解决 MAC 上 brew update 更新缓慢甚至卡死状态
MAC 下 Homebrew 默认的源是：
https://github.com/Homebrew/brew
执行以下命令 ，查看更新的详细过程：
$ brew update -verbose
Checking if we need to fetch /usr/local/Homebrew...
Checking if we need to fetch /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core...
Checking if we need to fetch /usr/local/Homebrew/Library/Taps/homebrew/homebrew-services...
Checking if we need to fetch /usr/local/Homebrew/Library/Taps/mongodb/homebrew-brew...
Fetching /usr/local/Homebrew...
Fetching /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core...
Fetching /usr/local/Homebrew/Library/Taps/mongodb/homebrew-brew...
Fetching /usr/local/Homebrew/Library/Taps/homebrew/homebrew-services...
Checking if we need to fetch /usr/local/Homebrew...
Checking if we need to fetch /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core...
Checking if we need to fetch /usr/local/Homebrew/Library/Taps/homebrew/homebrew-services...
Checking if we need to fetch /usr/local/Homebrew/Library/Taps/mongodb/homebrew-brew...
Fetching /usr/local/Homebrew...
Fetching /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core...
Fetching /usr/local/Homebrew/Library/Taps/mongodb/homebrew-brew...
Fetching /usr/local/Homebrew/Library/Taps/homebrew/homebrew-services...
执行过程会一直卡在 homebrew-services 处缓慢执行。
使用 brew doctor 查看源是哪里：
$ brew doctor
Please note that these warnings are just used to help the Homebrew maintainers
with debugging if you file an issue. If everything you use Homebrew for is
working fine: please don't worry or file an issue; just ignore this. Thanks!
Warning: A newer Command Line Tools release is available.
Update them from Software Update in System Preferences or run:
softwareupdate --all --install --force
If that doesn't show you an update run:
sudo rm -rf /Library/Developer/CommandLineTools
sudo xcode-select --install
Alternatively, manually download them from:
https://developer.apple.com/download/more/.
Please note that these warnings are just used to help the Homebrew maintainers
with debugging if you file an issue. If everything you use Homebrew for is
working fine: please don't worry or file an issue; just ignore this. Thanks!
Warning: A newer Command Line Tools release is available.
Update them from Software Update in System Preferences or run:
softwareupdate --all --install --force
If that doesn't show you an update run:
sudo rm -rf /Library/Developer/CommandLineTools
sudo xcode-select --install
Alternatively, manually download them from:
https://developer.apple.com/download/more/.
此处可以说明 homebrew 使用了默认的源，否则会提示使用以下命令替换源：
$ git -C "/usr/local/Homebrew" remote set-url origin https://github.com/Homebrew/brew
$ git -C "/usr/local/Homebrew/Library/Taps/homebrew/homebrew-core" remote set-url origin https://github.com/Homebrew/homebrew-core
$ git -C "/usr/local/Homebrew/Library/Taps/homebrew/homebrew-core" remote set-url origin https://github.com/Homebrew/homebrew-core
竟然默认源速度不佳，建议使用其他源替换。
以下是测试有效的源：
//替换 brew-git
$ cd "$(brew --repo)"
$ git remote set-url origin https://mirrors.ustc.edu.cn/brew.git
//替换homebrew-core.git
$ cd "$(brew --repo)/Library/Taps/homebrew/homebrew-core"
$ git remote set-url origin https://mirrors.ustc.edu.cn/homebrew-core.git
//替换homebrew-cask
$ cd "$(brew --repo)"/Library/Taps/homebrew/homebrew-cask
$ git remote set-url origin https://mirrors.ustc.edu.cn/homebrew-cask.git
$ cd "$(brew --repo)"
$ git remote set-url origin https://mirrors.ustc.edu.cn/brew.git
//替换homebrew-core.git
$ cd "$(brew --repo)/Library/Taps/homebrew/homebrew-core"
$ git remote set-url origin https://mirrors.ustc.edu.cn/homebrew-core.git
//替换homebrew-cask
$ cd "$(brew --repo)"/Library/Taps/homebrew/homebrew-cask
$ git remote set-url origin https://mirrors.ustc.edu.cn/homebrew-cask.git
使用 “brew update -verbose” 命令执行更新即可！