brew/Library/Homebrew/utils/fork.rb

# typed: strict
# frozen_string_literal: true

require "fcntl"
require "utils/socket"

module Utils
  sig { params(child_error: T::Hash[String, T.untyped]).returns(Exception) }
  def self.rewrite_child_error(child_error)
    inner_class = Object.const_get(child_error["json_class"])
    error = if child_error["cmd"] && inner_class == ErrorDuringExecution
      ErrorDuringExecution.new(child_error["cmd"],
                               status: child_error["status"],
                               output: child_error["output"])
    elsif child_error["cmd"] && inner_class == BuildError
      # We fill `BuildError#formula` and `BuildError#options` in later,
      # when we rescue this in `FormulaInstaller#build`.
      BuildError.new(nil, child_error["cmd"], child_error["args"], child_error["env"])
    elsif inner_class == Interrupt
      Interrupt.new
    else
      # Everything other error in the child just becomes a RuntimeError.
      RuntimeError.new <<~EOS
        An exception occurred within a child process:
          #{inner_class}: #{child_error["m"]}
      EOS
    end

    error.set_backtrace child_error["b"]

    error
  end

  # When using this function, remember to call `exec` as soon as reasonably possible.
  # This function does not protect against the pitfalls of what you can do pre-exec in a fork.
  # See `man fork` for more information.
  sig {
    params(directory: T.nilable(String), yield_parent: T::Boolean,
           _blk: T.proc.params(arg0: T.nilable(String)).void).void
  }
  def self.safe_fork(directory: nil, yield_parent: false, &_blk)
    require "json/add/exception"

    block = proc do |tmpdir|
      UNIXServerExt.open("#{tmpdir}/socket") do |server|
        read, write = IO.pipe

        pid = fork do
          # bootsnap doesn't like these forked processes
          ENV["HOMEBREW_NO_BOOTSNAP"] = "1"
          error_pipe = server.path
          ENV["HOMEBREW_ERROR_PIPE"] = error_pipe
          server.close
          read.close
          write.fcntl(Fcntl::F_SETFD, Fcntl::FD_CLOEXEC)

          Process::UID.change_privilege(Process.euid) if Process.euid != Process.uid

          yield(error_pipe)
        # This could be any type of exception, so rescue them all.
        rescue Exception => e # rubocop:disable Lint/RescueException
          error_hash = JSON.parse e.to_json

          # Special case: We need to recreate ErrorDuringExecutions
          # for proper error messages and because other code expects
          # to rescue them further down.
          if e.is_a?(ErrorDuringExecution)
            error_hash["cmd"] = e.cmd
            error_hash["status"] = if e.status.is_a?(Process::Status)
              {
                exitstatus: e.status.exitstatus,
                termsig:    e.status.termsig,
              }
            else
              e.status
            end
            error_hash["output"] = e.output
          end

          write.puts error_hash.to_json
          write.close

          exit!
        else
          exit!(true)
        end

        begin
          yield(nil) if yield_parent

          begin
            socket = server.accept_nonblock
          rescue Errno::EAGAIN, Errno::EWOULDBLOCK, Errno::ECONNABORTED, Errno::EPROTO, Errno::EINTR
            retry unless Process.waitpid(pid, Process::WNOHANG)
          else
            socket.send_io(write)
            socket.close
          end
          write.close
          data = read.read
          read.close
          Process.waitpid(pid) unless socket.nil?
        rescue Interrupt
          Process.waitpid(pid)
        end

        # 130 is the exit status for a process interrupted via Ctrl-C.
        raise Interrupt if $CHILD_STATUS.exitstatus == 130
        raise Interrupt if $CHILD_STATUS.termsig == Signal.list["INT"]

        if data.present?
          error_hash = JSON.parse(T.must(data.lines.first))
          raise rewrite_child_error(error_hash)
        end

        raise ChildProcessError, $CHILD_STATUS unless $CHILD_STATUS.success?
      end
    end

    if directory
      block.call(directory)
    else
      Dir.mktmpdir("homebrew-fork", HOMEBREW_TEMP, &block)
    end
  end
end