[GSoC] migration

[Joshua Root]

On 2017-6-25 06:29 , Bradley Giesbrecht wrote:
>> On Jun 22, 2017, at 8:15 PM, Joshua Root <jmr at macports.org> wrote:
>>
>> On 2017-6-23 11:29 , Bradley Giesbrecht wrote:
>>> For the migration functionality wouldn’t we only install “requested” ports? Dependencies could be different with a platform change.
>>> Also, if the installed variants for a given port are the default variants would we want to ignore variants?
>>
>> These two points are related. A requested flag for variants is needed to be able to do the right thing.
>>
>> We don't want to ignore variants exactly; we want to apply only the requested variants (positive and negative) when reinstalling ports.
> 
> Is a “default” variant requested? I believe some ports have different default variants depending on the platform, operation system or other environment factors. Shouldn’t we be replicating the actual user install commands as close as possible? Should we include variants the user did not supply to the port install command?

Default variants are unrequested (unless the user also asked for them 
explicitly). The idea is indeed to replicate what the user asked for, 
and allow default variants to be selected as normal after that.

> A snapshot would be a list of install commands that created the current installed state. Restoring a snapshot would deactivate the active ports and reproduce the install commands for the selected snapshot. Migrate would amount to creating a new snapshot, uninstalling installed ports and reproducing the install commands for the last snapshot.

OK, well that's a bit more involved than I expected and relies on 
information that is not currently kept in the registry.

- Josh