[RSpec] Cloning objects and leaking state


#1

G’day folks. I’ve been beating my head on this one problem for a
couple of hours, and have managed to figure out what’s causing it.
However, I don’t understand why it’s happening, nor do I know how to
solve or get around it.

One of my methods clones an arg, and it seems that doing so causes
state to leak out. I discovered this because changing this:
new_subtitle = subtitle.clone
to this:
new_subtitle = subtitle
causes the problem to disappear.

Before we get into the code snippets, are there any known caveats,
warnings, or problems with writing specs that cover the cloning or
duplication of objects?

Here’s the code, specs, and spec output:
http://gist.github.com/53482

As you can see, in the “should clone …” example, the expectation on
line 24 succeeds, but the same expectation on line 28 fails. After
that, the same expectation fails in the “should not leak state” example.

Any idea what’s going on? I’d appreciate any help at all. Thanks,
Nick


#2

On Tue, Jan 27, 2009 at 1:48 PM, Nick H. removed_email_address@domain.invalid
wrote:

Before we get into the code snippets, are there any known caveats, warnings,
or problems with writing specs that cover the cloning or duplication of
objects?

Here’s the code, specs, and spec output:
http://gist.github.com/53482

As you can see, in the “should clone …” example, the expectation on line
24 succeeds, but the same expectation on line 28 fails. After that, the same
expectation fails in the “should not leak state” example.

Would you please try doing the same thing without using the clone
method specifically. i.e. change the method name in the spec to :copy
and then call that in the subject code as well and see if you come up
w/ different results. I want to make sure this is really about the
clone method itself.


#3

On 27/01/2009, at 7:44 PM, David C. wrote:

state to
objects?
Would you please try doing the same thing without using the clone
method specifically. i.e. change the method name in the spec to :copy
and then call that in the subject code as well and see if you come up
w/ different results. I want to make sure this is really about the
clone method itself.

I removed the call to #clone from the instance method, and lo and
behold, the problem persists:
http://tinyurl.com/cl8rvy
I don’t know why, because when I posted the first email in this
thread, this seemed to resolve it.

I also tried replacing #clone with #dup, and the problem still occurs:
http://tinyurl.com/dzuvrd

Now I’m even more flummoxed. If the #clone isn’t causing this problem,
what is?

Can I do anything else to help others help me with this?

Thanks, guys!
Nick


#4

On Tue, Jan 27, 2009 at 2:48 PM, Nick H. removed_email_address@domain.invalid
wrote:

Before we get into the code snippets, are there any known caveats, warnings,
or problems with writing specs that cover the cloning or duplication of
objects?

Here’s the code, specs, and spec output:
http://gist.github.com/53482

As you can see, in the “should clone …” example, the expectation on line
24 succeeds, but the same expectation on line 28 fails. After that, the same
expectation fails in the “should not leak state” example.

What does your full SubtitleFile class look like? It looks like there
is an issue with how the #subtitles method is storing information.

Since you’re using ActiveRecord::BaseWithoutTable there are probably
not going to be any unique primary keys amongst your SubtitleFiles to
help differentiate them. So if #subtitles is a has_many, every call to
SubtitleFile#subtitles may lookup Subtitles in the exact same way w/o
any unique key to use to differentiate which subtitles belong to which
subtitle files which would produce the results you are seeing.

Is Subtitle a ActiveRecord::BaseWithoutTable or a real
ActiveRecord::Base model?

Any idea what’s going on? I’d appreciate any help at all. Thanks,
Nick


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Zach D.
http://www.continuousthinking.com
http://www.mutuallyhuman.com


#5

On Wed, Jan 28, 2009 at 8:30 AM, Nick H. removed_email_address@domain.invalid
wrote:

One of my methods clones an arg, and it seems that doing so causes state
objects?
What does your full SubtitleFile class look like? It looks like there
model?
ActiveRecord::BaseWithoutTable models, and there weren’t any mixups after
creating associations between different model instances.

Here’s the source for the SubtitleFile model. The #add_subtitle! in question
starts on line 74:
http://gist.github.com/54028

I’m not familiar with this library, so I can’t say for sure, but the
line
default :subtitles => []
looks very suspicious to me.

The underlying library code would have to do a .clone of that default
value, otherwise all instances will share the same array (meaning if
you push a value onto it, the next instance will have that array with
one element).

If it doesn’t automatically clone (it may, I don’t know), it probably
should provide a lambda syntax so you can do:
default :subtitles => lambda { [] }

Pat


#6

On 28/01/2009, at 7:50 AM, Zach D. wrote:

state to
objects?
What does your full SubtitleFile class look like? It looks like there
ActiveRecord::Base model?
Hi Zach. Subtitle is an ActiveRecord::BaseWithoutTable model; it does
not inherit from ActiveRecord::Base .

The SubtitleFile “subtitles” attribute is actually just an Array. I
didn’t bother to setup a has_many relationship with Subtitle.
SubtitleFile#subtitles is a standard getter method that ActiveRecord
sets up for me.

In my app’s console, I tested out a has_many relationship between two
ActiveRecord::BaseWithoutTable models, and there weren’t any mixups
after creating associations between different model instances.

Here’s the source for the SubtitleFile model. The #add_subtitle! in
question starts on line 74:
http://gist.github.com/54028

Thanks, mate.
Nick


#7

On 28/01/2009, at 12:12 PM, Pat M. wrote:

hours, and have managed to figure out what’s causing it. However,
to this:
http://gist.github.com/53482
there
which
The SubtitleFile “subtitles” attribute is actually just an Array. I

The underlying library code would have to do a .clone of that default
value, otherwise all instances will share the same array (meaning if
you push a value onto it, the next instance will have that array with
one element).

If it doesn’t automatically clone (it may, I don’t know), it probably
should provide a lambda syntax so you can do:
default :subtitles => lambda { [] }

Pat

Pat, the lambda fixed the problem. That’s a keen spidey sense you have
there!

To spell this out in more detail, having
defaults :subtitles => []
causes the SubtitleFile class to initialise an Array when the class is
defined. From then on, the “subtitles” attribute of all SubtitleFile
instances will refer to that one specific array.

Thanks for your help with this, mate. It’s much appreciated.
Nick


#8

On 28/01/2009, at 12:16 PM, Pat M. wrote:

If it doesn’t automatically clone (it may, I don’t know), it probably
should provide a lambda syntax so you can do:
default :subtitles => lambda { [] }

Yeah, I just looked at the code for ARD and this is the problem. You
can pass in immutable objects and they don’t get affected for obvious
reasons. But with a mutable object like an array, you need to pass it
a lambda so that it’s creating a new instance each time.

Pat

That makes a lot of sense.

The active_record_defaults README[1] shows strings being used for
default values. The problem that I experienced in this thread will
plague the README’s example if a non-assignment operation is performed
on the attribute.

[1]
http://svn.viney.net.nz/things/rails/plugins/active_record_defaults/README

Thanks again, Pat, David, and Zach!
Nick


#9

If it doesn’t automatically clone (it may, I don’t know), it probably
should provide a lambda syntax so you can do:
default :subtitles => lambda { [] }

Yeah, I just looked at the code for ARD and this is the problem. You
can pass in immutable objects and they don’t get affected for obvious
reasons. But with a mutable object like an array, you need to pass it
a lambda so that it’s creating a new instance each time.

Pat