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.
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Mutex#lock, Mutex#synchronize and any code using them are explicitly unsafe. This includes
Monitorin the standard library which uses Mutex to provide reentrancy. -
Dir.chdirwith block -
any
IOwrite operations whenIO#syncis false; includingIO#write,IO#write_nonblock,IO#puts. Pipes and sockets default to ‘IO#sync = true’, so it is safe to write to them unlessIO#syncwas disabled. -
File#flock, as the underlying flock(2) call is not specified by POSIX
Commonly safe operations inside Signal.trap blocks¶ ↑
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Assignment and retrieval of local, instance, and class variables
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Most object allocations and initializations of common types including
Array,Hash,String,Struct,Time. -
Common
Array,Hash,String,Structoperations which do not execute a block are generally safe; but beware if iteration is occurring elsewhere. -
Hash#[],Hash#[]=(unlessHash.newwas given an unsafe block) -
Thread::Queue#pushandThread::SizedQueue#push(since Ruby 2.1) -
Creating a new
ThreadviaThread.new/Thread.start can used to get around the unusability of Mutexes inside a signal handler -
Signal.trapis safe to use inside blocks passed toSignal.trap -
arithmetic on
IntegerandFloat(‘+’, ‘-’, ‘%’, ‘*’, ‘/’)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
IntegerandFloatclasses 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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Dir.chdir(without block arg)
…