Make (*WatchedFiles[T]).Clone propagate nil

[DanielRosenwasser]

Fixes #2248.

[Copilot]

Pull request overview

This PR fixes a panic that occurred when WatchEnabled is false but TypingsLocation is set. In this scenario, the typingsWatch field (and other watch fields) remain nil, but ATA can still attempt to clone them, causing a nil pointer dereference when acquiring the mutex.

• Added nil check to WatchedFiles[T].Clone() to propagate nil instead of panicking
• Added unit test to verify nil propagation behavior
• Added integration test to ensure ATA works correctly with WatchEnabled=false

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
internal/project/watch.go	Added nil check in Clone method to prevent panic when called on nil receiver
internal/project/watch_test.go	Added unit test verifying Clone returns nil when called on nil WatchedFiles
internal/project/ata/ata_test.go	Added integration test ensuring ATA doesn't panic with WatchEnabled=false but TypingsLocation set

[jakebailey]

Where do we end up trying to call clone on nil? I can imagine this fix works, but I'm not sure why we even get that far

[sheetalkamat]

typingsWatch

In DidUpdateATAState where we get files to watch for ATA and add them for watching

[DanielRosenwasser]

So this would be hard to emulate in VS Code (or it just doesn't happen) because it supports the capability workspace.didChangeWatchedFiles - but it causes issues for scripts written against our server which don't. Basically, when that capability is not set the server creates a session where WatchEnabled is false.

typescript-go/internal/lsp/server.go

Lines 1005 to 1008 in 7c0aa9e

	func (s *Server) handleInitialized(ctx context.Context, params *lsproto.InitializedParams) error {
	if s.clientCapabilities.Workspace.DidChangeWatchedFiles.DynamicRegistration {
	s.watchEnabled = true
	}

typescript-go/internal/lsp/server.go

Lines 1032 to 1049 in 7c0aa9e

	s.session = project.NewSession(&project.SessionInit{
	Options: &project.SessionOptions{
	CurrentDirectory: cwd,
	DefaultLibraryPath: s.defaultLibraryPath,
	TypingsLocation: s.typingsLocation,
	PositionEncoding: s.positionEncoding,
	WatchEnabled: s.watchEnabled,
	LoggingEnabled: true,
	DebounceDelay: 500 * time.Millisecond,
	PushDiagnosticsEnabled: !disablePushDiagnostics,
	Locale: s.locale,
	},
	FS: s.fs,
	Logger: s.logger,
	Client: s,
	NpmExecutor: s,
	ParseCache: s.parseCache,
	})

And I believe what happens is that when creating the first project, this conditionally sets typingsWatch (and other watchers).

typescript-go/internal/project/project.go

Lines 151 to 174 in d876d05

	if builder.sessionOptions.WatchEnabled {
	project.programFilesWatch = NewWatchedFiles(
	"non-root program files for "+configFileName,
	lsproto.WatchKindCreate|lsproto.WatchKindChange|lsproto.WatchKindDelete,
	core.Identity,
	)
	project.failedLookupsWatch = NewWatchedFiles(
	"failed lookups for "+configFileName,
	lsproto.WatchKindCreate,
	createResolutionLookupGlobMapper(builder.sessionOptions.CurrentDirectory, builder.sessionOptions.DefaultLibraryPath, project.currentDirectory, builder.fs.fs.UseCaseSensitiveFileNames()),
	)
	project.affectingLocationsWatch = NewWatchedFiles(
	"affecting locations for "+configFileName,
	lsproto.WatchKindCreate|lsproto.WatchKindChange|lsproto.WatchKindDelete,
	createResolutionLookupGlobMapper(builder.sessionOptions.CurrentDirectory, builder.sessionOptions.DefaultLibraryPath, project.currentDirectory, builder.fs.fs.UseCaseSensitiveFileNames()),
	)
	if builder.sessionOptions.TypingsLocation != "" {
	project.typingsWatch = NewWatchedFiles(
	"typings installer files",
	lsproto.WatchKindCreate|lsproto.WatchKindChange|lsproto.WatchKindDelete,
	core.Identity,
	)
	}
	}

So it's not always there.