Issue #19473 has been updated by mame (Yusuke Endoh). If `Mutex#lock` were permitted inside a trap handler, a program like the following would have a very hard-to-reproduce race condition bug. ```ruby m = Mutex.new trap(:INT) { m.synchronize { p :signalled } } while true sleep 1 m.synchronize { p :hi! } end ``` If you press Ctrl+C while this program is running, it would, in most cases, print `:signalled` and continue execution, which is an expected behavior. However, if the signal arrives at a particularly unlucky moment, it would cause a `deadlock; recursive locking (ThreadError)`. The problem isn't just that the bug is hard to reproduce; it's that programmers often don't even realize they've introduced such a race condition bug. To prevent this, `Mutex#lock` is designed to consistently raise an exception when called inside a trap handler. ---------------------------------------- Bug #19473: can't be called from trap context (ThreadError) is too limiting https://bugs.ruby-lang.org/issues/19473#change-113957 * Author: Eregon (Benoit Daloze) * Status: Open * ruby -v: ruby 3.2.1 (2023-02-08 revision 31819e82c8) [x86_64-linux] * Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN ---------------------------------------- Simple reproducer: ``` $ ruby -ve 'm=Mutex.new; trap(:HUP) { m.synchronize { p :OK } }; Process.kill :HUP, Process.pid; sleep 0.1' ruby 3.2.1 (2023-02-08 revision 31819e82c8) [x86_64-linux] -e:1:in `synchronize': can't be called from trap context (ThreadError) from -e:1:in `block in <main>' from -e:1:in `kill' from -e:1:in `<main>' ``` Expected behavior: ``` $ ruby -ve 'm=Mutex.new; trap(:HUP) { m.synchronize { p :OK } }; Process.kill :HUP, Process.pid; sleep 0.1' truffleruby 22.3.1, like ruby 3.0.3, GraalVM CE Native [x86_64-linux] :OK $ ruby -ve 'm=Mutex.new; trap(:HUP) { m.synchronize { p :OK } }; Process.kill :HUP, Process.pid; sleep 0.1' jruby 9.4.0.0 (3.1.0) 2022-11-23 95c0ec159f OpenJDK 64-Bit Server VM 17.0.6+10 on 17.0.6+10 +jit [x86_64-linux] :OK ``` This exception is highly problematic, for instance it breaks `Timeout.timeout` in `trap`: https://github.com/ruby/timeout/issues/17#issuecomment-1142035939 I suppose this behavior is because *sometimes* it's problematic to lock a Mutex in trap, e.g., if it's already locked by the main thread/fiber. But that would otherwise already raise `deadlock; recursive locking (ThreadError)`, so there is no point to fail early. And that's just one case, not all, so we should not always raise an exception. There seems to be no valid reason to prevent *all* `Mutex#synchronize` in `trap`. After all, if the Mutex for instance is only used in `trap`, it's well-defined AFAIK. For instance a given trap handler does not seem executed concurrently: ``` $ ruby -ve 'trap(:HUP) { puts "in trap\n"+caller.join("\n")+"\n\n"; sleep 0.1 }; pid = Process.pid; Process.wait fork { 20.times { Process.kill :HUP, pid } }; sleep 1' ruby 3.2.1 (2023-02-08 revision 31819e82c8) [x86_64-linux] in trap -e:1:in `wait' -e:1:in `<main>' in trap -e:1:in `wait' -e:1:in `<main>' in trap -e:1:in `wait' -e:1:in `<main>' in trap -e:1:in `wait' -e:1:in `<main>' in trap -e:1:in `wait' -e:1:in `<main>' in trap -e:1:in `wait' -e:1:in `<main>' ``` And if the trap handler using the Mutex is never called while the Mutex is held by the main thread/fiber, there is also no problem. -- https://bugs.ruby-lang.org/