Lock inside filestate.apply instead of Update/Delete/Import (#8806)

This makes Delete/Update/Import/Preview the same for filestate and
httpstate. They could just be plain functions now except that `apply` is
hidden.

This also catches that when Watch did an update it wouldn't take the
lock for the filestate backend because Watch called `apply` directly not
`Update`.

pkg/backend/filestate/backend.go

@@ -467,50 +467,22 @@ func (b *localBackend) Preview(ctx context.Context, stack backend.Stack,
 
 func (b *localBackend) Update(ctx context.Context, stack backend.Stack,
 	op backend.UpdateOperation) (engine.ResourceChanges, result.Result) {
-
-	err := b.Lock(ctx, stack.Ref())
-	if err != nil {
-		return nil, result.FromError(err)
-	}
-	defer b.Unlock(ctx, stack.Ref())
-
 	return backend.PreviewThenPromptThenExecute(ctx, apitype.UpdateUpdate, stack, op, b.apply)
 }
 
 func (b *localBackend) Import(ctx context.Context, stack backend.Stack,
 	op backend.UpdateOperation, imports []deploy.Import) (engine.ResourceChanges, result.Result) {
-
-	err := b.Lock(ctx, stack.Ref())
-	if err != nil {
-		return nil, result.FromError(err)
-	}
-	defer b.Unlock(ctx, stack.Ref())
-
 	op.Imports = imports
 	return backend.PreviewThenPromptThenExecute(ctx, apitype.ResourceImportUpdate, stack, op, b.apply)
 }
 
 func (b *localBackend) Refresh(ctx context.Context, stack backend.Stack,
 	op backend.UpdateOperation) (engine.ResourceChanges, result.Result) {
-
-	err := b.Lock(ctx, stack.Ref())
-	if err != nil {
-		return nil, result.FromError(err)
-	}
-	defer b.Unlock(ctx, stack.Ref())
-
 	return backend.PreviewThenPromptThenExecute(ctx, apitype.RefreshUpdate, stack, op, b.apply)
 }
 
 func (b *localBackend) Destroy(ctx context.Context, stack backend.Stack,
 	op backend.UpdateOperation) (engine.ResourceChanges, result.Result) {
-
-	err := b.Lock(ctx, stack.Ref())
-	if err != nil {
-		return nil, result.FromError(err)
-	}
-	defer b.Unlock(ctx, stack.Ref())
-
 	return backend.PreviewThenPromptThenExecute(ctx, apitype.DestroyUpdate, stack, op, b.apply)
 }
 
@@ -540,6 +512,15 @@ func (b *localBackend) apply(
 			colors.SpecHeadline+"%s (%s):"+colors.Reset+"\n"), actionLabel, stackRef)
 	}
 
+	// Take a lock unless this is just a preview
+	if kind != apitype.PreviewUpdate {
+		err := b.Lock(ctx, stack.Ref())
+		if err != nil {
+			return nil, result.FromError(err)
+		}
+		defer b.Unlock(ctx, stack.Ref())
+	}
+
 	// Start the update.
 	update, err := b.newUpdate(stackName, op)
 	if err != nil {