gopls/internal/golang/completion: return error if file is not new

The previous logic returned (nil, nil) in that case, without
the necessary documentation, and the caller failed to handle
that case, leading to a panic.

Fixes golang/go#74057

Change-Id: I9f0c6e98457354189845954bab53a2b848401b01
Reviewed-on: https://go-review.googlesource.com/c/tools/+/680259
Auto-Submit: Alan Donovan <adonovan@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: adonovan

gopls/internal/golang/completion/newfile.go

@@ -16,11 +16,15 @@ import (
 	"golang.org/x/tools/gopls/internal/protocol"
 )
 
-// NewFile returns a document change to complete an empty go file.
+// NewFile returns a document change to complete an empty Go source file.
 func NewFile(ctx context.Context, snapshot *cache.Snapshot, fh file.Handle) (*protocol.DocumentChange, error) {
-	if bs, err := fh.Content(); err != nil || len(bs) != 0 {
+	content, err := fh.Content()
+	if err != nil {
 		return nil, err
 	}
+	if len(content) != 0 {
+		return nil, fmt.Errorf("file is not empty")
+	}
 	meta, err := snapshot.NarrowestMetadataForFile(ctx, fh.URI())
 	if err != nil {
 		return nil, err

gopls/internal/server/workspace.go

@@ -160,6 +160,7 @@ func (s *server) DidCreateFiles(ctx context.Context, params *protocol.CreateFile
 		case file.Go:
 			change, err := completion.NewFile(ctx, snapshot, fh)
 			if err != nil {
+				// any error, including "it's not a new file"
 				continue
 			}
 			allChanges = append(allChanges, *change)