Proposal: Multi-project gopls workspaces

Author(s): Heschi Kreinick, Rebecca Stambler

Last updated: [Date]

Discussion at https://golang.org/issue/32394.

(Previously, https://golang.org/issue/37720)

Abstract

We propose a new workspace model for gopls that supports editing multiple projects at the same time, without compromising editor features.

Background

gopls users may want to edit multiple projects in one editor session. For example, a microservice might depend on a proprietary infrastructure library, and a feature might require working across both. In GOPATH mode, that's relatively trivial, because all code exists in the same context. In module mode, where multiple versions of dependencies are in play, it is much more difficult.

Consider the following application:

If I Find References on an errors.Wrapf call in app1, I expect to see references in lib as well. This is especially true if I happen to own lib, but even if not I may be looking for usage examples. In this situation, supporting that is easy.

Now consider a workspace with two applications.

Again, I would expect a Find References in either App1 or App2 to find all Wrapf calls, and there‘s no reason that shouldn’t work in this scenario. In module mode, things can be more difficult. Here's the next step in complexity:

At the level of the type checker, v1.0.0 and v1.0.1 of the library are completely unrelated packages that happen to share a name. We as humans expect the APIs to match, but they could be completely different. Nonetheless, in this situation we can simply load both, and if we do a Find References on Wrapf there should be no problem finding all of them. That goes away in the next step:

Now there are two versions of Wrapf. Again, at the type-checking level, these packages have nothing to do with each other. There is no easy way to relate Wrapf from v0.9.1 with its match from v0.9.0. We would have to do a great deal of work to correlate all the versions of a package together and match them up. (Wrapf is a simple case; consider how we'd match them if it was a method receiver, or took a complex struct, or a type from another package.) Worse yet, how would a multi-project rename work? Would it rename in all versions?

One final case:

Imagine I start in App1 and Go To Definition on a function from the utility library. So far, no problem: there‘s only one version of the utility library in scope. Now I Go To Definition on Wrapf. Which version should I go to? The answer depends on where I came from, but that information can’t be expressed in the filesystem path of the source file, so there's no way for gopls to keep track.

Proposal

We propose to require all projects in a multi-project workspace use the same set of dependency versions. For GOPATH, this means that all the projects should have the same GOPATH setting. For module mode, it means creating one super-module that forces all the projects to resolve their dependencies together. Effectively, this would create an on-the-fly monorepo. This rules out users working on projects with mutually conflicting requirements, but that will hopefully be rare. Hopefully gopls can create this super-module automatically.

The super-module would look something like:

module gopls-workspace

require (
    example.com/app1 v0.0.0-00010101000000-000000000000
    example.com/app2 v0.0.0-00010101000000-000000000000
)

replace (
    example.com/app1 => /abs/path/to/app1
    example.com/app2 => /abs/path/to/app2

    // Further replace directives included from app1 and app2
)

Note the propagation of replace directives from the constituent projects, since they would otherwise not take effect.

Rationale

For users to get the experience they expect, with all of the scenarios above working, the only possible model is one where there‘s one version of any dependency in scope at a time. We don’t think there are any realistic alternatives to this model. We could try to include multiple versions of packages and then correlate them by name and signature (as discussed above) but that would be error-prone to implement. And if there were any problems matching things up, features like Find References would silently fail.

Compatibility

No compatibility issues.

Implementation

The implementation involves (1) finding all of the modules in the view, (2) automatically creating the super-module, and (3) adjusting gopls's go/packages queries and go command calls to run in the correct modules.

When a view is created, we traverse the view's root folder and search for all of the existing modules. These modules will then be used to programmatically create the super-module. Once each view is created, it will load all of its packages (initial workspace load). As of 2020-08-13, for views in GOPATH or in a module, the initial workspace load takes the form of a go list ./... query. With the current design, the initial workspace load will need be a query of the form: go list example.com/app1/... example.com/app12/..., within the super-module. In GOPATH mode, we will not create the super-module.

All go/packages queries should be made from the super-module directory. Only go mod commands need to be made from the module to which they refer.

The super-module's go.mod file

As of 2020-08-13, gopls relies on the go command‘s -modfile flag to avoid modifying the user’s existing go.mod file. We will continue to use the -modfile flag when running the go command from within a module, but -modfile is no longer necessary when we run the go command from the super-module.

The go command does require that its working directory contain a go.mod file, but we want to run commands from the super-module without exposing super-module‘s go.mod file to the user. To handle this, we will create a temporary directory containing the super-module’s go.mod file, to act as the module root for any go/packages queries.

Configuration

gopls.mod

We should allow users to provide their own super-module go.mod file, for extra control over the developer experience. This can also be used to mitigate any issues with the automatic creation of the super-module. We should detect a gopls.mod in the view's root folder and use that as the super-module if present.

Additional Considerations

The authors have not yet considered the full implications of this design on:

• Nested modules
  • A workspace pattern of module/... will include packages in nested modules inside it, whether the user wants them or not.
• Modules with replace directives (mentioned briefly above)
• Views containing a single module within the view's root folder
  • Consider not creating a super-module at all

If any issues are noted during the implementation process, this document will be updated accordingly.

This design means that there is no longer any need to have multiple views in a session. The gopls team will need to reconsider whether there is value in offering users a standalone workspace for each workspace folder, rather than merging all workspace folders into one view.

Transition

Users have currently been getting support for multiple modules in gopls by adding each module as its own workspace folder. Once the implementation is complete, we will need to help users transition to this new model--otherwise they will find that memory consumption rises, as gopls will have loaded the same module into memory multiple times. We will need to detect if a workspace folder is part of multiple views and alert the user to adjust their workspace.

While the gopls team implements this design, the super-module functionality will be gated behind an opt-in flag.

Open issues (if applicable)

The go.mod editing functionality of gopls should continue to work as it does today, even in multi-project mode. Most likely it should simply continue to operate in one project at a time.