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

Mon Mar 21 15:12:47 CDT 2016

I like the proposal.

I have 1 concern. 
As a package author, like PromiseKit <https://github.com/mxcl/PromiseKit>  I often have a need to fix some of my dependencies and publish it to my package users.

With the current proposal I can do it:
  - Advise it in README, which is not really a solution done by swift package manager.
  - update Manifest.swift file. There is no convent way to do it now.

I would love to have some functionality to make it simpler for me to specify (modify Manifest.swift or smt else) that Dependency X should: 
  - use specific tag X
  - use commit X

But maybe that should be part of another proposal. The lock file would perfectly solve the problems for developing projects that uses packages.

- Kostiantyn

> On 20 Mar 2016, at 06:07, Daniel Dunbar via swift-build-dev <swift-build-dev at swift.org> wrote:
> 
> My thoughts on this proposal:
> 
> 1. I agree with some of the other comments that I would prefer the version file be adjacent to "Package.swift". When the Packages directory isn't being checked in, I really would like to think of it as an "implementation detail" and not embed functionality inside it that would make it hard to remove or change in the future.
> 
> 2. I like VersionLocks.json well enough, but would like to see a discussion about possible alternatives. My personal proposal (in line with #1) is to use "PackageVersions.json" which has a nice agreement with Package.swift and would mean two common metadata files show up adjacent. I don't really want to bike shed on the name, but I suspect whatever we pick first will last for a while so I would at least like to review the various alternatives. I also will throw out that my personal opinion is we don't need to pick a name that bears much resemblance with existing terminology, whatever we pick will eventually become "the standard" for the SwiftPM ecosystem so I would prefer to pick the most-descriptive-possible name up front, not one that alludes to the same concept in other systems.
> 
> 3. I like the terminology section here, I almost feel like we should adopt that as official terminology in our documentation (which I don't think we have yet, correct me if I am wrong).
> 
> 4. I would like it if the lock file recorded the exact SHA it received, and validate that when retrieving. This helps protect users against MITM attacks or unexpected changes if an upstream modifies a tag. It also can be used as part of safety checks when migrating to an alternate repository host which is expected to have the same content.
> 
> 5. The "workflow - build" sections #2,3,4 are rather complicated. Is this because the proposal is trying to work with existing Packages layouts, or because the proposal is trying to handle the various variations of what the user may have checked in inside the Packages subdirectory?
> 
> 6. I wonder if we should be defining, as Eloy alludes to, two different things:
>  - The version lock file, which defines the expected versions for the package manager to use when it is doing package resolution.
>  - The package state file (in Packages.swift), which is used by the package manager to track information on the Packages/ subdir state in order to provide useful features primarily focused at the scenarios when the user is modifying those files.
> Currently it seems like a lot of the behaviors in the proposal are focused at the latter case, but they feel like they should be decoupled problems to me.
> 
>  - Daniel
> 
>> On Mar 17, 2016, at 11:23 AM, Max Howell via swift-evolution <swift-evolution at swift.org <mailto: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:
>> 
>> To ensure they are all using the same versions of their dependencies for any given version-control commit.
>> Ensure they are all using the same versions of the underlying Swift toolchain
>> 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 <https://github.com/mxcl/PromiseKit>" "ref": "3.0.3" }, { "clone": "Packages/Alamofire-1.2.3", "origin": "https://github.com/a-fork-somewhere/Alamofire <https://github.com/a-fork-somewhere/Alamofire>" "ref": "crucial-fix" }, { "clone": "Packages/Quick-1.2.3", "origin": "https://github.com/Quick/Quick <https://github.com/Quick/Quick>" "ref": "1.2.3" } ] }
>> 
>> Workflow — Regular Build
>> 
>> User runs swift build
>> If Packages/ contains clones and a VersionLocks.jsonSwiftPM skips to 7.
>> If Packages/ contains clones and no VersionLocks.json the lockfile is generated from the clones
>> 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 *
>> If Packages/VersionLocks.json is present its dependency graph is used
>> If Packages doesn't exist or is empty the dependency graph is resolved, packages are cloned and the Lockfile is generated
>> 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
>> 
>> User makes local modification to a dependency’s sources
>> User runs swift build
>> swift build errors out.
>> 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
>> 
>> User steps into a Package directory eg. Packages/Foo-1.2.3
>> User changes the origin of Foo to their own fork
>> User alters HEAD to point to a fix in their own fork
>> swift build errors out.
>> 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:
>> 
>> 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.
>> 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.
>> 
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> 
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