This adds the concept of a 'scheme' to the merger. Up to this point,
the merger has used the 'golang' scheme in all cases. However it is
possible with Gerrit to create a set of git repositories which collide
with each other using that scheme:
root/example.com/component
root/example.com/component/subcomponent
The users which brought this to our attention intend to use their repos
in a flat layout, like:
root/component
root/subcomponent
To resolve this we need to do two things: avoid collisions in all cases
in the internal git repo caches of the mergers and executors, and give
users options to resolve collisions in workspace checkouts.
In this change, mergers are updated to support three schemes:
* golang (the current behavior)
* flat (new behavior described above)
* unique
The unique scheme is not intended to be user-visible. It produces a
truly unique and non-conflicting name by using urllib.quote_plus. It
sacrifices legibility in order to obtain uniqueness.
The mergers and executors are updated to use the unique scheme in their
internal repo caches.
A new job attribute, 'workspace-scheme' is added to allow the user to
select between 'golang' and 'flat' when Zuul prepares the repos for
checkout.
There is one more kind of repo that Zuul prepares: the playbook repo.
Each project that supplies a playbook to a job gets a copy of its repo
checked out into a dedicated directory (with no sibling repos). In that
case there is no risk of collision, and so we retain the current behavior
of using the golang scheme for these checkouts. This allows the playbook
paths to continue to be self-explanatory. For example:
trusted/project_0/example.com/org/project/playbooks/run.yaml
Documentation and a release note are added as well.
Change-Id: I3fa1fd3c04626bfb7159aefce0f4dcb10bbaf5d9
b9a6190a45