.runfiles/<repo>/_solib_k8/<package path?>/liblib<lib name>.so`
    #
    # since data deps *are* hoisted to the top-level target's runfiles tree, we
    # should be able to access our data deps at
    # `$ORIGIN/<.. for pkg path>/../../<repo>/`..
    #
    # EDIT: nevermind; see below
    linkopts = [
        # For direct usages of this library:
        #
        # (TODO: ideally we'd have top-level rdeps not use this flag...)
        "-Wl,-rpath='$ORIGIN/liba.so.runfiles/_main'",

        # For indirect usages of this library (refers to the top-level runfiles
        # tree):
        #
        # this is necessary because:
        #   - `dlopen` only consults this library's runpath
        #   - binaries that link dynamically to this library will find this
        #     library via a symlink in their runfiles trees' solib dir; this
        #     path is different than the `bazel-out/...` path at which this
        #     library actually resides
        #     + for the purpose of resolving `$ORIGIN` relative paths, the
        #       symlink path is what the runtime loader will use
        #     + additionally, while we can rely on the top-level target's
        #       runfiles tree containing our data deps, we cannot rely on *our*
        #       runfiles tree being present (this is usually not the case)
        #
        # so, we include an rpath entry that maps from the top-level target's
        # runfiles tree solib symlink of this library to the directory in the
        # top-level target's runfiles tree where we can expect to find our data
        # deps:
        #
        # NOTE: EDIT: this actually doesn't work because shared object deps in
        # final binaries are actually (sometimes) via the top-level target's
        # `$ORIGIN/_solib.../` runpath entry rather than via the
        # `$ORIGIN/<name>.runfiles/_main/_solib.../` entry; when this happens
        # the relative path specified here doesn't work. We once again have to
        # name the top-level target in order to construct a working relative
        # path which, as explained below, we cannot do.
        #
        # (TODO: ideally we'd have top-level rdeps not use this flag...)
        "-Wl,-rpath='$ORIGIN/../../_main'",

        # TODO: for usages where this library is linked directly (statically)
        # into a top-level shared-object, we need to add an rpath entry for
        # `$ORIGIN/<top-level name>.runfiles/<repo>`
        #
        # Unfortunately we cannot do that here — we do not know the top-level
        # target's name and there may very well be many top-level rdeps for this
        # library.
    ],
    # linkstatic = True,
)

cc_library(
    name = "b",
    deps = [":a"],
    srcs = [":b.cc", "common.h"],
    hdrs = [":b.h"],
    data = [":libruntime-b.so"],
    # linkstatic = False,
)

#-------------------------------------------------------------------------------

cc_binary(
    name = "main-shared",
    deps = [":b"],
    srcs = [":main.cc"],
    # shared binary but with `a.cc` and `b.cc` linked in via shared objects
    linkstatic = False,

    # FIXME: doesn't work because the `dlopen` calls in `liba.so`, `libb.so`
    # will not use the parent executable's runpath (or rpath) entries...
    # linkopts = ["-Wl,-rpath='$ORIGIN/main-shared.runfiles/_main/'"],

    # Forcing `--runpath` does not help:
    # linkopts = ["-Wl,--disable-new-dtags", "-Wl,-rpath='$ORIGIN/main-shared.runfiles/_main/'"],
)

cc_binary(
    name = "main-static",
    deps = [":b"],
    srcs = [":main.cc"],
    # shared binary but with `a.cc` and `b.cc` linked in directly
    linkstatic = True,

    # This works because the `dlopen` callsite is now part of the final binary.
    #
    # NOTE that this is *necessary*: see the comment on `:a`; because the
    # libraries using `dlopen` have been linked into `main-static` statically,
    # it's this target's RUNPATH that will be consulted when those `dlopen`s
    # are resolved.
    linkopts = ["-Wl,-rpath='$ORIGIN/main-static.runfiles/_main/'"],
)

common.h

#pragma once

#include <cstdio>
#include <cstdlib>
#include <iostream>

#include <dlfcn.h>

using Func = void(void);

#define CHECK(func_name, args) [&]{                          \
    if (auto var = func_name args) { return var; }           \
    else {                                                   \
        std::perror("Error in " # func_name);                \
        std::cerr << "dlerror: " <<  dlerror() << std::endl; \
        std::exit(1);                                        \
    }                                                        \
}()

defs.bzl

# See:
#   - subrule design doc: https://docs.google.com/document/d/1RbNC88QieKvBEwir7iV5zZU08AaMlOzxhVkPnmKDedQ/edit#heading=h.1rg4gw6b0c54
#   - extending rules design doc: https://docs.google.com/document/d/1p6z-shWf9sdqo_ep7dcjZCGvqN5r2jsPkJCqHHgfRp4/edit#heading=h.5mcn15i0e1ch

def _patchelf_add_rpath_impl(sctx, output_file, input_file, shared_objects):
    pass

patchelf_add_rpath = subrule(
    implementation = _patchelf_add_rpath_impl,
    attrs = dict(
        _patchelf_binary = attr.label(
            executable = True,
            default = Label("@patchelf"),
            cfg = "exec",
        ),
    ),
)

EXTRA_ATTRS = dict(
    runtime_shared_objects = attr.label_list(
        allow_empty = True,
        doc = "TODO",
        allow_files = [".so"],
        cfg = "target",
    ),
)
def _cc_child_impl(ctx):
    return ctx.super(
        # NOTE: we'd like to just pass in a computed `linkopts` flag that has
        # all the appropriate `rpaths` but unfortunately overriding parent rule
        # attributes during child rule analysis isn't (yet?) supported:
        # https://docs.google.com/document/d/1p6z-shWf9sdqo_ep7dcjZCGvqN5r2jsPkJCqHHgfRp4/edit?disco=AAAA3pixqsA
    )

cc_binary = rule(
    implementation = _cc_child_impl,
    attrs = EXTRA_ATTRS,
    parent = native.cc_binary,
    subrules = [patchelf_add_rpath],
)

cc_library = rule(
    implementation = _cc_child_impl,
    attrs = EXTRA_ATTRS,
    parent = native.cc_library,
    subrules = [patchelf_add_rpath],
)

flake.lock

{
  "nodes": {
    "flake-utils": {
      "inputs": {
        "systems": "systems"
      },
      "locked": {
        "lastModified": 1710146030,
        "narHash": "sha256-SZ5L6eA7HJ/nmkzGG7/ISclqe6oZdOZTNoesiInkXPQ=",
        "owner": "numtide",
        "repo": "flake-utils",
        "rev": "b1d9ab70662946ef0850d488da1c9019f3a9752a",
        "type": "github"
      },
      "original": {
        "owner": "numtide",
        "repo": "flake-utils",
        "type": "github"
      }
    },
    "nixpkgs": {
      "locked": {
        "lastModified": 1720542800,
        "narHash": "sha256-ZgnNHuKV6h2+fQ5LuqnUaqZey1Lqqt5dTUAiAnqH0QQ=",
        "owner": "NixOS",
        "repo": "nixpkgs",
        "rev": "feb2849fdeb70028c70d73b848214b00d324a497",
        "type": "github"
      },
      "original": {
        "owner": "NixOS",
        "ref": "nixos-unstable",
        "repo": "nixpkgs",
        "type": "github"
      }
    },
    "root": {
      "inputs": {
        "flake-utils": "flake-utils",
        "nixpkgs": "nixpkgs"
      }
    },
    "systems": {
      "locked": {
        "lastModified": 1681028828,
        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
        "owner": "nix-systems",
        "repo": "default",
        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
        "type": "github"
      },
      "original": {
        "owner": "nix-systems",
        "repo": "default",
        "type": "github"
      }
    }
  },
  "root": "root",
  "version": 7
}

flake.nix

{
  inputs.nixpkgs.url = github:NixOS/nixpkgs/nixos-unstable;
  inputs.flake-utils.url = github:numtide/flake-utils;

  outputs = { nixpkgs, flake-utils, self }: flake-utils.lib.eachDefaultSystem (sys: let
    np = nixpkgs.legacyPackages.${sys};
    inherit (np) lib;
  in {
    # We want to use a pre-release Bazel.
    packages.bazel = let
      # get the version and `dist.zip` SHA256 from `.bazelversion`:
      versionInfo = let
        lines = lib.splitString "\n" (builtins.readFile ./.bazelversion);
      in rec {
        version = builtins.head lines;
        sha256 = builtins.elemAt lines 1;

        majorVersion = builtins.head (lib.splitString "-" version);
      };

      # override the source, lockfile:
      src = with versionInfo; np.fetchurl {
        url = "https://releases.bazel.build/${majorVersion}/rolling/${version}/bazel-${version}-dist.zip";
        inherit sha256;
      };
      # the actual bazel nixpkg vendors this file to avoid IFD; we're okay with
      # IFD:
      lockfile = np.stdenvNoCC.mkDerivation {
        name = "bazel-${versionInfo.version}-MODULE.bazel.lock";
        inherit src;
        sourceRoot = ".";
        nativeBuildInputs = [ np.unzip ];
        buildPhase = "cp MODULE.bazel.lock $out";
      };

      # apply the overrides:
      base = np.bazel_7;
    in lib.pipe base [
      # bazel 8 wants JDK 21:
      (pkg: pkg.override rec {
        buildJdk = np.jdk21_headless;
        runJdk = buildJdk;
      })
      (pkg: pkg.overrideAttrs (old: {
        postPatch = builtins.replaceStrings
          ["java_runtime_version=local_jdk_17" "java_language_version=17" "--extra_toolchains=@bazel_tools//tools/jdk:all"]
          ["java_runtime_version=local_jdk_21" "java_language_version=21" ""]
          old.postPatch
        ;
      }))

      # version gets used in many places and is overrideable at the
      # `callPackage` level:
      (pkg: pkg.override { inherit (versionInfo) version; })

      # `src` and `lockfile` must be overriden at the `mkDerivation` level:
      (pkg: pkg.overrideAttrs (old: let
        # `lockfile` gets used in a couple of places:
        distDir = repoCache;
        repoCache = old.passthru.repoCache.override { inherit lockfile; };
      in {
        inherit src;

        postPatch = builtins.replaceStrings
          ["${old.passthru.repoCache}"]
          ["${repoCache}"]
          old.postPatch
        ;

        passthru = old.passthru // {
          tests = old.passthru.tests.override { inherit lockfile repoCache; };
          inherit lockfile distDir repoCache;
        };
      }))
    ];
    devShells.default = np.mkShell {
      nativeBuildInputs = with np; [
        self.packages.${sys}.bazel
        buildifier
      ];
    };
  });
}

main.cc

#include "a.h"
// #include "b.h"

auto main() -> int {
    A::run();
    // B::run();


    return 1;
}

MODULE.bazel

module(name = "rules_cc_subrule_playground")

bazel_dep(name = "patchelf", version = "0.18.0")

MODULE.bazel.lock

{
  "lockFileVersion": 11,
  "registryFileHashes": {
    "https://bcr.bazel.build/bazel_registry.json": "8a28e4aff06ee60aed2a8c281907fb8bcbf3b753c91fb5a5c57da3215d5b3497",
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    "https://bcr.bazel.build/modules/abseil-cpp/20211102.0/MODULE.bazel": "70390338f7a5106231d20620712f7cccb659cd0e9d073d1991c038eb9fc57589",
    "https://bcr.bazel.build/modules/abseil-cpp/20211102.0/source.json": "7e3a9adf473e9af076ae485ed649d5641ad50ec5c11718103f34de03170d94ad",
    "https://bcr.bazel.build/modules/bazel_features/1.11.0/MODULE.bazel": "f9382337dd5a474c3b7d334c2f83e50b6eaedc284253334cf823044a26de03e8",
    "https://bcr.bazel.build/modules/bazel_features/1.11.0/source.json": "c9320aa53cd1c441d24bd6b716da087ad7e4ff0d9742a9884587596edfe53015",
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runtime-lib-a.cc

#include <iostream>

extern "C" void print_a(void) {
    std::cout << "hello from a" << std::endl;
}

runtime-lib-b.cc

#include <iostream>

extern "C" void print_b(void) {
    std::cout << "hello from b" << std::endl;
}