[swift-build-dev] [swift-evolution] Proposal Discussion Thread: SwiftPM: Locking and Overriding Dependencies

[Honza Dvorsky]

Very happy this is coming! I really like the details of this proposal as
well.

Just one thing - having the `VersionLocks` in `Packages/VersionLocks` seems
like a bad idea to me. Couple of hurdles right away
- for people who don't want to check in their dependencies, we can't just
add `Packages` to our `.gitignore`, because that would make it impossible
to check in the `VersionLocks` file (users would need to add exceptions to
ignore Packages but keep the lockfile, which might result on users
accidentally not committing their lockfile at all, which would defeat its
whole purpose).
- `swift build --clean=dist` deletes the whole `Packages` directory,
removing the lockfile, while I can imagine just wanting to delete and
re-pull my dependencies with `swift build --clean=dist; swift build`. This
will potentially generate a new lockfile, even though there was a
completely valid one which I didn't explicitly say I want to overwrite.
Again, we can add code to work around it, but needing to add so much
exception code is a code smell to me.

The two points above are just the low-hanging fruit that came into my mind.
I prefer the approach CocoaPods takes with `Podfile.lock` being next to the
repo manifest, instead of in the `Pods` folder (which instead contains a
`Manifest.lock` AFAIK). So does Rubygems and others.

For me, a regular troubleshooting step is removing the `Packages` and
`.build` folders and rebuilding. In my opinion, this step should **not**
include the risk of altering the locked dependency graph, because both
folders are just "Derived Data", however the VersionLocks file is a source
of truth.

And mixing source of truth with generated files feels very wrong to me.

I'm happy to be proven wrong, but I'd suggest to move the location of the
lockfile next to `Package.swift`. Having it in `Packages` causes a couple
of issues right away without bringing any tangible benefits as far as I can
see (not to mention diverging from other package managers like CocoaPods
and Rubygems, which all keep the lockfile next to their manifest files -
and seem to be happy with it for years).

Honza



On Fri, Mar 18, 2016 at 9:45 AM Ben Rimmington via swift-evolution <
swift-evolution at swift.org> wrote:

> <
> https://github.com/apple/swift-package-manager/blob/master/Documentation/Internals/SwiftBasedManifestFormat.md#discussion
> >
>
> We decided to use a Swift-based format for the manifest because we believe
> it gives developers the best experience for working with and describing
> their project. The primary alternative we considered was to use a
> declarative format encoded in a common data format like JSON. Although that
> would simplify implementation of the tooling around the manifest, it has
> the downside that users must then learn this additional language, and the
> development of high quality tools for that (documentation, syntax coloring,
> parsing diagnostics) isn't aligned with our goal of building great tools
> for Swift. In contrast, using the Swift language means that we can leverage
> all of the work on Swift to make those tools great.
>
>
> Could you generate a similar Swift-based format for the lockfile?
>
> -- Ben
>
> On 17 Mar 2016, at 18:23, Max Howell via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> The following is a draft proposal, feedback welcome.
>
> ____________
> SwiftPM Dependency Version Locking
>
>    - Proposal: SE-NNNN
>    <https://github.com/apple/swift-evolution/blob/master/proposals/NNNN-swiftpm-dependency-lockfiles.md>
>    - Author(s): Ankit Agarwal <https://github.com/aciidb0mb3r>, Max Howell
>    <https://github.com/mxcl>
>    - Status: *Discussion*
>    - Review manager: Rick Ballard
>
> Introduction
>
> This proposal seeks to declare a new, generated file
> Packages/VersionLocks.json that describes the exact state of a package’s
> dependency graph and then by default will be respected when executing most
> package manager commands. Thus it is considered a “version lock” for a
> package’s dependency sources.
>
> Swift-evolution thread
> <https://lists.swift.org/pipermail/swift-build-dev/Week-of-Mon-20151214/000067.html>
> Terminology
>
>    - A *package* refers to a published, versioned git repository designed
>    to be consumed as a dependency by SwiftPM.
>    - A *project* refers to an end-user workspace that uses SwiftPM (via a
>    Package.swift and swift build) fetching and building packages as part
>    of its build
>
> Describing this distinction is required because both the above have the
> same form, but are used differently by an end-user. An end-user may publish
> packages, but will eventually consume those packages in a project.
>
> As justification for this confusion, it is considered a feature that
> projects can easily and trivially become packages when using SwiftPM.
> Encouraging a vibrant packaging ecosystem is one of our goals.
> Motivation
>
> In a vibrant packaging ecosystem, dependencies update continuously with
> bug-fixes and new features. A development team needs:
>
>    1. To ensure they are all using the same versions of their
>    dependencies for any given version-control commit.
>    2. Ensure they are all using the same versions of the underlying Swift
>    toolchain
>    3. Be able to override or modify dependency specifications for the
>    whole team for specific commits.
>
> Currently with SwiftPM it is possible to fulfill *1.* by committing the
> sources of a package’s dependencies with the package itself, but this is
> not always desirable. There is no way to achieve 2. and 3. with SwiftPM
> alone.
>
> Additionally, there is not currently a way to know which version of Swift
> a package requires to build. At this time this situation is particularly
> precarious because Swift itself is not backwards compatible. As a Swift
> developer at the very least recording which Swift version a package was
> built with by the package developer is essential information in order to
> assess a package's suitability. Practically the package manager could in
> the future use this information to aid an end-user or even fix the problem
> when packages fail to compile.
> Proposed Solution
>
> A file: Packages/VersionLocks.json will be created alongside the
> Package.swift file. Its contents will describe:
>
>    - The URL and versions of cloned dependencies
>    - An inline diff of any *local* modifications made to those packages
>    relative to their pristine cloned states
>    - The exact version of the Swift toolchain used as part of the last
>    successful build of the package
>
> This file is generated by SwiftPM.
>
> This file *should* be checked-in with projects.
>
> This file is *generated* and should not be edited by users. If the file
> is edited by users the behavior is undefined.
>
> This file *should* be checked-in with packages designed for consumption
> in projects, *however* SwiftPM will not use the checkout files of
> dependencies when determining a project’s dependency graph (this would make
> dependency graphs much less likely to resolve due to overly strict
> versioning requirements). In the future we may choose to make it possible
> for end-users to attempt to build a package using all checkout files since
> in certain deployment scenarios where an exact graph has already been
> tested, this is a solid reliabiity feature.
>
> Any local, modifications made to the clones in Packages are recorded in
> Packages/VersionLocks.json as part of the flow described in the next
> section. Modifications here means: changes to git remotes and the git-ref
> of the checked-out HEAD.
> Detailed Design
>
> In a fresh clone that does not contain a Packages directory swift build will
> determine the dependency graph, clone the packages into Packages and
> generate a Packages/VersionLocks.json file.
>
> The user can now step into the Packages directory and modify package
> sources. If the user then runs swift build again the package manager will
> error out:
>
> error: dependency sources have been modified
> execute `swift build --lock` or `swift build --ignore-lock`
>
> It is an error to build against an unlocked dependency graph, but to
> facilitate fixing bugs etc. an ignore flag can be specified.
>
> When swift build --lock is specified the package manager regenerates the
> lockfile detailing the active git remote and the SHA that is checked-out.
>
> Every time swift build completes a build the lockfile is updated (if
> necessary) recording the current version of the Swift toolchain that
> achieved the build.
> Packages/VersionLocks.json
>
> The exact design of the contents of this file will be explored during
> iterative development, but here is a possible example:
>
> json { "packages": [ { "clone": "Packages/PromiseKit-3.0.3", "origin": "
> https://github.com/mxcl/PromiseKit" "ref": "3.0.3" }, { "clone":
> "Packages/Alamofire-1.2.3", "origin": "
> https://github.com/a-fork-somewhere/Alamofire" "ref": "crucial-fix" }, {
> "clone": "Packages/Quick-1.2.3", "origin": "https://github.com/Quick/Quick"
> "ref": "1.2.3" } ] }
> Workflow — Regular Build
>
>    1. User runs swift build
>    2. If Packages/ contains clones and a VersionLocks.jsonSwiftPM skips
>    to 7.
>    3. If Packages/ contains clones and no VersionLocks.json the lockfile
>    is generated from the clones
>    4. If Packages/ contains checked out sources without git information
>    and no VersionLocks.json SwiftPM fetches the git information and
>    provided there is no diff, generates the Lockfile, if there is variation it
>    is an error *
>    5. If Packages/VersionLocks.json is present its dependency graph is
>    used
>    6. If Packages doesn't exist or is empty the dependency graph is
>    resolved, packages are cloned and the Lockfile is generated
>    7.
>
>    Build, if Packages are missing because we skipped from 2. the build
>    will error, it is the user's responsibility to instruct SwiftPM to
>    --update or to fix their dependency graph some other way.
>    -
>
>       This scenario is so users can check in their complete dependency
>       sources to their tree instead of / as well as the VersionLocks.json file:
>       a situation which sometimes is necessary if your dependencies are removed
>       from their third party online location, etc.
>
> Workflow — Making Modifications
>
>    1. User makes local modification to a dependency’s sources
>    2. User runs swift build
>    3. swift build errors out.
>    4. User must either lock the graph or run with --ignore-lock
>
> The error-out is likely to be considered tedious by users, however we
> consider it important that users are made aware and forced to act when they
> modify their dependencies and thus are exposing their team/users to
> so-called “dependency hell”.
>
> Runing swift build --lock regenerates the lockfile, but does not build.
>
> Modifications must be committed. This means that if the modifications are
> not uploaded to a location accessible to the rest of the team they will
> fail to build when they update their checkouts.
>
> The package manager could check for this by asking git if the specified
> origin has the current locked ref and error out as appropriate.
> Workflow — Overriding Packages
>
>    1. User steps into a Package directory eg. Packages/Foo-1.2.3
>    2. User changes the origin of Foo to their own fork
>    3. User alters HEAD to point to a fix in their own fork
>    4. swift build errors out.
>    5. User must either lock the graph or run with --ignore-lock
>
> Running swift build --lock regenerates the lockfile, the new origin and
> tag is stored. Thus a fresh clone of this project would use these overrides.
>
> It is important to note that this workflow will not be respected for
> dependencies, only for projects.
>
> If a package author requires an override they have a few options:
>
>    1. Change the Package.swift dependency specification. This should only
>    be done as a last resort, for example, a critical bug must be fixed in a
>    dependency and that dependency author is not being responsive. It is up to
>    the Package author to ensure this scenario goes well. SwiftPM itself wants
>    to guard against these conditions with our proposed “publish & lint” step
>    that validates such decisions before signing a published package tag. But
>    we are not there yet and thus package authors should be responsible.
>    2. Advise end-users in a package README that they should override the
>    dependency themselves.
>
> *2* is preferred, but *1* will happen. We consider it our responsibility
> to develop tooling that makes *1.* safe or unnecessary, but we are not
> there yet.
> Workflow — Updating Packages
>
> SwiftPM has no update mechanism yet, but once it does running swift build
> --update will fetch the latest versions of all dependencies and update
> the lockfile.
> Impact on existing code
>
> This proposal will have no impact on existing code.
> Alternatives Considered
>
> One alternative is to allow mentioning refs in manifest file while
> declaring a dependency but as discussed in this
> <http://markdownlivepreview.com/%22https://lists.swift.org/pipermail/swift-build-dev/Week-of-Mon-20151214/> thread
> it might not be the best idea.
>
> Using Git submodules for this feature was considered. However something
> additionally would be required to specify swift version and record local
> diffs. Also this would lock us into git, and despite the fact that
> currently we only use git, we have not yet ruled out supporting other
> version control systems.
>
>
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
>
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