修复 Emacs 在 macOS 下最大文件数为 1024 的问题

不知道从哪个版本开始，macOS 最大文件数（max open files）改成了 1024，这对于使用 lsp 进行开发来说，显得有些小。而且这个问题并不能简单通过调大 ulimit 解决，在这个 reddit 帖子[1]里，rpluim 用户提到：

Emacs uses pselect, which is limited to FD_SETSIZE file descriptors, usually 1024. I suspect you've got one of the file-watching utilities enabled in emacs, which tends to use up a lot of file descriptors. … Increasing the maxfiles limit will not change the value of FD_SETSIZE compiled into emacs and the macOS libraries. Emacs would have to move to using poll or kqueue to fully solve this issue.

在 macOS 的开发者文档[2]里也能找到印证：

The default size FD_SETSIZE (currently 1024) is some-what somewhat what smaller than the current kernel limit to the number of open files. However, in order to accommodate programs which might potentially use a larger number of open files with select, it is possible to increase this size within a program by providing a larger definition of FD_SETSIZE before the inclusion of <sys/types.h>.

但是文档里没提到怎么改，搜了下找到了一个 erlang 的类似问题[3]，里面有提到怎么修改：

CFLAGS="-DFD_SETSIZE=10000 -DDARWIN_UNLIMITED_SELECT"

Great！经过一番测试，成功将 emacs 的最大文件数改成 10000，这里总结下步骤：

1. 1. 调大系统级别 ulimit 的限制，可参考这个 gist[4] 或 Mac OS X下的资源限制[5]。新建文件 limit.maxfiles.plist

    <?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN"
            "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
    <plist version="1.0">
      <dict>
        <key>Label</key>
        <string>limit.maxfiles</string>
        <key>ProgramArguments</key>
        <array>
          <string>launchctl</string>
          <string>limit</string>
          <string>maxfiles</string>
          <string>64000</string>
          <string>524288</string>
        </array>
        <key>RunAtLoad</key>
        <true/>
        <key>ServiceIPC</key>
        <false/>
      </dict>
    </plist>

上面的 xml 含义是把最大文件打开数的 soft/hard 限制分别改成 64000 与 524288。

     sudo chown root:wheel /Library/LaunchDaemons/limit.maxfiles.plist
     sudo chmod 644 /Library/LaunchDaemons/limit.maxfiles.plist
    # 加载plist文件
     sudo launchctl load -w /Library/LaunchDaemons/limit.maxfiles.plist
    # 确认已经生效
    launchctl limit maxfiles
    ulimit -n

1. 1. 下载 emacs 源码，在 configure 时指定 CFLAGS 参数。参考命令：

    git clone https://git.savannah.gnu.org/emacs && cd emacs
    git checkout emacs-28
    git clean -xf
    
    ./autogen.sh
    ./configure "CFLAGS=-DFD_SETSIZE=10000 -DDARWIN_UNLIMITED_SELECT"
    
    export CPATH=`xcrun --show-sdk-path`/usr/include:`xcrun --show-sdk-path`/usr/include/libxml2
    
    make -j 4 && make install

之后打开新编译的 Emacs 进行测试：

    (shell-command-to-string "ulimit -n")
    ;; 10000
    
    (dotimes (i 2000)
      (make-process
       :name (format "Sleep-%s" i)
       :buffer nil
       :command '("sleep" "60000")
       :connection-type 'pipe))

上面的 dotimes 代码块创建了 2000 个进程，之后用 lsof -p ${emacs_pid} | wc -l 查看打开的文件数，可以看到是 4000 多个，应该是一个 process 会打开两个文件：stdout 与 stderr。

这样，Emacs 最大文件打开数就成功修改了！

引用链接

[1] 这个 reddit 帖子: https://www.reddit.com/r/emacs/comments/mq2znn/comment/gudivjv/?utm_source=share&utm_medium=web2x&context=3 [2] 开发者文档: https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man2/select.2.html [3] 类似问题: https://github.com/Homebrew/legacy-homebrew/issues/6143 [4] 这个 gist: https://gist.github.com/skylock/0117ec637d468f91260927b43b816eda [5] Mac OS X下的资源限制: https://wudaijun.com/2017/02/max-osx-ulimit/