internal/lsp/source: handle empty strings in directoryFilters

Avoid a panic when a directoryFilters entry is empty.

Fixes golang/go#51843

Change-Id: I50948894ce60d83441c3fc698c2e7d80d1d2b50e
Reviewed-on: https://go-review.googlesource.com/c/tools/+/395675
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

gopls/internal/regtest/misc/settings_test.go

@@ -0,0 +1,36 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package misc
+
+import (
+	"testing"
+
+	. "golang.org/x/tools/internal/lsp/regtest"
+)
+
+func TestEmptyDirectoryFilters_Issue51843(t *testing.T) {
+	const src = `
+-- go.mod --
+module mod.com
+
+go 1.12
+-- main.go --
+package main
+
+func main() {
+}
+`
+
+	WithOptions(
+		EditorConfig{
+			Settings: map[string]interface{}{
+				"directoryFilters": []string{""},
+			},
+		},
+	).Run(t, src, func(t *testing.T, env *Env) {
+		// No need to do anything. Issue golang/go#51843 is triggered by the empty
+		// directory filter above.
+	})
+}

internal/lsp/source/options.go

@@ -820,7 +820,7 @@ func (o *Options) set(name string, value interface{}, seen map[string]struct{})
 		var filters []string
 		for _, ifilter := range ifilters {
 			filter := fmt.Sprint(ifilter)
-			if filter[0] != '+' && filter[0] != '-' {
+			if filter == "" || (filter[0] != '+' && filter[0] != '-') {
 				result.errorf("invalid filter %q, must start with + or -", filter)
 				return result
 			}