spawn-bang

In Crystal, an unhandled exception in the main Fiber causes the process to die with exit(1), while an unhandled exception in a spawn-ed Fiber will print the exception to STDERR and stop that Fiber, but the process will otherwise keep running. This makes it far too easy to have an unnoticed, unhandled exception. This spawn-bang shard makes available a top-level method and macro spawn! which is identical to spawn but will terminate the process with exit(1) if there is an unhandled exception.

In general, you can always replace spawn with spawn!. You are still free to (and encouraged to) implement your own exception handling within spawn!, especially for exceptions you know are likely to happen and you'd like to ignore or handle without killing the process. Using spawn! simply changes the default behavior for unhandled exceptions.

spawn-bang is currently used in production on Heii On-Call.

How it works

The code:

require "spawn-bang"

spawn! do
  # YOUR CODE GOES HERE
end

is 100% equivalent to:

spawn do
  begin
    # YOUR CODE GOES HERE
  rescue ex
    STDERR.print "Unhandled exception in spawn!: "
    ex.inspect_with_backtrace(STDERR)
    STDERR.print "spawn!: Fatal. Dying..."
    STDERR.flush
    exit(1)
  end
end

A macro version is also provided for compatibility with the macro version of spawn. See Spawning a call.

Proposed for inclusion in Crystal standard library

I would like to see spawn! included in the Crystal standard library, rather than requiring this shard.

If you agree, feel free to comment on: Proposal for spawn! on Crystal forum

Installation

1. Add the dependency to your shard.yml:

   dependencies:
     spawn-bang:
       github: compumike/spawn-bang

2. Run shards install

Usage

1. Add require "spawn-bang"

2. Replace all occurences of spawn with spawn!

See also

• crystal/src/concurrent.cr
• Crystal Concurrency Guide
• Proposal for spawn! on Crystal forum