From: eregontp@... Date: 2017-11-29T19:07:30+00:00 Subject: [ruby-core:83980] [Ruby trunk Feature#14143] Thread.report_on_exception should be true by default Issue #14143 has been updated by Eregon (Benoit Daloze). I think it is time to do this, and most rubyists using threads with Ruby seem to agree with this (see #6647 and I can say at least JRuby & TruffleRuby implementers want this). Other threads should write a backtrace just like the main thread when they die due to an exception, and not hope there will be a Thread#join soon enough. Threads are heavyweight in Ruby, they are rarely if ever used as an isolation mechanism, and we cannot assume Thread#join will always be called. Proper exception handling in threads should anyway not rely on Thread#join semantics but deal with the exception and communicate the failure with other threads explicitly. A one-liner example to illustrate: $ ruby -e 'q=Queue.new; t=Thread.new{ q.push "abc".starts_with?("foo") }; p q.pop; t.join' This small example will show a deadlock message, but not what is the source of the problem: -e:1:in `pop': No live threads left. Deadlock? (fatal) 1 threads, 1 sleeps current:0x00005599e5464bf0 main thread:0x00005599e512e5e0 * # rb_thread_t:0x00005599e512e5e0 native:0x00007f4493ac8700 int:0 -e:1:in `pop' -e:1:in `
' from -e:1:in `
' Thread.report_on_exception=true by default would clearly show the misspelled method name: # terminated with exception: -e:1:in `block in
': undefined method `starts_with?' for "abc":String (NoMethodError) Did you mean? start_with? -d/--debug is not a good fit here because: * it changes the semantics to abort_on_exception. * it outputs a lot of extra exceptions (in bigger programs), yet without showing the backtrace of exceptions. * it does not differentiate between exceptions which killed a Thread and exceptions which are rescue'd. report-on-GC as proposed in #6647 is of no use here since the Thread might be GC-reachable long after it dies, and it adds extra non-determinism which has no place in error reporting. ---------------------------------------- Feature #14143: Thread.report_on_exception should be true by default https://bugs.ruby-lang.org/issues/14143#change-68059 * Author: Eregon (Benoit Daloze) * Status: Open * Priority: Normal * Assignee: * Target version: ---------------------------------------- Extracted from #6647 to focus on the default value now that the feature is implemented. I strongly believe we should have Thread.report_on_exception = true by default. It only adds some extra stderr output for apps which let threads die, which is very rarely intended. If it is intended, then one can use Thread.current.report_on_exception = false to clarify it's OK for that thread to die and the failure is handled by the app on Thread#join. I enabled Thread.report_on_exception=true by default in ruby/spec, see https://github.com/ruby/spec/pull/517, the only cases needing Thread.current.report_on_exception=false are the specs testing report_on_exception itself and Thread#join/value/status/raise. Enabling it for test-all shows a fair amount of extra output and failures, which I would bet some of them are bugs in the tests (I already found one, r60854 & r60870), and other tests should simply more carefully test what they expect (for instance assert_raise() inside the Thread just around the code raising an exception and join the Thread). I am willing to help to reduce the extra output and failures in test-all, but I would like a OK from Matz to try enabling Thread.report_on_exception by default. Dear Matz, do you think it is reasonable to show exceptions killing threads on stderr by default, instead of silently swallowing them until Thread#join ? (if there is ever a Thread#join ..., often not or too late, when the rest of the application has crashed) -- https://bugs.ruby-lang.org/ Unsubscribe: