Added support for PERL_CPM_MIRROR to set default mirror

[christopherraa]

Adding the environment variable $ENV{PERL_CPM_MIRROR} so that it is possible to set a mirror without invoking cpm directly with --mirror.

A specific usecase for this:

You create a base docker image where you have PERL_CPM_MIRROR set to a sensible default value (other than metacpan.org). Containers based on this image picks that up automatically, so that a developer just running cpm install ... in their Dockerfile will get packages pulled from the correct repository. This is especially useful in environments where you have many images that are based on one base image and you would like to avoid duplication of urls. If the url needs to change you can do that once and not have to go through all Dockerfiles and change them like you would have to if everyone was forced to specify --mirror.

This change is however not limited to the above usecase. It also makes it convenient to do a quick export PERL_CPM_MIRROR=... in your environment and from that point on not have to remember to run cpm with --mirror all the time.

I've made the change in such a way that --mirror always has precedence if specified on the commandline, with a fallback to $ENV[PERL_CPM_MIRROR if set, or lastly https://cpan.metacpan.org as a last resort.

[christopherraa]

Oh, and the tests run fine locally by the way, but your approval @skaji is required to get the Github actions to run.

[skaji]

Thanks for the PR.
I have a plan to deprecate --mirror option, and to introduce --source option.

[christopherraa]

Will --source also be overridable from $ENV? Also do you have an approximate timeline for when --mirror would be deprecated and --source introduced?

[skaji]

I'm not sure yet.

[christopherraa]

Would you be open to accepting this pull request until such time as there is a set timeline for --source? Having this feature would simplify things a great deal.

[skaji]

Just out of curiosity, why do you want to change mirror?

[christopherraa]

Two main reasons:

• The default metacpan mirror is very slow compared to what we can get having a local mirror. Orders of magnitude slower.
• It is convenient to have our internal packages in the same mirror. These are packages that cannot be released to cpan.

[skaji]

OK, thanks.

It is convenient to have our internal packages in the same mirror. These are packages that cannot be released to cpan.

Then I recommend you to use --resolver, not --mirror.

[christopherraa]

Does resolver support being set through an environment variable? I can't seem to see any trace of this in the current code.

The main purpose of this pull request is being able to not specify mirror / resolver on every invocation of cpm. This is especially useful in the scenarios I described initially in this pull request.

For cpanm we have PERL_CPANM_OPT wherein one can specify all options, but I must say I prefer having a greater granularity and that is why I propose using the variable the way I do. I also feel this (using PERL_CPM_MIRROR as a variable) is more in line with your previous use of overriding through environment variables (eg. PERL_CPM_PREBUILT).

To reiterate:

• I am not trying to change resolving with this pull request. Resolving is already working just fine.
• The only issue I am trying to address is to be able to set which mirror to use through the environment

But to change things around: what is your main concern in accepting support for this? And is there anything I can change in the pull request so that your concerns are addressed?

[christopherraa]

Just wanted to follow up on this. Is there no interest in being able to set parameters through environment variables like this merge request enables?

[skaji]

I have a plan to deprecate --mirror option, so I don't want to add a new feature around mirror.

[christopherraa]

I have a plan to deprecate --mirror option, so I don't want to add a new feature around mirror.

Are you open to getting support for letting resolvers be set by env-variables? If so I am more than happy to change this merge request so that mirror is not changed but resolvers get the added functionality.