internal/lsp/cache: lock in snapshot.knownFilesInDir

We were not locking while iterating s.files in snapshot.knownFilesInDir.
All other accesses of s.files appear to be locked, so this should fix
golang/go#43972.

Fixes golang/go#43972

Change-Id: I01184c3992c91f8beb4a3239f70cc4487a528ec0
Reviewed-on: https://go-review.googlesource.com/c/tools/+/287573
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

internal/lsp/cache/snapshot.go

@@ -32,7 +32,6 @@ import (
 	"golang.org/x/tools/internal/packagesinternal"
 	"golang.org/x/tools/internal/span"
 	"golang.org/x/tools/internal/typesinternal"
-	"golang.org/x/xerrors"
 	errors "golang.org/x/xerrors"
 )
 
@@ -718,6 +717,9 @@ func (s *snapshot) allKnownSubdirs(ctx context.Context) map[span.URI]struct{} {
 // the given directory. It does not respect symlinks.
 func (s *snapshot) knownFilesInDir(ctx context.Context, dir span.URI) []span.URI {
 	var files []span.URI
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
 	for uri := range s.files {
 		if source.InDir(dir.Filename(), uri.Filename()) {
 			files = append(files, uri)
@@ -1010,7 +1012,7 @@ func (s *snapshot) awaitLoaded(ctx context.Context) error {
 
 func (s *snapshot) GetCriticalError(ctx context.Context) *source.CriticalError {
 	loadErr := s.awaitLoadedAllErrors(ctx)
-	if xerrors.Is(loadErr, context.Canceled) {
+	if errors.Is(loadErr, context.Canceled) {
 		return nil
 	}