Puppet users - Oct 2012 - Duplicate class declaration because of counterintuitive class scoping

Hi,

I bumped into the following this afternoon (on a 2.7.19 puppet master/agent
combo):

consider a class profile::tomcat in module profile with the following
content:

$ cat modules/profile/manifests/tomcat.pp
class profile::tomcat {

    class { ''tomcat'': }
    notice(''Class profile::tomcat in module profile'')
}

and

$ cat modules/tomcat/manifests/init.pp
class tomcat {

    notice(''Class tomcat in module tomcat'')
    ...
}

When I include this class profile::tomcat in a node definition, I would
expect it to apply the resources
contained in the main class of the tomcat module (in
modules/tomcat/manifests/init.pp).

Instead I get:

$ puppet apply --modulepath modules --verbose site.pp
Duplicate declaration: Class[Profile::Tomcat] is already declared; cannot
redeclare at /home/bkkrw/Desktop/test/modules/profile/manifests/tomcat.pp:3
on node XXXXXX

If I instead use

class profile::tomcat {

    include tomcat
    notice(''Class profile::tomcat in module profile'')
}

then the catalog compiles successfully, but instead of applying the tomcat
class from modules/tomcat/manifests/init.pp,
puppet applies the profile::tomcat class:

$ puppet apply --modulepath modules --verbose site.pp
notice: Scope(Class[Profile::Tomcat]): Class profile::tomcat in module
profile
info: Applying configuration version ''1350936327''
notice: Finished catalog run in 0.04 seconds

So, apparently the tomcat class included in class profile::tomcat seems to
resolve to the profile::tomcat class itself
instead of the class defined in modules/tomcat/manifests/init.pp.

I can force the behaviour I want by using:

class { ''::tomcat'':}

or

include ::tomcat

$ puppet apply --modulepath modules --verbose site.pp
notice: Scope(Class[Tomcat]): Class tomcat in module tomcat
notice: Scope(Class[Profile::Tomcat]): Class profile::tomcat in module
profile
info: Applying configuration version ''1350936684''
notice: Finished catalog run in 0.05 seconds

I haven''t come across this syntax for class declaration before, and
although it seems to work, it is at least
quite counterintuitive, and possibly a bug.
Everything I find seems to confirm that a declaration like "include
tomcat"
should just apply the resources
contained in the main class in module tomcat (in manifests/init.pp).

What do you guys think ?

Kind regards,

k

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On Monday, October 22, 2012 3:21:57 PM UTC-5, Kristof Willaert
wrote:
>
> Hi,
>
> I bumped into the following this afternoon (on a 2.7.19 puppet 
> master/agent combo):
>
> consider a class profile::tomcat in module profile with the following 
> content:
>
> $ cat modules/profile/manifests/tomcat.pp 
> class profile::tomcat {
>
>     class { ''tomcat'': }
>     notice(''Class profile::tomcat in module profile'')
> }
>
> and
>
> $ cat modules/tomcat/manifests/init.pp
> class tomcat {
>
>     notice(''Class tomcat in module tomcat'')
>     ...
> }
>
> When I include this class profile::tomcat in a node definition, I would 
> expect it to apply the resources
> contained in the main class of the tomcat module (in 
> modules/tomcat/manifests/init.pp).
>
> Instead I get: 
>
> $ puppet apply --modulepath modules --verbose site.pp 
> Duplicate declaration: Class[Profile::Tomcat] is already declared; cannot 
> redeclare at /home/bkkrw/Desktop/test/modules/profile/manifests/tomcat.pp:3
> on node XXXXXX
>
> If I instead use 
>
> class profile::tomcat {
>
>     include tomcat
>     notice(''Class profile::tomcat in module profile'')
> }
>
> then the catalog compiles successfully, but instead of applying the tomcat 
> class from modules/tomcat/manifests/init.pp,
> puppet applies the profile::tomcat class:
>
> $ puppet apply --modulepath modules --verbose site.pp 
> notice: Scope(Class[Profile::Tomcat]): Class profile::tomcat in module 
> profile
> info: Applying configuration version ''1350936327''
> notice: Finished catalog run in 0.04 seconds
>
> So, apparently the tomcat class included in class profile::tomcat seems to 
> resolve to the profile::tomcat class itself
> instead of the class defined in modules/tomcat/manifests/init.pp.
>
> I can force the behaviour I want by using:
>
> class { ''::tomcat'':} 
>
> or
>
> include ::tomcat
>
> $ puppet apply --modulepath modules --verbose site.pp 
> notice: Scope(Class[Tomcat]): Class tomcat in module tomcat
> notice: Scope(Class[Profile::Tomcat]): Class profile::tomcat in module 
> profile
> info: Applying configuration version ''1350936684''
> notice: Finished catalog run in 0.05 seconds
>
> I haven''t come across this syntax for class declaration before,
and
> although it seems to work, it is at least
> quite counterintuitive, and possibly a bug.
> Everything I find seems to confirm that a declaration like "include 
> tomcat" should just apply the resources
> contained in the main class in module tomcat (in manifests/init.pp).
>
> What do you guys think ?
>
>
I think the behavior you discovered is a natural and expected consequence 
of Puppet''s name scoping and resolution rules. The current namespace is
searched first for unqualified class names.  Only if no matching class is 
found there will other namespaces be considered.

The difference between the behavior with parametrized-class syntax and 
''include'' for declaring the tomcat class is also normal.  It
turned out
well in your case by alerting you to the problem, but usually it is a 
problem that you need to work around.  It is only a bug to declare a class 
multiple times when the second or some later declaration is made in the 
parametrized style, and then only because of the possibility of 
inconsistent parametrization.  You can ''include'' a class as
many times as
you want.

Finally, although it is usually unnecessary to use absolute class names 
(e.g. "::tomcat") to declare classes, that''s because name
clashes are
fairly rare.  Although you found Puppet''s behavior surprising, I
wouldn''t
call it buggy.


John

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Hi John,

I think the behavior you discovered is a natural and expected
consequence
> of Puppet''s name scoping and resolution rules. The current
namespace is
> searched first for unqualified class names.  Only if no matching class is
> found there will other namespaces be considered.
>
> The difference between the behavior with parametrized-class syntax and
> ''include'' for declaring the tomcat class is also normal. 
It turned out
> well in your case by alerting you to the problem, but usually it is a
> problem that you need to work around.  It is only a bug to declare a class
> multiple times when the second or some later declaration is made in the
> parametrized style, and then only because of the possibility of
> inconsistent parametrization.  You can ''include'' a class
as many times as
> you want.
>
I understand the difference between the parameterized style and the include
syntax. That is something that is explained thoroughly in the documentation.

> Finally, although it is usually unnecessary to use absolute class names
> (e.g. "::tomcat") to declare classes, that''s because
name clashes are
> fairly rare.  Although you found Puppet''s behavior surprising, I
wouldn''t
> call it buggy.
>
What I found surprising, was the fact that the "include tomcat" or
"class {
''tomcat'': }" syntax is documented everywhere as the
declaration of the main
class of a module, without any mention of it possibly being
"overridden" by
this kind of namespace clash. For variables it seems to be well documented
that we should use namespacing and top-scoping to avoid these clashes.

I used a similar setup to what is described by Craig Dunn (now a Puppetlabs
employee) at http://www.craigdunn.org/2012/05/239/, which happens to have
the exact same invocation as I did:

class profile::tomcat {
  class { "jdk": }
  class { "tomcat": }
}

As this is a blog post, it probably wasn''t tested, but it shows there
are
others who find this kind of invocation intuitive.

While it is easy to work around this by top-scoping the class, some form of
documented warning might prevent some hairpulling :)

Kind regards,

k

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[snip]

While it is easy to work around this by top-scoping the class, some form
of
> documented warning might prevent some hairpulling :)
>
Mostly for posterity, this behaviour is indeed considered buggy ("a major
outstanding design issue") and is in fact documented:

*
http://docs.puppetlabs.com/puppet/2.7/reference/lang_namespaces.html#relative-name-lookup-and-incorrect-name-resolution
* https://projects.puppetlabs.com/issues/2053

Kind regards,

k

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