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Feature #6647

Exceptions raised in threads should be logged

Added by Charles Nutter almost 4 years ago. Updated 3 days ago.

Status:
Assigned
Priority:
Normal
[ruby-core:45864]

Description

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:

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"

END

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

History

#1 [ruby-core:45865] Updated by Charles Nutter almost 4 years ago

FWIW, precedent: Java threads log their exceptions by default. I have never found the feature to be a bother, and it makes it nearly impossible to ignore fatally-flawed thread logic that spins up and fails lots of threads.

#2 [ruby-core:45866] Updated by Eero Saynatkari almost 4 years ago

headius (Charles Nutter) wrote:

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.

I have had to set .abort_on_exception more times than I care to remember.

  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}"}

$stderr/warn, but this would improve the current situation significantly.

Can significant upgrade problems be expected if .abort_on_exception defaulted to true? This would seem to be the behaviour to suit most users.

#3 [ruby-core:45881] Updated by Alex Young almost 4 years ago

On 25/06/12 23:44, rue (Eero Saynatkari) wrote:

Issue #6647 has been updated by rue (Eero Saynatkari).

headius (Charles Nutter) wrote:

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.

I have had to set .abort_on_exception more times than I care to remember.

Agreed. It's one of the things I check for in code review. Consider
this a +1 from me.

   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}"}

$stderr/warn, but this would improve the current situation significantly.

Can significant upgrade problems be expected if .abort_on_exception defaulted to true? This would seem to be the behaviour to suit most users.

That sounds a little extreme, although I wouldn't object. I'd be happy
with them not being silently swallowed.

--
Alex


Bug #6647: Exceptions raised in threads should be logged
https://bugs.ruby-lang.org/issues/6647#change-27456

Author: headius (Charles Nutter)
Status: Open
Priority: Normal
Assignee:
Category:
Target version:
ruby -v: head

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:

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"

END

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

#4 [ruby-core:45906] Updated by Eric Wong almost 4 years ago

Alex Young alex@blackkettle.org wrote:

On 25/06/12 23:44, rue (Eero Saynatkari) wrote:

Issue #6647 has been updated by rue (Eero Saynatkari).

$stderr/warn, but this would improve the current situation significantly.

Can significant upgrade problems be expected if .abort_on_exception defaulted to true? This would seem to be the behaviour to suit most users.

That sounds a little extreme, although I wouldn't object. I'd be
happy with them not being silently swallowed.

I think aborting the whole process is extreme (though, I usually do
it myself).

I would very much like to see this via $stderr/warn, though.

#5 [ruby-core:45913] Updated by Eero Saynatkari almost 4 years ago

normalperson (Eric Wong) wrote:

Alex Young alex@blackkettle.org wrote:

On 25/06/12 23:44, rue (Eero Saynatkari) wrote:

Issue #6647 has been updated by rue (Eero Saynatkari).

$stderr/warn, but this would improve the current situation significantly.

Can significant upgrade problems be expected if .abort_on_exception defaulted to true? This would seem to be the behaviour to suit most users.

That sounds a little extreme, although I wouldn't object. I'd be
happy with them not being silently swallowed.

I think aborting the whole process is extreme (though, I usually do
it myself).

You are probably correct. Reconsidering the issue, the benefit of raising
is probably not enough to offset that, thus leaving the $stderr/warn as the
better choice.

--
Eero

#6 [ruby-core:46444] Updated by Hiroshi Nakamura almost 4 years ago

  • Status changed from Open to Assigned
  • Target version set to 2.0.0
  • Category set to core
  • Assignee set to Yukihiro Matsumoto

Discussions ad CRuby dev meeting at 14th July.

  • We can understand the requirement. (We understood that the requirement is dumping something without raising when Thread#abort_on_exception = false)
  • Writing to STDERR could cause problem with existing applications so we should take care about it.
  • rb_warn() instead of puts would be good because we already using rb_warns.

Matz, do you mind if we dump Thread error with rb_warn if Thread#abort_on_exception = false?

#7 [ruby-core:47466] Updated by Charles Nutter over 3 years ago

Any update on this?

#8 [ruby-core:47988] Updated by Charles Nutter over 3 years ago

Ping! This came up in JEG's talk at Aloha RubyConf as a recommendation (specifically, set abort_on_exception globally to ensure failed threads don't quietly disappear). Ruby should not allow threads to quietly fail.

#9 [ruby-core:47994] Updated by Motohiro KOSAKI over 3 years ago

I think "exception raised" callback is better way because an ideal output (both format and output device) depend on an application. It should be passed a raised exception.

#10 [ruby-core:48000] Updated by Alex Young over 3 years ago

On 15/10/12 03:24, kosaki (Motohiro KOSAKI) wrote:

Issue #6647 has been updated by kosaki (Motohiro KOSAKI).

I think "exception raised" callback is better way because an ideal output (both format and output device) depend on an application. It should be passed a raised exception.

This, along with a sensible default that displays something to stderr,
would be absolutely ideal from my point of view.

--
Alex


Bug #6647: Exceptions raised in threads should be logged
https://bugs.ruby-lang.org/issues/6647#change-30689

Author: headius (Charles Nutter)
Status: Assigned
Priority: Normal
Assignee: matz (Yukihiro Matsumoto)
Category: core
Target version: 2.0.0
ruby -v: head

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:

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"

END

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

#11 [ruby-core:48057] Updated by Charles Nutter over 3 years ago

I started prototyping a callback version and ran into some complexities I could not easily resolve:

  • How does abort_on_exception= interact with a callback system? ** I tried implementing abort_on_exception=true to use a builtin callback that raises in Thread.main, but should abort_on_exception=true blow away a previously-set callback? ** Similarly: should abort_on_exception=false reset to a do-nothing callback? ** If neither of these, how do we combine callback and abort_on_exception behavior?
  • Seems like there should be a Thread.default_exception_handler you can set once for all future threads.

My concern is that bikeshedding a callback API -- as useful as it might be -- will cause further delays in the more important behavior of having threads report that they terminated due to an exception.

#13 [ruby-core:50046] Updated by Charles Nutter over 3 years ago

Checking in on this again. Can we at least agree it should happen for 2.0.0? Perhaps Matz should review this?

#14 Updated by Yusuke Endoh over 3 years ago

  • Tracker changed from Bug to Feature

headius (Charles Nutter) wrote:

Can we at least agree it should happen for 2.0.0?

No, objection. This looks to me nothing except for a feature request.
I cannot estimate the impact how writing to stderr affects existing applications.
There is a workaround.
I don't think that your concern is so significant that we should address by changing the spec from now.

Moving to the feature tracker and setting to next minor.

--
Yusuke Endoh mame@tsg.ne.jp

#15 [ruby-core:50055] Updated by Yusuke Endoh over 3 years ago

  • Target version changed from 2.0.0 to next minor

#16 [ruby-core:57437] Updated by Charles Nutter over 2 years ago

So, can we do this for 2.1? I have heard from many other users that really would like exceptions bubbling out of threads to be reported in some way. We have had numerous bug reports relating to code where threads disappear without a trace.

#17 [ruby-core:57468] Updated by Avdi Grimm over 2 years ago

This would indeed eliminate a huge amount of confusion for people getting
started with threads. Or for people years of experience with threads, for
that matter...

On Fri, Sep 27, 2013 at 7:18 AM, headius (Charles Nutter) <
headius@headius.com> wrote:

Issue #6647 has been updated by headius (Charles Nutter).

So, can we do this for 2.1? I have heard from many other users that really
would like exceptions bubbling out of threads to be reported in some way.
We have had numerous bug reports relating to code where threads disappear

without a trace.

Feature #6647: Exceptions raised in threads should be logged
https://bugs.ruby-lang.org/issues/6647#change-42043

Author: headius (Charles Nutter)
Status: Assigned
Priority: Normal
Assignee: matz (Yukihiro Matsumoto)
Category: core
Target version: next minor

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:

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"

END

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

--
http://bugs.ruby-lang.org/

--
Avdi Grimm
http://avdi.org

I only check email twice a day. to reach me sooner, go to
http://awayfind.com/avdi

#18 [ruby-core:57473] Updated by Koichi Sasada over 2 years ago

  • Target version changed from next minor to 2.1.0

#19 [ruby-core:57472] Updated by Koichi Sasada over 2 years ago

(2013/09/27 20:18), headius (Charles Nutter) wrote:

So, can we do this for 2.1? I have heard from many other users that really would like exceptions bubbling out of threads to be reported in some way. We have had numerous bug reports relating to code where threads disappear without a trace.

I'll ask matz.

Does JRuby log it already?
Any problem on your experience if you have?

--
// SASADA Koichi at atdot dot net

#20 [ruby-core:57493] Updated by Charles Nutter over 2 years ago

We do not currently log it, but the patch to do so is trivial.

https://gist.github.com/6764310

I'm running tests now to confirm it doesn't break anything.

#21 [ruby-core:57494] Updated by Charles Nutter over 2 years ago

Testing seems to indicate this is a pretty safe change, and it just makes the debug-logged exception output be logged any time abort_on_exception is not true.

#22 [ruby-core:57576] Updated by Akira Tanaka over 2 years ago

In the yesterday's meeting,
https://bugs.ruby-lang.org/projects/ruby/wiki/DevelopersMeeting20131001Japan
we discussed this issue.

We found that message at thread exiting with exception have a problem.
The thread can be joined after exit and the exception may be handled by joined thread.

% ruby -e '
t = Thread.new {
raise "foo"
}
sleep 1 # the thread exits with an exception.
begin
t.join
rescue
p $! # something to do with the exception
end
'
#

If thread exiting with exception outputs a message,
there is no way to disable to it.

So, the message should be delayed until Ruby is certain that
the thread is not joined.
This means the message should be output at the thread is collected by GC.

#23 [ruby-core:57586] Updated by Charles Nutter over 2 years ago

akr (Akira Tanaka) wrote:

In the yesterday's meeting,
https://bugs.ruby-lang.org/projects/ruby/wiki/DevelopersMeeting20131001Japan
we discussed this issue.

We found that message at thread exiting with exception have a problem.
The thread can be joined after exit and the exception may be handled by joined thread.
...
If thread exiting with exception outputs a message,
there is no way to disable to it.

So, the message should be delayed until Ruby is certain that
the thread is not joined.
This means the message should be output at the thread is collected by GC.

GC is a pretty fuzzy time boundary, but it's not terrible. Handling it will mean some finalization requirement for threads to say "hey, I just GCed this thread that died due to an unhandled exception". I feel like something more explicit is needed.

I guess I need to think about this. Some of the cases I want to fix -- where threads are spun up and left to do their own work -- this might be acceptable. But many users will keep references to worker threads they start in order to explicitly stop them on shutdown or other events. In those cases, the thread will be hard referenced and never GCed...and there will be no indication that the thread has died.

Perhaps this could be an on-by-default flag? It would require very little work to add something like:

class Thread
def report_on_exception=(report) ..
end

...where the default is true. Going forward, this would be like having the debug output of a thread-killing exception always happen, but you could turn it off. That would address your concern about not being able to silence it.

The workflow would go like this:

If you are spinning up a thread to do background work and don't plan to check on it...

  • Spin up the thread
  • Store it in a list if you like
  • A message will be reported if the thread dies in an exceptional way

If you are spinning up a thread you plan to join on at some time in the future...

  • Spin up the thread
  • Set Thread#report_on_exception = false
  • Join at your leisure...no message will be reported

This at least allows users to say "I mean to pick up this thread's results later...don't report an error" without having hard-referenced threads die silently.

Is this a reasonable compromise?

#24 [ruby-core:57594] Updated by Koichi Sasada over 2 years ago

FYT:
On pthread, there is pthread_detach() which declares nobody join on this thread.
In other words, pthread_detach() is same as Thread#report_on_exception=true.

#25 [ruby-core:57595] Updated by Koichi Sasada over 2 years ago

Sorry, it is not same, but we can consier that.

BTW, I think it true as default is good idea.

IMO, inter-thread communication via exception with Thread#join should be bad idea.

#26 [ruby-core:57617] Updated by Charles Nutter over 2 years ago

ko1 (Koichi Sasada) wrote:

Sorry, it is not same, but we can consier that.

BTW, I think it true as default is good idea.

So to summarize:

  • Exceptions will log when they bubble out of a thread, as with -d, unless Thread#report_on_exception == false
  • Thread#report_on_exception defaults to true

Can we do this for 2.1?

IMO, inter-thread communication via exception with Thread#join should be bad idea.

+1

I had originally wanted something similar to Java, where you can set an "unhandled exception handler" for any thread. That would cover all cases, and the default case would be to just report the error. I was unsuccessful in specifying it because I wasn't sure how it should interact with abort_on_exception=.

#27 [ruby-core:60265] Updated by Hiroshi SHIBATA over 2 years ago

  • Target version changed from 2.1.0 to current: 2.2.0

#28 [ruby-core:60375] Updated by Koichi Sasada over 2 years ago

Restart for 2.2.
Matz, do you have any idea?

#29 [ruby-core:69107] Updated by Lin Jen-Shin 12 months ago

Not sure if a +1 would do anything, but I like the idea of
Thread#report_on_exception defaults to true.

For quick and one time scripts, it's tedious to write
Thread.current.abort_on_exception = true all the time,
and it shouldn't be set to true by default, either.
So at least make debugging easier by default is a good idea,
and who doesn't like to see warnings anyway? :P

I was referred from yahns mailing list:
http://yhbt.net/yahns-public/m/20150508170311.GA1260%40dcvr.yhbt.net.html
Which some worker threads were dead silently and it's puzzling
if I don't even know there's an exception was raised.

#30 [ruby-core:69109] Updated by Eric Wong 12 months ago

I have an actual patch which is only 2 lines, but there's some test
failures and MANY warnings I don't feel motivated to fix just yet
unless matz approves the feature:

http://80x24.org/spew/m/0a12f5c2abd2dfc2f055922a16d02019ee707397.txt

#31 [ruby-core:69110] Updated by Charles Nutter 12 months ago

Eric Wong wrote:

I have an actual patch which is only 2 lines, but there's some test
failures and MANY warnings I don't feel motivated to fix just yet
unless matz approves the feature:

Hot diggity! I bet there's several of these that indicate bugs to be fixed. At the very least, they indicate exceptions that are being raised and not dealt with.

I think this is great evidence that this IS the right change to make.

#32 [ruby-core:74743] Updated by Benoit Daloze about 1 month ago

Akira Tanaka wrote:

In the yesterday's meeting,
https://bugs.ruby-lang.org/projects/ruby/wiki/DevelopersMeeting20131001Japan
we discussed this issue.

We found that message at thread exiting with exception have a problem.
The thread can be joined after exit and the exception may be handled by joined thread.

% ruby -e '
t = Thread.new {
raise "foo"
}
sleep 1 # the thread exits with an exception.
begin
t.join
rescue
p $! # something to do with the exception
end
'
#

If thread exiting with exception outputs a message,
there is no way to disable to it.

So, the message should be delayed until Ruby is certain that
the thread is not joined.
This means the message should be output at the thread is collected by GC.

I am strongly in favor of having something like Thread#report_on_exception, defaulting to true.
If a Thread can support known exceptions, it can rescue them explicitly.
If a Thread is used as some sort of isolation, it can disable #report_on_exception.

Thread#join is not enough, and because of the lack of reporting it's very easy to end in a deadlock with no other way to notice than dumping the thread stacks.
Imagine a simple actor framework where actors are just Threads with a Queue.
Actor1 waits for a message from Actor2, and Actor2 crashes because for instance it calls a method which does not exist.
Actor1 is blocked, and the user has absolutely no knowledge of what's happening, unless Thread.abort_on_exception is set before creating any thread.

So, in summary, Thread.abort_on_exception is not always appropriate,
Thread#join is not enough,
and silently swallowing exceptions can lead to deadlocks that the programmer has a hard time to notice.
Let's give a chance to users to see problems in their code with Thread!

#33 [ruby-core:75235] Updated by Shyouhei Urabe 7 days ago

I remember this topic was looked at in the developer meeting this month. Matz was positive to have Thread#report_on_exception, but not default true. Sorry I don't remember the reason why he was not comfortable with defaulting this.

#34 [ruby-core:75250] Updated by Benoit Daloze 6 days ago

Shyouhei Urabe wrote:

I remember this topic was looked at in the developer meeting this month. Matz was positive to have Thread#report_on_exception, but not default true.

Thanks for the reply.

Sorry I don't remember the reason why he was not comfortable with defaulting this.

That's unfortunate.
The main reason to have it by default is to give a chance when developing with Threads to notice the error in the sub-thread.
When trying out threads, the program will of course have very little chance to have Thread.abort_on_exception or Thread.report_on_exception in it,
particularly if the author is not extremely familiar with current Ruby thread exception pitfalls.
Debug mode (-d) would help but it's also a feature most people ignore or do not think to use (and it outputs much more).

That's why I think report_on_exception by default is the only reasonable choice for people not extremely familiar with Ruby thread and their exception handling.

I would guess the argument is about Thread#join or Thread#value.
But threads in Ruby are OS threads, using them for just one computation (like a future) is inefficient as it incurs spawning a OS thread every time.
For this and many other reasons, joining threads is often done much later than when the exception happens (if ever, for instance with a dead/livelock it would not).
Here is another example: Communication with threads is done most often with Queue, yet if the main Thread pops from the queue to get results from the sub-thread (producer) and the sub-thread throws an exception, the program will deadlock with no clue given to the programmer.

So relying on #join or #value is very brittle and I believe causes much more harm than a few extra exceptions printed on stdout, which can be easily handled with Thread.current.report_on_exception = false.

#35 [ruby-core:75313] Updated by Charles Nutter 3 days ago

I remember this topic was looked at in the developer meeting this month. Matz was positive to have Thread#report_on_exception, but not default true. Sorry I don't remember the reason why he was not comfortable with defaulting this.

I would guess it's for all the badly-behaved code out there that's just letting threads die silently when an error is raised.

Having it default to off defeats the purpose of this feature request. My request is that threads report when an exception bubbles out before being handled.

I understand the concern about Thread#join. If we report by default then we might have exceptions reported as unhandled when a subsequent Thread#join would handle them. The idea about reporting on GC of the thread is interesting but it might mean we still never get any indication if the thread never GCs. I'd expect this is the typical case, since most people don't fire off threads without having a hard reference to them.

Re: Thread#join

Thread#join always re-raises its exception no matter whether abort_on_exception is set, so I don't see this as an issue. If you expect you'll be handling a Thread's last exception via #join, you would just specify that it should be quiet.

Thread#join works this way right now for abort_on_exception:

2.3.0 :001 > Thread.abort_on_exception = true
 => true 
2.3.0 :002 > go = false
 => false 
2.3.0 :003 > t = Thread.new { Thread.pass until go; raise }
 => #<Thread:0x007fc8920abe98@(irb):3 run> 
2.3.0 :004 > begin; go = true; sleep; rescue Exception; p $!; end
RuntimeError
 => RuntimeError 
2.3.0 :005 > t.join
RuntimeError: 
    from (irb):3:in `block in irb_binding'

I also made the larger suggestion of having this all wire in as a per-thread exception handler API.

There would be at least four default handlers. In all cases I think #join and #value should still produce the original exception.

  • Silent: Do not propagate the exception and do not report it. This is current behavior.
  • Report: Do not propagate the exception but report that it ended a thread. This is my requested behavior.
  • Reraise: Propagate the exception to the main thread but do not otherwise report it. This is abort_on_exception = true.
  • Custom: Provide your own proc/block to handle any exception raised from the thread.

Note also we could blunt some compatibility concerns by making these settable as Thread.new keyword args:

t = Thread.new(exception: :raise)

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