Inconsistent Timeout::timeout and rescue Timeout::Error behavior - ruby-on-rails

Question

I'm using Timeout::timeout(1) for a process that takes longer than 1 second, though it only occasionally triggers a timeout. When it does, rescue captures it in different ways each time. Here's a sample of my code:
require 'timeout'
...
begin
status = Timeout::timeout(1) {
open(file_url) do |foo|
feed = RSS::Parser.parse(foo)
some_method_call(arg1, arg2)
#other stuff
end
}
rescue Timeout::Error
Rails.logger.debug "Timeout"
return nil
rescue Exception => ex
Rails.logger.debug "EXCEPTION - #{ex.message}"
return nil
end
Here are the three scenarios I encounter with the same input:
Process runs to completion and takes longer than 60 seconds
Process times out and hangs, printing only execution expired in development.log
Process times out, is rescued properly, prints "Timeout" in development.log, and returns nil
Why is this so inconsistent?
UPDATE
After reducing the timeout to 0.0001s, the process is timing out consistently and as expected. It seems that the open(file_url) block was opening faster than 1 second, and despite everything within the block taking more than 1 second, the Timeout was only triggered if the opening itself took longer than 1 second.
This however did not explain the execution expired exception. To test this, I moved the Timeout::timeout(0.0001) to within the open block. The code looks like the following:
require 'timeout'
...
begin
open(file_url) do |foo|
status = Timeout::timeout(0.0001) do
begin
feed = RSS::Parser.parse(foo)
some_method_call(arg1, arg2)
#other stuff
rescue Timeout::Error
Rails.logger.debug "Timeout 2"
rescue Exception => ex
Rails.logger.debug "EXCEPTION 2 - #{ex.message}"
end
end
end
rescue Timeout::Error
Rails.logger.debug "Timeout"
return nil
rescue Exception => ex
Rails.logger.debug "EXCEPTION - #{ex.message}"
return nil
end
Now, I'm consistently receiving the output EXCEPTION 2 - execution expired. Why is it that the Timeout::Error is not being triggered here?

Answer 1

Your inner
rescue Exception => ex
Rails.logger.debug "EXCEPTION 2 - #{ex.message}"
end
keeps the outer timeoutblock from raising Timeout::Error.
Removing that rescuestatement should do the trick.
If you really need to catch any exception, replace it with:
rescue StandardError => ex
Rails.logger.debug "EXCEPTION 2 - #{ex.message}"
end

Answer 2

Internally (within the Timeout block) Timeout does not use Timeout::Error. If it did, then every garden-variety rescue would catch it, and you don't want that. So it creates a new Exception and uses that, so that it hopefully blows through all normal error handling and actually makes the code stop running.
Check out the timeout.rb code in ruby200/lib/ruby/2.0.0. It's quite short, and pretty informative.
In particular, you can pass your own Exception in as the second parameter of Timeout::timeout, and Timeout will use that. So you can catch it inside your code, if you want.
Note that Logger currently traps all exceptions that happen while writing, and doesn't re-raise, so it breaks Timeout. I've filed a bug report.