[swift-build-dev] Proposal: Git Branch and ref support for dependencies in Swift Package Manager

[Ankit Agarwal]

Hi,

Here is a proposal of the adding git branch support feature in SPM

*Introduction*

Pointing to branch or a commit ref for dependencies in Package.swift as
opposed to only a tagged release.

*Motivation*

* Try a package which is almost stable or useable but not yet ready for a
release/pre-release so not tagged (eg: new feature being introduced by a
library)
* While developing packages, one would want to point a package that uses
the package to a develop branch (eg: Developing Foo package, Bar uses Foo
and wants to point Foo dep to develop branch)
* One would want to point to his own fork but not create a release while
developing/testing (eg: Fork a library not compatible with SPM to make it
compatible)
* One wants to point to some commit but doesn't have a branch/tag created
for that

*Proposed solution*

Allow refs and branch in Package.swift

let package = Package(
    name: "Hello",
    dependencies: [
        .Package(url: "ssh://git@example.com/Greeter.git", branch:
"develop", shouldFastForward: true),
        .Package(url: "ssh://git@example.com/FooBar.git", commit:
"d8ec7ca398a3ac3990477028117384d05ca7734e"),
    ]
)

*Detailed design*

* Only the root Package.swift would be able to use branch/ref feature to
avoid dependency hell, any other dependency fetched in current Package
should not compile if that dependency contains another dependency pointing
to a branch/ref
* This feature should strictly be used for testing/developing purpose and
should not be deployed to production environments

SPM could have the following behavior when running `swift build` :

* If pointed to a branch, there might be two use cases
Since there is a high probability that user wants to point a branch due to
active development of that dep and wants latest ref available in that branch
If a dependency is not cloned, clone it and checkout that branch
If shouldFastForward is on -> Always try to be on the latest ref, disregard
any local changes made to the checked out package
If shouldFastForward is false -> Always try to be on the latest ref unless
any local changes made to the checked out package

* If pointed to a ref :
If that dependency is not cloned, clone it and checkout that ref.
Consecutive `swift build` will not affect the cloned package
If changes are made in the cloned repo, rebuild that package with those
changes

*Impact on existing code*

None as this will be a new functionality

*Alternatives considered*

One option is to only allow a commit ref and not a branch so SPM will not
have to worry about fast forwarding but this is a desired feature.

On Tue, Dec 8, 2015 at 4:24 AM, Rick Ballard <rballard at apple.com> wrote:

> > On Dec 5, 2015, at 5:59 AM, Ankit Agarwal <ankit at ankit.im> wrote:
> >
> > Hi,
> >
> > Is pointing to a branch instead of version for a package in scope of SPM?
> > if it is, I'd love to try to implement it
>
> Hi Ankit,
>
> This is in scope, though not yet designed. Prior to anyone working on an
> implementation, we should agree on a design for how you'd do this. While
> this isn't at the top of our priority list at the moment, we'd welcome both
> design contributions and eventual implementation.
>
> If you'd like to put a proposal together for this, please see the Swift
> evolution process at
> https://github.com/apple/swift-evolution/blob/master/process.md. We'd be
> happy to discuss this here as part of your process for putting a proposal
> together. Some things to think about in this area are:
>
> – How should refs (branches or tags) that aren't simple version numbers be
> specified?
>
> – Right now we require you to tag something as a versioned "release".
> Should we require that you tag a branch before someone can make a package
> depend on it? It could be convenient to be able to just depend on a branch,
> but the meaning of depending on a branch changes over time as more commits
> come in. Is it harmful to allow packages to depend on something that's not
> an identified commit?
>
>         – Note that we have yet to design our security story (
> https://github.com/apple/swift-package-manager/blob/master/Documentation/PackageManagerCommunityProposal.md#security-and-signing);
> what we settle on there might require dependencies to be specified as a
> specific tagged commit, so that it can be signed.
>
> – Should it be possible to override a package's dependency to use a
> different branch, without having to modify and commit a change to that
> package's Package.swift?
>
> – We may want to design a way for packages to support different versions
> of the Swift language, as the language continues to change – e.g. a branch
> of the package for the last released swift vs the current under development
> swift snapshot. Is supporting dependencies on package branches a part of
> how we'll do that?
>
> Thanks,
>
>         - Rick
>
>


-- 
Ankit
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