ruby-core

Issue #6647 has been updated by Benoit Daloze.


Akira Tanaka wrote:
> How about introducing Thread[.#]report_on_exception=(bool) ?
> 
> false by default for compatibility.
> message should be printed at thread exit (not GC).

Agreed, except that it should be true by default otherwise it misses the whole point of Ruby Threads dying silently by default!
Extra output is very rarely a big problem and can be easily solved:

    Thread.current.report_on_exception = false if Thread.current.respond_to? :report_on_exception

Not so nice, so maybe we could use keyword args on Thread.new as Charles suggested.
This could conflict with existing arguments to Thread.new but it seems mostly harmless
(it would just yield an extra argument to the block which would just get ignored on older versions).

But more importantly, it seems to always be wrong to ignore exceptions like that,
and Threads which can allow such exceptions should handle them explicitly by adding a begin/rescue/end around the whole body so they can perform a useful recovery strategy.

Ruby Threads are not "futures", they are heavy concurrent OS threads and therefore I believe the current behavior (dying silently) is counter productive for everyone.

----------------------------------------
Feature #6647: Exceptions raised in threads should be logged
https://bugs.ruby-lang.org/issues/6647#change-58691

* Author: Charles Nutter
* Status: Assigned
* Priority: Normal
* Assignee: Yukihiro Matsumoto
----------------------------------------
Many applications and users I have dealt with have run into bugs due to Ruby's behavior of quietly swallowing exceptions raised in threads. I believe this is a bug, and threads should always at least log exceptions that bubble all the way out and terminate them.

The implementation should be simple, but I'm not yet familiar enough with the MRI codebase to provide a patch. The exception logging should be logged in the same way top-level exceptions get logged, but perhaps with information about the thread that was terminated because of the exception.

Here is a monkey patch that simulates what I'm hoping to achieve with this bug:

```ruby
class << Thread
  alias old_new new

  def new(*args, &block)
    old_new(*args) do |*bargs|
      begin
        block.call(*bargs)
      rescue Exception => e
        raise if Thread.abort_on_exception || Thread.current.abort_on_exception
        puts "Thread for block #{block.inspect} terminated with exception: #{e.message}"
        puts e.backtrace.map {|line| "  #{line}"}
      end
    end
  end
end

Thread.new { 1 / 0 }.join
puts "After thread"   
```

Output:

```
system ~/projects/jruby $ ruby thread_error.rb 
Thread for block #<Proc:0x000000010d008a80@thread_error.rb:17> terminated with exception: divided by 0
  thread_error.rb:17:in `/'
  thread_error.rb:17
  thread_error.rb:7:in `call'
  thread_error.rb:7:in `new'
  thread_error.rb:5:in `initialize'
  thread_error.rb:5:in `old_new'
  thread_error.rb:5:in `new'
  thread_error.rb:17
After thread
```



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