处理 Ruby 线程中引发的异常

Question

我正在寻找异常处理经典问题的解决方案。考虑以下代码：

def foo(n)
  puts " for #{n}"
  sleep n
  raise "after #{n}"
end

begin
  threads = []
  [5, 15, 20, 3].each do |i|
    threads << Thread.new do
      foo(i)
    end
  end

  threads.each(&:join)      
rescue Exception => e
  puts "EXCEPTION: #{e.inspect}"
  puts "MESSAGE: #{e.message}"
end

此代码在 5 秒后捕获异常。

但是如果我将数组更改为[15, 5, 20, 3]，上面的代码会在 15 秒后捕获异常。简而言之，它总是捕获第一个线程中引发的异常。

任何想法，为什么会这样。为什么每次 3 秒后都没有捕捉到异常？如何捕获任何线程引发的第一个异常？

Answer 1

如果您希望任何线程中的任何未处理异常导致解释器退出，您需要将Thread::abort_on_exception= 设置为true。未处理的异常导致线程停止运行。如果您不将此变量设置为 true，则仅当您为线程调用 Thread#join 或 Thread#value 时才会引发异常。如果设置为 true，它将在发生时引发并传播到主线程。

Thread.abort_on_exception=true # add this

def foo(n)
    puts " for #{n}"
    sleep n
    raise "after #{n}"
end

begin
    threads = []
    [15, 5, 20, 3].each do |i|
        threads << Thread.new do
            foo(i)
        end
    end
    threads.each(&:join)

rescue Exception => e

    puts "EXCEPTION: #{e.inspect}"
    puts "MESSAGE: #{e.message}"
end

输出：

 for 5
 for 20
 for 3
 for 15
EXCEPTION: #<RuntimeError: after 3>
MESSAGE: after 3

注意：但如果您希望任何特定线程实例以这种方式引发异常，则有类似的abort_on_exception= Thread instance method：

t = Thread.new {
   # do something and raise exception
}
t.abort_on_exception = true

Answer 2

Thread.class_eval do
  alias_method :initialize_without_exception_bubbling, :initialize
  def initialize(*args, &block)
    initialize_without_exception_bubbling(*args) {
      begin
        block.call
      rescue Exception => e
        Thread.main.raise e
      end
    }
  end
end

Answer 3

延迟的异常处理（受@Jason Ling 启发）

class SafeThread < Thread

  def initialize(*args, &block)
    super(*args) do
      begin
        block.call
      rescue Exception => e
        @exception = e
      end
    end
  end

  def join
    raise_postponed_exception
    super
    raise_postponed_exception
  end

  def raise_postponed_exception
    Thread.current.raise @exception if @exception
  end

end


puts :start

begin
  thread = SafeThread.new do
    raise 'error from sub-thread'
  end

  puts 'do something heavy before joining other thread'
  sleep 1

  thread.join
rescue Exception => e
  puts "Caught: #{e}"
end

puts 'proper end'

Answer 4

这将等待第一个线程引发或返回（并重新引发）：

require 'thwait'
def wait_for_first_block_to_complete(*blocks)
  threads = blocks.map do |block|
    Thread.new do
      block.call
    rescue StandardError
      $!
    end
  end
  waiter = ThreadsWait.new(*threads)
  value = waiter.next_wait.value
  threads.each(&:kill)
  raise value if value.is_a?(StandardError)
  value
end

Answer 5

Jason Ling's answer 将丢失任何传递给 Thread.new 的参数。这将打破 Puma 和其他宝石。为避免此问题，您可以使用：

Thread.class_eval do
  alias_method :initialize_without_exception_bubbling, :initialize
  def initialize(*args, &block)
    initialize_without_exception_bubbling(*args) {
      begin
        block.call(*args)
      rescue Exception => e
        Thread.main.raise e
      end
    }
  end
end