flutter engine 编译步骤(mac环境)

1、安装gclient环境、git、python2.7以上
2、使用翻GFW代理并导出配置代理环境变量：
suke-2:engine suke$ export http_proxy="http://127.0.0.1:8001"; export HTTP_PROXY="http://127.0.0.1:8001"; export https_proxy="http://127.0.0.1:8001"; export HTTPS_PROXY="http://127.0.0.1:8001"

3、配置git代理：
suke-2:engine suke$ git config --global http.proxy $HTTP_PROXY
suke-2:engine suke$ git config --global https.proxy $HTTPS_PROXY
    PS.>>>git拉取完成后记得关闭:
        suke-2:engine suke$ git config --global --unset http.proxy
        suke-2:engine suke$ git config --global --unset https.proxy
       >>>查看git代理设置：
        suke-2:engine suke$ git config --get http.proxy
        suke-2:engine suke$ git config --get https.proxy

4、github上fork Flutter Engine项目，github拉取代码太慢，可以使用国内码云(https://gitee.com)fork。例如我的https://gitee.com/xuangyang/engine


5、在本地创建文件夹engine.
6、在engine里面创建文件.gclient,并填写内容（gclient是谷歌搞的一个基于python脚本编写项目依赖管理的工具，类似git，但是更复杂）：
    solutions = [
      {
        "managed": False,
        "name": "src/flutter",
        "url": "https://gitee.com/xuangyang/engine.git",
        "custom_deps": {},
        "deps_file": "DEPS",
        "safesync_url": "",
      },
    ]

7、在终端cd到engine目录下，执行gclient sync。等待依赖拉取完成，时间比较长，依赖各种项目太多,skia、dart等等，在engine的git下面有个DEPS文件中查看具体依赖项目，以及拉取地址等。

8、经过漫长的等待，报错了：
>> .....
[0:38:09] Still working on:
[0:38:09]   src/third_party/dart
Syncing projects: 100% (94/94), done.                                                      

________ running 'cipd ensure -log-level error -root /Users/suke/development/flutter_build/engine -ensure-file /var/folders/jd/8wrnwyf53kj3tq8wdyqd_xg40000gn/T/tmp2ARsaR.ensure' in '.'
curl: (6) Could not resolve host: chrome-infra-packages.appspot.com
Error: Command 'cipd ensure -log-level error -root /Users/suke/development/flutter_build/engine -ensure-file /var/folders/jd/8wrnwyf53kj3tq8wdyqd_xg40000gn/T/tmp2ARsaR.ensure' returned non-zero exit status 6

目测是国内无法访问https://chrome-infra-packages.appspot.com造成的，先不管，继续尝试编译
编译ios framework:
suke-2:engine suke$ ./src/flutter/tools/gn --ios
>> gn gen --check in out/ios_debug
Traceback (most recent call last):
  File "./src/flutter/tools/gn", line 361, in <module>
    sys.exit(main(sys.argv))
  File "./src/flutter/tools/gn", line 336, in main
    gn_call_result = subprocess.call(command, cwd=SRC_ROOT)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py", line 522, in call
    return Popen(*popenargs, **kwargs).wait()
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py", line 710, in __init__
    errread, errwrite)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py", line 1335, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory

是gn脚本报错，查看代码336行：
gn_call_result = subprocess.call(command, cwd=SRC_ROOT)
是python调用gn命令，打印command数组内容为：['/Users/suke/development/flutter_build/engine/src/buildtools/mac-x64/gn', 'gen', '--check', '--ide=xcode', 'out/ios_debug', '--args=skia_enable_pdf=false enable_lto=true full_dart_sdk=false use_clang_static_analyzer=false use_ios_simulator=false dart_runtime_mode="develop" skia_use_fontconfig=false skia_use_dng_sdk=false skia_enable_flutter_defines=true use_goma=false dart_custom_version_for_pub="flutter" embedder_for_target=false is_official_build=true android_full_debug=false is_clang=true skia_use_sfntly=false dart_target_arch="dbc" dart_component_kind="static_library" flutter_runtime_mode="debug" goma_dir="None" target_os="ios" mac_sdk_path="" skia_use_x11=false enable_coverage=false target_cpu="arm64" skia_use_expat=false dart_lib_export_symbols=false is_debug=false']

对比发现buildtools/目录下没有mac-x64/文件夹，刚刚gclient sync最后报错应该是buildtool里面配置失败了。
接着查看update.sh脚本，发现其执行cipd脚本，里面调用curl https://chrome-infra-packages.appspot.com
应该就是这的问题，之前配置了代理并没有对curl有效，于是利用alias 重命名curl。
执行 vim ~/.bash_profile
增加一行内容： alias curl='curl -I -x "http://127.0.0.1:8001"'
保存并source ~/.bash_profile 
测试是否生效 curl https://www.google.com 
ok
继续回到buildtool目录下，执行./update.sh
>> Bootstrapping cipd client for mac-amd64...
>> ######################################################################## 100.0%
发现buildtool下多了mac-64/目录，下面相应的构建工具都有了，ok，回到engine目录，重新尝试编译。
执行
>> ERROR at //third_party/dart/build/dart/dart_action.gni:101:3: Assertion failed.
  assert(_is_fuchsia || prebuilt_dart_exe_works,
  ^-----
A pre built dart executable needs to be present
See //third_party/dart/build/dart/dart_action.gni:180:3: whence it was called.
  _prebuilt_tool_action(target_name) {
  ^-----------------------------------
See //third_party/dart/utils/compile_platform.gni:42:3: whence it was called.
  prebuilt_dart_action(target_name) {
  ^----------------------------------
See //flutter/lib/snapshot/BUILD.gn:235:1: whence it was called.
compile_platform("non_strong_platform") {
^----------------------------------------
See //flutter/BUILD.gn:21:5: which caused the file to be included.
    "$flutter_root/lib/snapshot:generate_snapshot_bin",
    ^-------------------------------------------------

mac 上 运行
git clone https://chromium.googlesource.com/chromium/tools/depot_tools.git
输入gclient version报错，
fatal error: failed to build venv: failed to build package : cipkg.Derivation{Name:"virtualenv", Platform:"", Builder:"builtin:cipdExport", Args:[]string{"infra/3pp/tools/virtualenv version:2@16.7.12.chromium.7\n"}, Env:[]string(nil), Inputs:[]string(nil)}: failed to build derivation: failed to export packages: exec: "cipd": cannot run executable found relative to current directory
请问这是什么问题？