internal/lsp/debug: fix early closure of logfile

As of https://golang.org/cl/218457, logs are not being captured because
the logfile is prematurely closed due to the scope of the deferred
closure changing.

Change-Id: I1754e5555025c7b2a5da58f621184d6740fd03cb
Reviewed-on: https://go-review.googlesource.com/c/tools/+/219080
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

internal/lsp/cmd/serve.go

@@ -54,9 +54,11 @@ func (s *Serve) Run(ctx context.Context, args ...string) error {
 		return tool.CommandLineErrorf("server does not take arguments, got %v", args)
 	}
 
-	if err := s.app.debug.SetLogFile(s.Logfile); err != nil {
+	err, closeLog := s.app.debug.SetLogFile(s.Logfile)
+	if err != nil {
 		return err
 	}
+	defer closeLog()
 	s.app.debug.ServerAddress = s.Address
 	s.app.debug.DebugAddress = s.Debug
 	s.app.debug.Serve(ctx)

internal/lsp/debug/serve.go

@@ -258,21 +258,27 @@ func (i *Instance) Prepare(ctx context.Context) {
 	export.AddExporters(i.ocagent, i.prometheus, i.rpcs, i.traces)
 }
 
-func (i *Instance) SetLogFile(logfile string) error {
+func (i *Instance) SetLogFile(logfile string) (error, func()) {
+	// TODO: probably a better solution for deferring closure to the caller would
+	// be for the debug instance to itself be closed, but this fixes the
+	// immediate bug of logs not being captured.
+	closeLog := func() {}
 	if logfile != "" {
 		if logfile == "auto" {
 			logfile = filepath.Join(os.TempDir(), fmt.Sprintf("gopls-%d.log", os.Getpid()))
 		}
 		f, err := os.Create(logfile)
 		if err != nil {
-			return fmt.Errorf("Unable to create log file: %v", err)
+			return fmt.Errorf("Unable to create log file: %v", err), nil
+		}
+		closeLog = func() {
+			defer f.Close()
 		}
-		defer f.Close()
 		stdlog.SetOutput(io.MultiWriter(os.Stderr, f))
 		i.LogWriter = f
 	}
 	i.Logfile = logfile
-	return nil
+	return nil, closeLog
 }
 
 // Serve starts and runs a debug server in the background.