Caveats for implementing Signal.trap callbacks

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#lock and Mutex#synchronize which are commonly used for thread-safety even prevent reentrancy.

An implementation detail of the Ruby VM

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.

Unsafe methods to call in Signal.trap blocks

When in doubt, consider anything not listed as safe below as being unsafe.

Mutex#lock, Mutex#synchronize and 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#sync is 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

Commonly safe operations inside Signal.trap blocks

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#[], Hash#[]= (unless Hash.new was given an unsafe block)
Thread::Queue#push and 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.

System call wrapper methods which are safe inside Signal.trap

Since Ruby has wrappers around many async-signal-safe C functions the corresponding wrappers for many IO, File, Dir, and Socket methods are safe.

(Incomplete list)

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Caveats for implementing Signal.trap callbacks

An implementation detail of the Ruby VM

Unsafe methods to call in Signal.trap blocks

Commonly safe operations inside Signal.trap blocks

System call wrapper methods which are safe inside Signal.trap