internal/lsp/fake: hold the mutex for all of SaveBufferWithoutActions

This addresses the race seen in
https://storage.googleapis.com/go-build-log/5ba1c3f2/windows-386-2008_2bf3acc7.log.

Paraphrased from Rob in chat:

The race is triggered because of the additional asynchronous handling of
file changes added to sync non-dirty files on disk changes. The buffer
is marked as non-dirty, THEN the on-disk change is processed, sending a
didChange, THEN the didSave is sent (https://golang.org/cl/267577).

Also, don't send didChange for formatting if nothing has changed.

Change-Id: Ibc6a55d35dee99c04dadf5470c2d68ca937f849b
Reviewed-on: https://go-review.googlesource.com/c/tools/+/271578
Trust: Rebecca Stambler <rstambler@golang.org>
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

Author: stamblerre

internal/lsp/fake/editor.go

@@ -397,9 +397,9 @@ func (e *Editor) SaveBuffer(ctx context.Context, path string) error {
 
 func (e *Editor) SaveBufferWithoutActions(ctx context.Context, path string) error {
 	e.mu.Lock()
+	defer e.mu.Unlock()
 	buf, ok := e.buffers[path]
 	if !ok {
-		e.mu.Unlock()
 		return fmt.Errorf(fmt.Sprintf("unknown buffer: %q", path))
 	}
 	content := buf.text()
@@ -408,7 +408,6 @@ func (e *Editor) SaveBufferWithoutActions(ctx context.Context, path string) erro
 	if ok {
 		includeText = syncOptions.Save.IncludeText
 	}
-	e.mu.Unlock()
 
 	docID := e.textDocumentIdentifier(buf.path)
 	if e.Server != nil {
@@ -423,10 +422,8 @@ func (e *Editor) SaveBufferWithoutActions(ctx context.Context, path string) erro
 		return errors.Errorf("writing %q: %w", path, err)
 	}
 
-	e.mu.Lock()
 	buf.dirty = false
 	e.buffers[path] = buf
-	e.mu.Unlock()
 
 	if e.Server != nil {
 		params := &protocol.DidSaveTextDocumentParams{
@@ -812,6 +809,9 @@ func (e *Editor) FormatBuffer(ctx context.Context, path string) error {
 		return fmt.Errorf("before receipt of formatting edits, buffer version changed from %d to %d", version, versionAfter)
 	}
 	edits := convertEdits(resp)
+	if len(edits) == 0 {
+		return nil
+	}
 	return e.editBufferLocked(ctx, path, edits)
 }