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Excerpts from corvus's message of 2017-10-24 15:22:51 -0700:
> Hi,
> 
> A number of issues related to how jobs are defined and run on projects
> with stable branches have come up recently.  I believe they are all
> related, and they, as well as the solutions to them, must be considered
> together.
> 
> 
> The Problems
> ============
> 
> I've identified the following five problems:
> 
> 1) Parents should have variants applied
> 
> Currently inheritance and variance are distinct.  Inheritance modifies a
> job's configuration statically at the time the configuration is loaded.
> Variants are applied dynamically right before the job runs.
> 
> That means that when a job starts, we pick a single job to start with,
> then apply variants of that particular job.  But since the inheritance
> has already been applied, any variants which a user may expect to apply
> to a parent job will not be applied.  When the inheritance chain is
> created at configuration time, only the "reference" definition of the
> job is used -- that is, the first appearance of the job.
> 
> In other words, more than likely, most jobs defined in a stable branch
> are going to inherit from a parent job defined on the master branch --
> even if that parent job has a stable branch variant.
> 
> 2) Variants may cause parents to be ignored
> 
> We currently ignore the parent attribute on a job variant.  If we did
> not, then when the variant is applied, it would pull in all of the
> attributes of its parent, which is likely to be on the master branch
> (since its parent will, as in #1, be the reference definition).
> 
> This ignoring of the parent attribute happens at configuration time
> (naturally, since that is when inheritance is applied).
> 
> This means that if the first job that matches a change is a variant
> (i.e., the reference definition of a job has a non-matching branch
> matcher), this top-level variant job will not actually have a parent.

In the discussion yesterday, and in the emails today, you've implied
that there is an ordering to job definitions beyond inheritance. Is that
discovery order documented somewhere? If not, is it simple enough to
describe in a few sentences here? Are repositories scanned in a
particular order, for example? Or is it based on something else?

Doug

> 
> 3) Variants may add duplicate pre/post playbooks
> 
> Currently, the master branch does not have an implied branch matcher, so
> jobs that exist on master and stable branches will generally be derived
> from both.
> 
> If such a job adds a pre or post playbook, the job that is ultimately
> created and run for a change on the stable branch may have those added
> by both the variant defined on the master branch as well as that defined
> on the stable branch (since pre and post playbooks are cumulative).
> 
> 4) Variants on branches without explicit playbooks should use branch
>    playbooks
> 
> Whenever a job includes a pre-run, run (including the implicit run), or
> post-run playbook, Zuul remembers where and uses that branch of that
> repo to run that playbook.  If a job were constructed from the master
> branch, and then had a stable branch variant applied but did not repeat
> the pre-run, run, or post-run attributes from the master, then Zuul
> would end up attempting to run the playbook from the master branch
> rather than the stable.
> 
> 5) The master branch should have implied branch matchers
> 
> Currently jobs defined in an untrusted project on any branch other than
> 'master' have an implicit branch matcher applied to them.  This is what
> allows the version of a job in a stable branch to only affect the stable
> branch.  The fact that there is no implicit branch matcher applied to
> the master branch is what allows jobs defined in zuul-jobs to run on
> changes to any branch.
> 
> However, this also means that jobs on stable branches are frequently
> built from variants on both the master and stable branch.  This may work
> for a short while, but will fail as soon as someone wants to add
> something to the master branch which should not exist on the stable
> branch (e.g., enabling a new service by default).
> 
> 
> The Design Considerations
> =========================
> 
> In looking at these, I believe they have come about because of two
> design goals which we did not appreciate were in moderate tension with
> each other:
> 
> A) In-repo configuration for a project should apply to that branch.
> Changing the behavior of a job on a stable branch should merely involve
> changing the configuration or playbook in that stable branch.  When a
> project branches master to create a new stable branch, both branches
> should initially have the same content and behavior, but then evolve
> independently.
> 
> B) We should be able to define jobs not only in a central repository
> such as project-config, but a central *untrusted* repository such as
> zuul-jobs or openstack-zuul-jobs.
> 
> I think these are valid and important to keep in mind as we consider
> solutions.
> 
> 
> The Changes
> ===========
> 
> I believe the following changes will address all five problems and
> achieve both design goals:
> 
> a) Apply inheritance at the same time as variance
> 
> Rather than applying inheritance at configuration time, apply it at the
> time the job is frozen before being run.  We can perform a depth-first
> traversal up the hierarchy of parents, applying all of the matching
> variants at each level as we return down.  With the following graph
> (where lines indicate parent relationships):
> 
>         0 base
>          /    \
> 1 devstack  2 devstack  4 altbase
>          \    /             |
>        3 tempest        5 tempest
>           /  \
>      6 foo  7 foo
> 
> Would have the following jobs applied in order:
> 
> 0 base, 1 devstack, 2 devstack, 3 tempest, 4 altbase,
> 5 tempest, 6 foo, 7 foo
> 
> b) Add an implicit branch matcher to the master branch
> 
> Generally this will add clarity to projects with multiple branches,
> however, if we always add the an implicit branch matcher, then it makes
> it difficult to use repos like zuul-jobs to define jobs that run
> everywhere.  So do this only if the project has multiple branches.  If
> the project only has a single branch, omit the implicit branch matcher.
> 
> c) Add a config option to disable the implicit branch matcher
> 
> There are some times when an implicit branch matcher on master may be
> undesirable.  For example when tempest was becoming branchless, it had
> multiple branches, but we would have wanted the jobs defined on master
> to be applicable everywhere.  Or if, for some reason, we wanted to
> create a feature branch on zuul-jobs.  For these cases, it's necessary
> to have an option to disable the implicit branch matcher.  We can add a
> new kind of configuration object to zuul.yaml, for example:
> 
>   - meta:
>       implicit-branch-matcher: False
> 
> Which would be intended only to apply to the current branch.  Or we
> could add an option to the tenant config file, so the admin can indicate
> that a certain project should not have an implicit branch matcher
> applied to certain branches.
> 
> 
> The Solutions
> =============
> 
> Reconsidering the problems above with those three changes:
> 
> 1) Parents should have variants applied
> 
> Change (a) is a straightforward solution to this problem.
> 
> Note that simply applying change (a) to our current configuration is
> likely to, in some cases, bring in new parents on the master branch
> which are not intended to apply to stable branches, and so change (a) is
> best made with change (b) at the same time to reconcile this.
> 
> 2) Variants may cause parents to be ignored
> 
> Change (a) means that it is safe to honor the parent attribute in all
> variants.  Typically, all variants will have the same parent, and the
> resulting job application will be straightforward.  In the example in
> change (a), that would mean base, devstack, tempest, foo.  However, if a
> variant did have a different parent, (e.g., tempest -> altbase in the
> example), that case is handled as well.
> 
> In all cases, we will either be honoring the parent specified by the
> user, or defaulting to the tenant-configured default base job.
> 
> 3) Variants may add duplicate pre/post playbooks
> 
> Change (b) means that variants on the master branch will no longer (by
> default) apply to variants on stable branches.  This is the most likely
> route for duplicate pre/post playbooks to occur.  They can still occur
> if a user explicitly adds them in two matching variants, however, this
> will no longer automatically happen when a project creates a new branch,
> which is the main concern.
> 
> 4) Variants on branches without explicit playbooks should use branch
>    playbooks
> 
> The main reason for a variant not to repeat the playbook is in an
> attempt to solve problem (3).  With that solved by (b), there should be
> little reason not to have explicit playbook attributes set on the job in
> all branches.  By doing so, Zuul will use the appropriate
> branch-specific playbook.
> 
> 5) The master branch should have implied branch matchers
> 
> Changes (b) and (c) are straightforward solutions to this problem.
> 
> 
> The Bonus Change
> ================
> 
> (d) Remove the implicit run playbook
> 
> This is not required to solve any of the problems stated, however, it
> does make the solution to problem (4) even more explicit.
> 
> Moreover, despite being an initial proponent of the implicit playbook, I
> have found that in practice, we have so many jobs that do not have
> playbooks at all (i.e., we're making heavy use of inheritance and
> variance) that it is becoming difficult to determine where to look for a
> job's run playbook.  Declaring the run playbook explicitly will help
> with discoverability.
> 
> 
> The Proposal
> ============
> 
> I have been experimenting with late-binding inheritance (i.e., the
> change described as (a)) in [1].  Change (b) was also required[2] in
> order to reconcile all of the tests (as described in the solution to
> problem (1)).
> 
> I have worked through these changes enough to feel comfortable that it
> is a workable solution that addresses the issues without bringing new
> problems to the table.
> 
> I think we should implement changes a-d as soon as possible -- ideally
> before additional use of in-repo stable branch jobs cause more issues in
> production.  At this stage, I don't think we need to make any allowances
> for backwards compatibility -- the way jobs are constructed and being
> ported into projects and across branches is largely compatible with
> these changes already.  We would just be catching the implementation up
> to the implicit understanding.
> 
> We do need a change to the infra-manual Zuul v3 migration page to
> indicate that not only job definitions, but playbooks and roles should
> appear in all branches of in-repo configuration.
> 
> I hope I have articulated the problems sufficiently to facilitate
> discussion.  I'm happy to discuss them more in depth, either via email
> or IRC.  Of course, these may not be the only solutions to these
> problems.  If you see other possible solutions, I'm happy to discuss
> them as well.
> 
> Thanks,
> 
> Jim
> 
> [1] https://review.openstack.org/511352
> [2] https://review.openstack.org/514459
>