Bazel中无法基于平台选择依赖问题求助

问题描述 #

我正在学习Bazel，尝试构建第三方库。有时使用Mac电脑，有时使用Linux电脑，因此构建opencv时希望根据平台使用不同的linkopts。查阅资料得知应该使用select和@platforms，但按配置在Linux机器上构建时出现如下错误：

ERROR: Target //thirdparty/opencv:opencv is incompatible and cannot be built, but was explicitly requested.
Dependency chain:
//thirdparty/opencv:opencv (442d8d)
//thirdparty/opencv:opencv_deps (442d8d) <-- target platform (@local_config_platform//:host) didn't satisfy constraint @platforms//os:osx
INFO: Elapsed time: 0.284s

看起来配置的选择逻辑没有生效，我的配置代码如下：

load("@rules_cc//cc:defs.bzl", "cc_library")
load("@rules_foreign_cc//foreign_cc:defs.bzl", "cmake")

cc_library(
    name = "opencv_deps",
    linkopts = select({
        "@platforms//os:macos": [
        "-ldl", 
        "-lz", 
        "-lpthread",
        "-lavutil",
        "-lavformat", 
        "-lavcodec", 
        "-lswscale", 
        "-L/opt/homebrew/opt/ffmpeg/lib", 
        "-L/opt/homebrew/Cellar/libx11/1.8.6/lib", 
        "-framework Cocoa",
        "-framework Accelerate",
        ],
        "@platforms//os:linux": [
            "-ldl",
            "-lz",
            "-lpthread"
        ],
    }),
    target_compatible_with = [
        "@platforms//os:macos",
        "@platforms//os:linux"
    ],
)

cmake(
    name = "opencv",
    build_args = [
        "-j3",
    ],
    cache_entries = {
        "OPENCV_FORCE_3RDPARTY_BUILD": "ON",
        "BUILD_SHARED_LIBS": "OFF",
        "BUILD_TESTS": "OFF",
        "BUILD_PERF_TESTS": "OFF",
        "BUILD_opencv_apps": "OFF",
        "BUILD_opencv_calib3d": "OFF",
        "BUILD_opencv_core": "ON",
        "BUILD_opencv_dnn": "OFF",
        "BUILD_opencv_features2d": "OFF",
        "BUILD_opencv_flann": "OFF",
        "BUILD_opencv_gapi": "OFF",
        "BUILD_opencv_highgui": "ON",
        "BUILD_opencv_imgcodecs": "ON",
        "BUILD_opencv_imgproc": "ON",
        "BUILD_opencv_java_bindings_generator": "OFF",
        "BUILD_opencv_js": "OFF",
        "BUILD_opencv_ml": "OFF",
        "BUILD_opencv_objdetect": "OFF",
        "BUILD_opencv_photo": "OFF",
        "BUILD_opencv_python_bindings_generator": "OFF",
        "BUILD_opencv_stitching": "OFF",
        "BUILD_opencv_ts": "OFF",
        "BUILD_opencv_video": "OFF",
        "BUILD_opencv_videoio": "ON",
        "BUILD_opencv_world": "ON",
        "BUILD_IPP_IW": "ON",
        "WITH_ADE": "OFF",
        "WITH_CAROTENE": "OFF",
        "WITH_IPP": "OFF",
        "WITH_ITT": "ON",
        "WITH_OPENEXR": "OFF",
        "WITH_JASPER": "OFF",
        "WITH_JPEG": "OFF",
        "WITH_OPENJPEG": "ON",
        "WITH_TIFF": "OFF",
        "WITH_WEBP": "ON",
        "WITH_QUIRC": "ON",
        "WITH_GTK": "OFF",
        "WITH_OPENBLAS": "ON",
        "WITH_OPENCL": "OFF",
    },
    env = {
        "CMAKE_BUILD_TYPE": "RelWithDebInfo",
        "CMAKE_BUILD_PARALLEL_LEVEL": "16",
    },
    lib_source = "@opencv//:opencv_all",
    out_include_dir = "include/opencv4",
    out_static_libs = [
        "libopencv_world.a",
        "opencv4/3rdparty/libittnotify.a",
        "opencv4/3rdparty/liblibopenjp2.a",
        "opencv4/3rdparty/liblibwebp.a",
        "opencv4/3rdparty/liblibpng.a",
    ],
    targets = [
        "install",
    ],
    visibility = ["//visibility:public"],
    deps = [":opencv_deps"]
)

问题原因及解决方法 #

报错的核心是target_compatible_with配置错误。Bazel中该字段的列表是逻辑与关系，要求目标平台同时满足所有约束。当前配置同时指定了@platforms//os:macos和@platforms//os:linux，这两个约束不可能同时成立，导致Linux平台判定opencv_deps不兼容。

解决方法有两种：

1. 用select为不同平台设置对应兼容性约束，确保每个平台只匹配自身的OS约束：

cc_library(
    name = "opencv_deps",
    linkopts = select({
        "@platforms//os:macos": [
        "-ldl", 
        "-lz", 
        "-lpthread",
        "-lavutil",
        "-lavformat", 
        "-lavcodec", 
        "-lswscale", 
        "-L/opt/homebrew/opt/ffmpeg/lib", 
        "-L/opt/homebrew/Cellar/libx11/1.8.6/lib", 
        "-framework Cocoa",
        "-framework Accelerate",
        ],
        "@platforms//os:linux": [
            "-ldl",
            "-lz",
            "-lpthread"
        ],
    }),
    target_compatible_with = select({
        "@platforms//os:macos": ["@platforms//os:macos"],
        "@platforms//os:linux": ["@platforms//os:linux"],
    }),
)

2. 直接删除target_compatible_with字段。因为linkopts中的select已经明确该库仅适用于macOS和Linux，Bazel会自动根据select条件推断兼容性，无需额外声明。

修改后重新构建，Linux平台就能正常匹配对应的linkopts，兼容性错误即可消除。

内容的提问来源于stack exchange，提问作者El_Loco