Zero-config gopls workspaces

This issue describes a change to gopls' internal data model that will allow it to "Do The Right Thing" when the user opens a Go file. By decoupling the relationship between builds and workspace folders, we can eliminate complexity related to configuring the workspace (hence "zero-config"), and lay the groundwork for later improvements such as better support for working on multiple sets of build tags simultaneously (#29202).

After this change, users can work on multiple modules inside of a workspace regardless of whether they are related by a go.work file or explicitly open as separate workspace folders.

Background

Right now, gopls determines a unique build (called a View) for each workspace folder. When a workspace folder is opened, gopls performs the following steps:

Request configuration for each workspace folder using the workspace/configuration request with scopeUri set to the folder URI.
Using this configuration (which may affect the Go environment), resolve a root directory for this folder: a. Check go env GOWORK. b. Else, look for go.mod in a parent directory (recursively). c. Else, look for go.mod in a nested directory, if there is only one such nested directory. This was done to support polyglot workspaces where the Go project is in a nested directory, but is a source of both confusion and unpredictable startup time. d. Else, use the folder as root.
Load package metadata for the workspace by calling go/packages.Load.
Type check packages. We "fully" type-check packages that are inside a workspace module, and attempt to type-check only the exported symbols of packages in dependencies outside the workspace.

Problems

There are several problems with this model:

gopls startup involves scanning the entire workspace directory to find modules. If a user opens a home directory with millions of files, we pay significant a startup penalty (#56496).
The layout of gopls' internal data model depends on which directories are opened. Users must understand which directory to open, and gopls has to try to provide useful error messages when the workspace is misconfigured. This has been a significant source of confusion, and has led to various workarounds such as the "experimentalWorkspaceModule" setting, "expandWorkspaceToModule" setting, and "directoryFilters" setting.
Confusingly, if there is only one module in a nested directory, gopls will work. But if there are two modules, gopls won’t work.
If the user opens a file in a module that is not included in the workspace, gopls will simply not work, even though the go command may function properly when run from the file’s directory (as in the case where there are two nested modules but no go.work file).
gopls does a lot of work eagerly when initialized, before the user opens a file or makes any request. This may not be desirable, particularly in polyglot workspaces.
ad-hoc packages (packages outside of GOPATH, with no go.mod) do not work well with gopls. They have limited support if the ad-hoc package directory is opened as a workspace folder, but have several bugs and don’t work as expected when multiple ad-hoc directories are present.

New Model

We can address these problems by decoupling Views from workspace folders. The set of views will be dynamic, depending on both the set of open folders and the set of open files, and will be chosen to cover all open files.

Specifically, define new View and Folder types approximately as follows:

type Session struct {
    views   []*View
    folders []*Folder

    // other per-session fields
}

type View struct {
    viewType ViewType  // workspace (go.work), module (go.mod), GOPATH, or adhoc
    source   URI       // go.work file, go.mod file, or directory
    modules  []URI     // set of modules contained in this View, if any
    options   *Options // options derived from either session options, or folder options

    // …per-view state, such as the latest snapshot
}

type ViewType int

const (
    workspace ViewType = iota // go.work
    module ViewType           // go.mod
    gopath ViewType           // GOPATH directory
    adhoc ViewType            // ad-hoc directory – see below
)

type Folder struct {
    dir     URI      // workspace folder
    options *Options // configuration scoped to the workspace folder
}

A Session consists of a set of View objects describing modules (go.mod files), workspaces (go.work files), GOPATH directories or ad-hoc packages that the user is working on. This set is determined by both the workspace folders specified by the editor and the set of open files.

View types

A workspace View is defined by a go.work file. source is the path to the go.work file.
A module View is defined by a single go.mod file. source is the path to the go.mod file.
A GOPATH View is defined by a folder inside a GOPATH directory, with GO111MODULE=off or GO111MODULE=auto and no go.mod file. source is the path to the directory.
An adhoc View is defined by a folder outside of GOPATH, with no enclosing go.mod file. In this case, we consider files in the same directory to be part of a package, and source is the path to the directory.

The set of Views

We define the set of Views to ensure that we have coverage for each open folder, and each open file.

For each workspace folder, determine a View using the the following algorithm:

If go env GOWORK is set, create a workspace View.
Else, look for go.mod in a parent directory. If found, create a module View.
Else, if the workspace folder is inside GOPATH, and GO111MODULE is not explicitly set to on, create a GOPATH View. If GO111MODULE=on explicitly, fail.
Else, create an adhoc View for the workspace folder. This may not be desirable for the user if they have modules contained in nested directories. In this case we could either prompt the user, or scan for modules in nested directories, creating Views for each (but notably if we do decide to scan the filesystem, we would create a View for each go.mod or go.work file encountered, rather than fail if there are more than one).

For each open file, apply the following algorithm:

Match to an existing View

Find the enclosing module corresponding to the file by searching parent directories for go.mod files.
If a go.mod file is found, search for existing workspace or module type Views containing this module in their modules set.
Search for existing GOPATH type Views whose source directory contains the file.
Search for existing adhoc type Views whose source is equal to filepath.Dir(file).

If no existing View matches the file, create a new one
- Find a workspace folder containing the file, if any. If none is found, use a nil Folder (and therefore assume the default configuration). Note that if a workspace folder is found, the file is either in a module that is not included in the go.work file, or the folder is ad-hoc.
- If the file is in a module, define a new View of module type. Apply an explicit GOWORK=off to the View configuration to ensure that we can load the module.
- If the file is not in a module, define a new ad-hoc View.

Initializing views

Initialize views using the following logic. This essentially matches gopls’ current behavior.

For workspace Views, load modulepath/... for each workspace module.
For module Views, load modulepath/... for the main module.
For GOPATH Views, load ./... from the View dir.
For ad-hoc Views, load ./ from the View dir.

Type-check packages (and report their compiler diagnostics) as follows:

For workspace Views, type-check any package whose module is a workspace module.
For module Views, type-check any package whose module is the main module.
For GOPATH Views, type-check any package contained in dir.
For adhoc Views, type-check the ad-hoc package.

Resolving requests to Views

When a file-oriented request is handled by gopls (a request prefixed with textDocument/, such as textDocument/definition), gopls must usually resolve package metadata associated with the file.

In most cases, gopls currently chooses an existing view that best applies to the file (cache.bestViewForURI), but this is already problematic, because it can lead to path-dependency and incomplete results (c.f. #57558). For example: when finding references from a package imported from multiple views, gopls currently only shows references in one view.

Wherever possible, gopls should multiplex queries across all Views and merge their results. This would lead to consistent behavior of cross references. In a future where gopls has better build-tag support, this could also lead to multiple locations for jump-to-definition results.

In some cases (for example hover or signatureHelp), we must pick one view. In these cases we can apply some heuristic, but it should be of secondary significance (any hover or signatureHelp result is better than none).

Updating Views

Based on the algorithms used to determine Views above, the following notifications may affect the set of Views:

didOpen and didClose cause gopls to re-evaluate Views, ensuring that we have a View for each open file contained in a workspace folder.
didChangeConfiguration and didChangeWorkspaceFolderscauses gopls to update Folder layout and configuration. Note that configuration may affect e.g. GOWORK values and therefore may lead to a new set of Views.
didChange or didChangeWatchedFile may cause gopls to re-evaluate Views if the change is to a go.mod or go.work file (for example, a go.mod file deleted or added, or a go.work file changed in any way).

Following these changes, gopls will re-run the algorithm above to determine a new set of Views. It will re-use existing Views that have not changed.

Whenever new Views are created, they are reinitialized as above.

Differences from the current model

The algorithms described above are not vastly divergent from gopls’ current behavior. The significant differences may be summarized as follows:

Rather than having one view per folder, we may have multiple Views per folder (or even multiple folders per view, such as the case where several folders resolve to a common go.work file).
The set of ‘workspace modules’ in a given View is static. As go.mod files are added or removed, or go.work files changed, we reconfigure the set of Views. This simplifies the logic of handling metadata invalidation in each view.

Downsides

While this change will make gopls “do the right thing” in more cases, there are a several notable downsides:

Users may experience increased memory usage simply due to the fact that gopls successfully loads more packages. We are working on a separate redesign that will allow us to hold significantly less information in memory per view.
When opening a new workspace, if there are no open files gopls may resolve less information, for example if there is no go.work or go.mod in a parent directory of the workspace folder. This means that workspace/symbols requests may return empty results, or results that depend on the set of open files. Users can mitigate this by using a go.work file.
Workspace-wide queries such as “find references” may become more confusing to users. For example if the user has modules a and b in their workspace, and a depends on a version of b in the module cache, find reference on a symbol in the b directory of the workspace will not include references in a. Users can mitigate this by using a go.work file. It would also be possible for us to implement looser heuristics in our references search.

Future extension to build tags

By decoupling Views from workspace folders, it becomes possible for gopls to support working on multiple sets of build tags simultaneously. One can imagine that the algorithm above to compute views based on open files could be extended to GOOS and GOARCH: if an open file is not included in an existing view because of its GOOS or GOARCH build constraints, create a new view with updated environment.

The downsides above apply: potentially increased memory, and potentially confusing UX as the behavior of certain workspace-wide queries (such as references or workspace symbols) depends on the set of open files. We can work to mitigate these downsides, and in my opinion they do not outweigh the upsides, as these queries simply don't work in the current model.

Task List

Here's an approximate plan of attack for implementing this feature, which I'm aiming to complete by the end of the year. (migrated from https://github.com/golang/go/issues/57979#issuecomment-1787445478).

This is inside baseball, but may be interesting to @adonovan and @hyangah.

Phase 1: making Views immutable:

[x] Port all the completion tests to the new marker framework. The old marker framework toggles options before every single completion assertion, and if that causes the views to be recreated, the tests run far too slow.
[x] Remove the "minorOptionsChange" logic: every new configuration should result in a new view. The existing logic to avoid recreating the view is almost certainly a premature optimization.
[x] Extract the Session.getWorkspaceInformation logic to be unit-testable, and rename/refactor. This will be the basis of the new workspace algorithm.
[x] Isolate "folder" information into the Folder type, to reuse across new/multiple views.
[x] Invert the control of NewView, which should no longer query workspace information, but rather should be provided immutable workspace and folder information.
[x] Move management of the go.work and workspace mod files outside of the view, and into what is currently called getWorkspaceInformation. Whenever a go.work file changes, create a new view (at least if the new go.work parses and is saved to disk).
[x] Move all the mutable view state (such as vulnerabilities) into the snapshot. This is actually necessary for correctness anyway.

At this point, gopls should still behave identically, but Views will have immutable options and main modules. There may be a bit more churn when configuration or go.work files change, but such events should be very infrequent, and it seems reasonable to reload the workspace when they occur.

Phase 2: supporting multiple views per folder

[ ] Where it makes sense, multiplex LSP queries across all views. We currently do this only for WorkspaceSymbols, but it makes sense for all workspace-wide queries such as finding references or implementations.
[x] Rewrite the diagnostic logic to merge diagnostics from different views. I'm not sure exactly how this will work: the current logic enforces freshness by tracking a monotonic snapshot counter, but that ends up being rather complicated, and we should do better.
[x] Change the bestViewForURI logic to return nil if no view matches, and lift up snapshot.contains to the View, since views are now immutable.
[x] On any didOpen/didClose/didChangeConfiguration/didChange*(of go.work files), recompute the set of Views necessary to cover all open files (computeViews). Compute the diff with the current set, and minimally (re)create views that are necessary.
[ ] Move memoizedFS to each view, so that it is naturally reset whenever views change. Since views affect the set of file watchers, changes to the set of views may pull in files whose changes weren't observed, so we need to re-read.

Phase 3: support for multiple GOOS/GOARCH combinations

[x] Extend the computeViews algorithm to consider GOOS and GOARCH combinations. I'm not sure exactly how this algorithm will work: presumably if the current GOOS/GOARCH combination doesn't match an open file, we'll pick another, but the algorithm to pick another is non-trivial.

golang / go

x/tools/gopls: zero-config gopls workspaces #57979