On Wed, May 24, 2017 at 4:04 PM, James E. Blair <cor...@inaugust.com> 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 > > 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. > > As the main massager of the Github patches, I like this approach. However I haven't used Depends-On much in the Gerrit land, so it is completely green field for us over in Github land. - jlk
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