Help begin/rescue/ensure

[S Kanakakorn]

I have this simple code.

require 'rubygems'
require 'net/ssh'

begin
session = Net::SSH.start('aaaa-lnx', assword=>'xyz',
:username=>'root',:timeout=>5)
rescue
puts 'cannot connect '
ensure
puts 'reach ensure point'
end
puts 'at last'

--------------------------------------

If I put in non existing host name (aaaa-lnx), then the statement
"puts 'at last'" won't print out.
The ensure blocks work fine. Somehow, the "rescue" block can't catch
the error. This is my output.

reach ensure point
/Applications/Locomotive2/Bundles/standardRailsJan2007.locobundle/i386/lib/ruby/1.8/timeout.rb:54:in
`open': execution expired (Timeout::Error)
from /Applications/Locomotive2/Bundles/standardRailsJan2007.locobundle/i386/lib/ruby/gems/1.8/gems/net-ssh-1.0.10/lib/net/ssh/transport/session.rb:88:in
`initialize'

If I put in a good hostname with wrong password, the "rescue" block
will catch the error just fine. Here is the output:

cannot connect
reach ensure point
at last


What did I do wrong ?

Thanks,
-Nick

 

[Brian Candler]

require 'rubygems'
require 'net/ssh'

begin
session = Net::SSH.start('aaaa-lnx', assword=>'xyz',
:username=>'root',:timeout=>5)
rescue
puts 'cannot connect '
ensure
puts 'reach ensure point'
end
puts 'at last' ...
What did I do wrong ?

A bare "rescue" doesn't catch all exceptions, only those which are
subclasses of StandardError. Change to:

rescue Exception

HTH,

Brian.

 

[ChrisH]

I have this simple code.
...
rescue
puts 'cannot connect '
ensure
puts 'reach ensure point'
end
puts 'at last'

--------------------------------------

If I put in non existing host name (aaaa-lnx), then the statement
"puts 'at last'" won't print out.
The ensure blocks work fine. Somehow, the "rescue" block can't catch
the error. This is my output.

reach ensure point
/Applications/Locomotive2/Bundles/standardRailsJan2007.locobundle/i386/lib/ruby/1.8/timeout.rb:54:in
`open': execution expired (Timeout::Error)
from /Applications/Locomotive2/Bundles/standardRailsJan2007.locobundle/i386/lib/ruby/gems/1.8/gems/net-ssh-1.0.10/lib/net/ssh/transport/session.rb:88:in
`initialize'

If I put in a good hostname with wrong password, the "rescue" block
will catch the error just fine. Here is the output:

cannot connect
reach ensure point
at last

What did I do wrong ?

rescue without a specified exception defaults to StandardError.
Tiimeout::Error is a subclass of SignalException, which is a sibling
of StandardError, not a subclass. Thus that exception doesn't match
your rescue, so the exception gets passed up the call stack, the
ensure
gets executed, but the code after it is bypassed.

For a picture of the exception class hierarchy see http://
www.insula.cz/dali/material/rubycl/RubyExceptionClasses.jpg

cheers
Chris

 

[Daniel Berger]

A bare "rescue" doesn't catch all exceptions, only those which are
subclasses of StandardError. Change to:

rescue Exception

Given how often this issue appears on the mailing list, I'm beginning
to think that a bare rescue should just rescue Exception. That seems
to be what most people expect it to do.

Is there a downside to this idea?

Dan

 

[Brian Candler]

Given how often this issue appears on the mailing list, I'm beginning
to think that a bare rescue should just rescue Exception. That seems
to be what most people expect it to do.

Is there a downside to this idea?

Well, it would be very confusing if a rescue intended to catch an
invalid-data sort of error also caught an invalid program error (such as a
"method missing" exception caused by a typo in a method name). At best,
these would end up finding their way into a log somewhere. At worst, they
would be silently ignored, making debugging very difficult.