[ruby-core:76390] [Ruby trunk Feature#6647] Exceptions raised in threads should be logged

Issue #6647 has been updated by John Backus.


Akira Tanaka wrote:
> It seems true-by-default and report-on-GC needs more discussion.

I'm not sure how helpful my input is here but I think `true-by-default` makes this change useful. Otherwise it is probably not very useful. Right now it seems like the main problems are:

1. New ruby developers don't know to about `Thread.abort_on_exception` and end up confused when their code doesn't work but they aren't seeing exceptions

2. More experienced ruby developers can still easily forget to set `Thread.abort_on_exception` and be confused until they realize

These problems are not solved at all if the default value for `Thread.report_on_exception` is `false`:

1. New ruby developers wont know to set this configuration

2. Experienced ruby developers are probably going to forget `report_on_exception` if they forgot about `abort_on_exception`. 

----------------------------------------
Feature #6647: Exceptions raised in threads should be logged
https://bugs.ruby-lang.org/issues/6647#change-59636

* Author: Charles Nutter
* Status: Closed
* 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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