Bug #13239: Bug with "special exceptions" when they are thrown in context of a rescue clause.

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: 2.4.x, 2.3.x, etc
• Backport: 2.2: UNKNOWN, 2.3: DONE, 2.4: DONE

[nvashchenko (Nikolay Vashchenko)]

I've stumbled upon a case when ruby is supposed to throw "IOError: stream closed"(https://github.com/ruby/ruby/blob/trunk/thread.c#L4823) because there was a retained FD lock by another thread, but I'm was getting this instead of it:

RuntimeError: can't modify frozen IOError
  /home/vagrant/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/puma-3.6.2/lib/puma/server.rb:877:in `write'
  /home/vagrant/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/puma-3.6.2/lib/puma/server.rb:877:in `<<'
  /home/vagrant/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/puma-3.6.2/lib/puma/server.rb:877:in `stop'

I've done some digging and it appeared to a be a ruby bug with how such exceptions(so called "special exceptions") are handled. This exception is being frozen right after creation: https://github.com/ruby/ruby/blob/trunk/vm.c#L2078 but later, when it's thrown, it's being handled exactly as regular exception that is not frozen, which leads to a problem here:
https://github.com/ruby/ruby/blob/trunk/eval.c#L511 - it is an attempt to assign the "#cause" attribute on it when it all happens inside a rescue clause, since the exception itself is frozen.
I've created a script to reproduce it:

rd, wr = IO.pipe
Thread.new do
  IO.select [rd]
  wr.close
end

begin
  raise 'any exception'
rescue
  wr << 'A' 
end 

It works with this ruby fork where I've added sleep for couple of seconds to imitate slow system call response, to keep the FD locked for a while and produce initial exception: https://github.com/ruby/ruby/blob/trunk/eval.c#L511

[nvashchenko (Nikolay Vashchenko)]

Oops, wrong link to the ruby fork.
Here's the right one: https://github.com/NickolasVashchenko/ruby/commits/special_exc_cause_bug

[nobu (Nobuyoshi Nakada)]

• Description updated (diff)
• Status changed from Open to Feedback

It has been fixed at r57415, I think.

[nvashchenko (Nikolay Vashchenko)]

Yes, that appears to be the case, thanks you.

[matthewd (Matthew Draper)]

Should this be closed for backporting?

[shyouhei (Shyouhei Urabe)]

• Status changed from Feedback to Closed
• Backport changed from 2.2: UNKNOWN, 2.3: UNKNOWN, 2.4: UNKNOWN to 2.2: UNKNOWN, 2.3: REQUIRED, 2.4: REQUIRED

matthewd (Matthew Draper) wrote:

Should this be closed for backporting?

OK, closing.

[nagachika (Tomoyuki Chikanaga)]

• Backport changed from 2.2: UNKNOWN, 2.3: REQUIRED, 2.4: REQUIRED to 2.2: UNKNOWN, 2.3: REQUIRED, 2.4: DONE

Backported into ruby_2_4 at r57939.

[usa (Usaku NAKAMURA)]

• Backport changed from 2.2: UNKNOWN, 2.3: REQUIRED, 2.4: DONE to 2.2: UNKNOWN, 2.3: DONE, 2.4: DONE

ruby_2_3 r59538 merged revision(s) 57415,57474.

[MSP-Greg (Greg L)]

Obviously, 2.2 changes are soon to be stopped.

Recently in Puma, Travis was changed from 2.2.8 to 2.2.9, and in code using a pipe, the "can't modify frozen IOError" occurred.

This was not an issue with 2.2.8, which was used in Puma / Travis testing since 2017-09-18.

So, the question is, might this be backported to 2.2?

Thanks, Greg