[chef] Re: The dangers of prepend

[Lamont Granquist < >]

Yeah you are monkey patching core chef resources.  That is alway bad 
practice, you aren't adhering to the public APIs of those classes and we 
can make no guarantees that we won't break you in minor or patch 
releases of core chef.

A slightly better approach would be subclassing, but in ruby there's no 
private methods which are inaccessible to the subclass, so its still 
dependent upon you to only use the public APIs (and we don't necessarily 
do the best job, or sometimes any job, of documenting the public APIs 
correctly and expect you to be able to read the code and infer what the 
public APIs are).  A better solution is to use composition and use the 
resources as they were intended.  There's retry logic around the core 
resource class which you could be using instead of monkeypatching the 
action in the resource.  If that doesn't get you what you need, you 
should really submit a PR to add a feature so that you have a public API 
to use.

In general cookbook code should never, ever under any circumstances open 
up core chef classes and hack them (although a few use cases like 
including modules into Chef::Recipe are supported idioms, but if its not 
one of those idioms it is probably bad).  It is also likely that 
subclassing core chef providers  is the wrong approach as well (that can 
be used, but I'd say it requires expert level understanding of the 
inheritance-vs-composition tradeoff).

On 08/25/2015 11:11 AM, Yoshi Spendiff wrote:
> Hi,
>
> As an FYI to anyone that uses the varnish cookbook >= 2.0.0, an update 
> was just released (2.2.1) that fixes a bug in the service resource and 
> is probably worth updating to sooner rather than later.
>
> One of the libraries in the varnish cookbook used a prepend statement 
> to altered the behaviour of the Provider::Service::Init and 
> Provider::Service::Systemd classes, resulting in service resource 
> declarations attempting starts/restarts 5 times and failing silently. 
> Because it was in a loaded library simply including the cookbook in a 
> dependency chain would cause this.
> This was a bug (it was only supposed to affect services of a certain 
> name) but I think it's a pretty clear demonstration of the dangers of 
> using prepend on core Chef libraries.
>
> Perhaps some sort of a warning should be issue if a prepend is used, 
> as the user may be signing up for something they aren't aware of.
>
> --
> Yoshi Spendiff
> Ops Engineer
> Indochino
> Mobile: +1 778 952 2025
> Email: 
>
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