[ruby-core:75576] [Ruby trunk Feature#6647] Exceptions raised in threads should be logged

Issue #6647 has been updated by B Kelly.


Daniel Ferreira wrote:
> Why do we need `Thread#report_on_exception` ?
> As I see it `Thread.report_on_exception` should be enough.
> 
> Is there a use case scenario for `Thread#report_on_exception` ?

Yes, because `Thread#value` passes the exception to the caller.

Granted ruby recently no longer supports $SAFE=4 sandboxing,
the following was a common sandbox idiom:

  `result = Thread.new {$SAFE=4; do_some_sandbox_work()}.value`

In such cases, it is intended by design that any exception be
passed out of the sandbox to the caller.  Automatic logging
at that boundary would not be desired.

By the way, I'm personally fine with logging by default, and
I support this feature as way to catch unexpected unhandled
thread exceptions.

But I would argue there indeed needs to be a way to disable
logging on a per-thread basis, where exceptions are intended
to be passed via `Thread#value` by design.


Regards,

Bill



----------------------------------------
Feature #6647: Exceptions raised in threads should be logged
https://bugs.ruby-lang.org/issues/6647#change-58709

* 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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