gopls/internal/cmd: headless mcp server command

Adds a gopls subcommand that runs the Go MCP
server in "headless" mode - independently from a
real LSP client. Uses the fake client implementation
used by other gopls subcommands to initialize
a new gopls session.

`gopls mcp` - start an stdio MCP server
`gopls mcp -listen=localhost:3000" - start an SSE MCP server
running over http on the given address

Change-Id: I64ea79196a8d1d01df02757e693354beb8e52197
Reviewed-on: https://go-review.googlesource.com/c/tools/+/675935
Reviewed-by: Hongxiang Jiang <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: madelinekalil

gopls/internal/cmd/call_hierarchy.go

@@ -39,7 +39,7 @@ func (c *callHierarchy) Run(ctx context.Context, args ...string) error {
 		return tool.CommandLineErrorf("call_hierarchy expects 1 argument (position)")
 	}
 
-	conn, err := c.app.connect(ctx)
+	conn, _, err := c.app.connect(ctx)
 	if err != nil {
 		return err
 	}

gopls/internal/cmd/check.go

@@ -66,7 +66,7 @@ func (c *check) Run(ctx context.Context, args ...string) error {
 		opts.RelatedInformationSupported = true
 	}
 
-	conn, err := c.app.connect(ctx)
+	conn, _, err := c.app.connect(ctx)
 	if err != nil {
 		return err
 	}

gopls/internal/cmd/cmd.go

@@ -287,6 +287,7 @@ func (app *Application) featureCommands() []tool.Application {
 		&fix{app: app}, // (non-functional)
 		&foldingRanges{app: app},
 		&format{app: app},
+		&headlessMCP{app: app},
 		&highlight{app: app},
 		&implementation{app: app},
 		&imports{app: app},
@@ -305,20 +306,24 @@ func (app *Application) featureCommands() []tool.Application {
 	}
 }
 
-// connect creates and initializes a new in-process gopls session.
-func (app *Application) connect(ctx context.Context) (*connection, error) {
+// connect creates and initializes a new in-process gopls LSP session.
+func (app *Application) connect(ctx context.Context) (*connection, *cache.Session, error) {
 	client := newClient(app)
-	var svr protocol.Server
+	var (
+		svr  protocol.Server
+		sess *cache.Session
+	)
 	if app.Remote == "" {
 		// local
 		options := settings.DefaultOptions(app.options)
-		svr = server.New(cache.NewSession(ctx, cache.New(nil)), client, options)
+		sess = cache.NewSession(ctx, cache.New(nil))
+		svr = server.New(sess, client, options)
 		ctx = protocol.WithClient(ctx, client)
 	} else {
 		// remote
 		netConn, err := lsprpc.ConnectToRemote(ctx, app.Remote)
 		if err != nil {
-			return nil, err
+			return nil, nil, err
 		}
 		stream := jsonrpc2.NewHeaderStream(netConn)
 		jsonConn := jsonrpc2.NewConn(stream)
@@ -329,7 +334,7 @@ func (app *Application) connect(ctx context.Context) (*connection, error) {
 				protocol.ClientHandler(client, jsonrpc2.MethodNotFound)))
 	}
 	conn := newConnection(svr, client)
-	return conn, conn.initialize(ctx, app.options)
+	return conn, sess, conn.initialize(ctx, app.options)
 }
 
 func (c *connection) initialize(ctx context.Context, options func(*settings.Options)) error {

gopls/internal/cmd/codeaction.go

@@ -108,7 +108,7 @@ func (cmd *codeaction) Run(ctx context.Context, args ...string) error {
 		return tool.CommandLineErrorf("codeaction expects at least 1 argument")
 	}
 	cmd.app.editFlags = &cmd.EditFlags
-	conn, err := cmd.app.connect(ctx)
+	conn, _, err := cmd.app.connect(ctx)
 	if err != nil {
 		return err
 	}

gopls/internal/cmd/codelens.go

@@ -83,7 +83,7 @@ func (r *codelens) Run(ctx context.Context, args ...string) error {
 		opts.Codelenses[settings.CodeLensTest] = true
 	}
 
-	conn, err := r.app.connect(ctx)
+	conn, _, err := r.app.connect(ctx)
 	if err != nil {
 		return err
 	}

gopls/internal/cmd/definition.go

@@ -73,7 +73,7 @@ func (d *definition) Run(ctx context.Context, args ...string) error {
 			o.PreferredContentFormat = protocol.Markdown
 		}
 	}
-	conn, err := d.app.connect(ctx)
+	conn, _, err := d.app.connect(ctx)
 	if err != nil {
 		return err
 	}

gopls/internal/cmd/execute.go

@@ -71,7 +71,7 @@ func (e *execute) Run(ctx context.Context, args ...string) error {
 
 	e.app.editFlags = &e.EditFlags // in case command performs an edit
 
-	conn, err := e.app.connect(ctx)
+	conn, _, err := e.app.connect(ctx)
 	if err != nil {
 		return err
 	}

gopls/internal/cmd/folding_range.go

@@ -36,7 +36,7 @@ func (r *foldingRanges) Run(ctx context.Context, args ...string) error {
 		return tool.CommandLineErrorf("folding_ranges expects 1 argument (file)")
 	}
 
-	conn, err := r.app.connect(ctx)
+	conn, _, err := r.app.connect(ctx)
 	if err != nil {
 		return err
 	}

gopls/internal/cmd/format.go

@@ -42,7 +42,7 @@ func (c *format) Run(ctx context.Context, args ...string) error {
 		return nil
 	}
 	c.app.editFlags = &c.EditFlags
-	conn, err := c.app.connect(ctx)
+	conn, _, err := c.app.connect(ctx)
 	if err != nil {
 		return err
 	}

gopls/internal/cmd/highlight.go

@@ -38,7 +38,7 @@ func (r *highlight) Run(ctx context.Context, args ...string) error {
 		return tool.CommandLineErrorf("highlight expects 1 argument (position)")
 	}
 
-	conn, err := r.app.connect(ctx)
+	conn, _, err := r.app.connect(ctx)
 	if err != nil {
 		return err
 	}