[ruby-core:83596] [Ruby trunk Feature#6647] Exceptions raised in threads should be logged

Issue #6647 has been updated by Eregon (Benoit Daloze).


Eregon (Benoit Daloze) wrote:
> FWIW, I enabled Thread.report_on_exception = true by default in ruby/spec.
> I had to change a few specs, but I think it really improved the specs rather than being cumbersome.
> See https://github.com/ruby/spec/pull/517 for details.

And it already proved useful!
https://travis-ci.org/ruby/spec/jobs/294063354
No way to know why we get ECONNREFUSED without seeing the other thread dying with EBADF.

akr:
> So, I think we should try report-on-GC.

I don't think report-on-GC is good, it just shows the error too late, if it shows it at all.
This is adding extra non-determinism on top of non-deterministic thread bugs, so I'm against it.

I argue that 99% of rubyists expect their threads to not die, or they plan for it and use Thread.new{begin ... rescue ... end} to log/handle exceptions.
Who wants a part of the program crashing silently? Nobody.

Thread#join is often too late, and it is a tiny price to pay to use Thread.report_on_exception = false in the rare case of using short-lived threads with no exception handling except #join.
Long-running threads are typically only joined at the end of the program, but the error, the fact that the thread died, should be shown to the programmer much earlier.

----------------------------------------
Feature #6647: Exceptions raised in threads should be logged
https://bugs.ruby-lang.org/issues/6647#change-67623

* Author: headius (Charles Nutter)
* Status: Closed
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
* Target version: 
----------------------------------------
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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