Open Stack

On Wed, May 24, 2017 at 04:04:20PM -0700, James E. Blair wrote:
> Hi,
> 
> As part of Zuul v3, we're adding support for GitHub (and later possibly
> other systems).  We want these systems to have access to the full power
> of cross-project-dependencies in the same way as Gerrit.  However, the
> current syntax for the Depends-On footer is currently the
> Gerrit-specific change-id.
> 
> We chose this in an attempt to be future-compatible with some proposed
> changes to Gerrit itself to support cross-project dependencies.  Since
> then, Gerrit has gone in a different direction on this subject, so I no
> longer think we should weigh that very heavily.
> 
> While Gerrit change ids can be used to identify one or more changes
> within a Gerrit installation, there is no comparable identifier on
> GitHub, as pull request numbers are unique only within a project.
> 
> The natural way to identify a GitHub pull request is with its URL.
> 
> This can be used to identify Gerrit changes as well, and will likely be
> well supported by other systems.  Therefore, I propose we support URLs
> as the content of the Depends-On footers for all systems.  E.g.:
> 
>   Depends-On: https://review.openstack.org/12345
>   Depends-On: https://github.com/ansible/ansible/pull/12345
> 
Hopefully not to off-topic, would it also be possible to support the reverse of
this?  I know we've unofficially used the Needed-By footer for some governance
patches. It has been helpful when looking at git logs to see the other direction
dependency from time to time.

Not a big deal if it is a no, just something that popped into my head when
reading this topic.

> Similarly to the Gerrit change IDs, these identifiers are easily
> navigable within Gerrit (and Gertty), so that reviewers can traverse the
> dependency chain easily.
> 
> One substantial aspect of this change is that it is more specific about
> projects and branches.  A single Gerrit change ID can refer to more than
> one branch, and even more than one project.  Zuul interprets this as
> "this change depends on *all* of the changes that match".  Often times
> that is convenient, but sometimes it is not.  Frequently users ask "how
> can I make this depend only on a change to master, not the backport of
> the change to stable?" and the answer is, "you can't".
> 
> URLs have the advantage of allowing users to be specific as to which
> instances of a given change are actually required.  If, indeed, a change
> depends on more than one, of course a user can still add multiple
> Depends-On headers, one for each.
> 
> It is also easy for Zuul connections to determine whether a given URL is
> referring to a change on that system without actually needing to query
> it.  A Zuul connected to several code review systems can easy determine
> which to ask for the change by examining the hostname.
> 
> URLs do have two disadvantages compared to Gerrit change IDs: they can
> not be generated ahead of time, and they are not as easily found in
> offline git history.
> 
> With Gerrit change IDs, we can write several local changes, and before
> pushing them to Gerrit, add Depends-On headers since the change id is
> generated locally.  URLs are not known until the changes are pushed to
> Gerrit (or GitHub pull requests opened).  So in some cases, editing of
> an already existing commit message may be required.  However, the most
> common case of a simple dependency chain can still be easily created by
> pushing one change up at a time.
> 
> Change IDs, by virtue of being in the commit message of the dependent as
> well as depending change, become part of the permanent history of the
> project, no longer tied to the code review system, once they merge.
> This is an important thing to consider for long-running projects.  URLs
> are less suitable for this, since they acquire their context from
> contemporaneous servers.  However, Gerrit does record the review URL in
> git notes, so while it's not as convenient, with some additional tooling
> it should be possible to follow dependency paths with only the git
> history.
> 
> Of course, this is not a change we can make instantaneously -- the
> change IDs have a lot of inertia and developer muscle memory.  And we
> don't want changes that have been in progress for a while to suddenly be
> broken with the switch to v3.  So we will need to support both syntaxes
> for some time.
> 
> We could, indeed, support both syntaxes indefinitely, but I believe it
> would be better to plan on deprecating the Gerrit change ID syntax with
> an eye to eventually removing it.  I think that ultimately, the URL
> syntax for Depends-On is more intuitive to a new user, especially one
> that may end up being exposed to a Zuul which connects to multiple
> systems.  Having a Gerrit change depend on a GitHub pull request (and
> vice versa) will be one of the most powerful features of Zuul v3, and
> the syntax for that should be approachable.
> 
> In short, I think the value of consistency across multiple backends and
> ease of use for new users outweighs the small loss of functionality for
> Gerrit power users in this case.
> 
> I propose we adopt support for URLs in all source drivers in v3, and
> declare Gerrit change IDs deprecated.  We will continue to support both
> for a generous deprecation period (at least 6 months after the initial
> Zuul 3.0 release), and then remove support for them.
> 
> How does that sound?
> 
> -Jim
> 
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