As with implementing signal handlers in C or most other languages, all code passed to Signal.trap must be reentrant. If you are not familiar with reentrancy, you need to read up on it at Wikipedia or elsewhere before reading the rest of this document.
Most importantly, “thread-safety” does not guarantee reentrancy; and methods such as
Mutex#synchronize which are commonly used for thread-safety even prevent reentrancy.
The Ruby VM defers Signal.trap callbacks from running until it is safe for its internal data structures, but it does not know when it is safe for data structures in YOUR code. Ruby implements deferred signal handling by registering short C functions with only async-signal-safe functions as signal handlers. These short C functions only do enough tell the VM to run callbacks registered via Signal.trap later in the main Ruby Thread.
When in doubt, consider anything not listed as safe below as being unsafe.
Mutex#synchronizeand any code using them are explicitly unsafe. This includes Monitor in the standard library which uses Mutex to provide reentrancy.
Dir.chdir with block
any IO write operations when
IO#syncis false; including
IO#write, IO#write_nonblock, IO#puts. Pipes and sockets default to
IO#sync = true, so it is safe to write to them unless IO#sync was disabled.
File#flock, as the underlying flock(2) call is not specified by POSIX
Assignment and retrieval of local, instance, and class variables
Most object allocations and initializations of common types including Array, Hash, String, Struct, Time.
Common Array, Hash, String, Struct operations which do not execute a block are generally safe; but beware if iteration is occurring elsewhere.
Hash#=(unless Hash.new was given an unsafe block)
Thread::SizedQueue#push(since Ruby 2.1)
Creating a new Thread via Thread.new/Thread.start can used to get around the unusability of Mutexes inside a signal handler
Signal.trap is safe to use inside blocks passed to Signal.trap
arithmetic on Integer and Float (
+',-‘, ‘%’, ‘*’, ‘/’)
Additionally, signal handlers do not run between two successive local variable accesses, so shortcuts such as
+=' and-=’ will not trigger a data race when used on Integer and Float classes in signal handlers.
Since Ruby has wrappers around many async-signal-safe C functions the corresponding wrappers for many IO, File, Dir, and Socket methods are safe.
- Dir.chdir (without block arg)