如何避免CMake ExternalProject切换配置时重复克隆大型外部库

我用CMake构建跨平台库，通过CMakePresets.txt存储配置与构建预设，搭配Visual Studio Code的CMake Tools扩展，涉及Win x32、Win x64、UWP ARM、UWP ARM64、Android ARM、Android ARM64等多种配置。该库依赖一个20GB的外部库，我用ExternalProject模块克隆并构建它。为了只克隆一次、多平台复用，我把SOURCE_DIR设为my_library/external/external_library，但切换配置预设时，已经克隆好的库会被删除并重新克隆，耗时极长，给团队协作带来很大困扰。

请问能不能在SOURCE_DIR已存在时，让ExternalProject不再重新克隆这个外部库？如果可以，我希望继续用ExternalProject提供的Git下载命令。

代码示例 #

my_library/CMakePresets.txt #

{
    "version": 4,
    "cmakeMinimumRequired": {
        "major": 3,
        "minor": 23,
        "patch": 0
    },
    "configurePresets": [
        {
            "name": "win_x64",
            "binaryDir": "${sourceDir}/build/win_x64",
            "generator": "Ninja Multi-Config"
        },
        {
            "name": "android_arm64",
            "binaryDir": "${sourceDir}/build/android_arm64",
            "generator": "Ninja Multi-Config"
        },
        ...
    ],
    "buildPresets": [
        {
            "name": "rel_win_x64",
            "configurePreset": "win_x64",
            "configuration": "Release"
        },
        {
            "name": "rel_android_arm64",
            "configurePreset": "android_arm64",
            "configuration": "Release"
        },
        ...
    ]
}

my_library/CMakeLists.txt #

cmake_minimum_required(VERSION 3.24)

project(my_library VERSION 1.0)

add_subdirectory(external)

add_library(my_library src/example.cc include/example.h)

target_link_libraries(my_library external_library)

my_library/external/CMakeLists.txt #

include(ExternalProject)

ExternalProject_Add(external_library_src
    SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/external_library
    GIT_REPOSITORY https://github.com/ExternalLibrary/external_library.git
    GIT_TAG version_tag
    UPDATES_DISCONNECTED true
    CONFIGURE_COMMAND configure_command
    BUILD_COMMAND build_command
)

add_library(external_library STATIC IMPORTED GLOBAL)
add_dependencies(external_library external_library_src)

你遇到的问题是因为ExternalProject默认会在每次配置时检查Git仓库状态，甚至可能因不同配置的构建目录隔离导致误删源目录。可以通过以下设置解决，同时保留Git下载能力：

1. 自定义下载命令，仅在源码目录不存在时执行克隆
   通过判断源码目录的.git文件夹是否存在，决定是否执行Git克隆命令，替换掉ExternalProject默认的Git参数：

修改my_library/external/CMakeLists.txt中的ExternalProject配置：

include(ExternalProject)

# 检查外部库源码目录是否已存在Git仓库
if(NOT EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/external_library/.git)
    set(DOWNLOAD_CMD git clone --branch version_tag https://github.com/ExternalLibrary/external_library.git ${CMAKE_CURRENT_SOURCE_DIR}/external_library)
else()
    set(DOWNLOAD_CMD "")
endif()

ExternalProject_Add(external_library_src
    SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/external_library
    # 清空默认Git参数，改用自定义下载命令
    GIT_REPOSITORY ""
    GIT_TAG ""
    DOWNLOAD_COMMAND ${DOWNLOAD_CMD}
    UPDATES_DISCONNECTED true
    CONFIGURE_COMMAND configure_command
    BUILD_COMMAND build_command
    # 给每个配置单独设置构建目录，避免产物互相覆盖
    BINARY_DIR ${CMAKE_BINARY_DIR}/external_library_build
)

add_library(external_library STATIC IMPORTED GLOBAL)
add_dependencies(external_library external_library_src)

2. 保留UPDATES_DISCONNECTED true
   这个参数会阻止CMake主动检查外部库的远程更新，避免不必要的网络交互和目录操作。

3. 单独设置外部库的构建目录
   给BINARY_DIR指定${CMAKE_BINARY_DIR}下的子目录，这样不同平台配置的编译产物会分开存放，不会互相干扰，同时源码目录始终复用。

内容的提问来源于stack exchange，提问作者Aidan Howie