Avoid hanging: ensure we always close f.filesCh

The contract seems to be that the `walkFilesystem` goroutine is
responsible for closing `f.filesCh`, but before this change, there were
codepaths that could result in the gorouting exiting without closing
`f.filesCh`. That shouldn't be possible anymore, so long as we keep this
statement at the top of the function =)

This fixes numtide#406

Author: jfly

cli/format.go

@@ -201,6 +201,9 @@ func (f *Format) Run() (err error) {
 
 func (f *Format) walkFilesystem(ctx context.Context) func() error {
 	return func() error {
+		// close the files channel when we're done walking the file system
+		defer close(f.filesCh)
+
 		eg, ctx := errgroup.WithContext(ctx)
 		pathsCh := make(chan string, BatchSize)
 
@@ -261,9 +264,6 @@ func (f *Format) walkFilesystem(ctx context.Context) func() error {
 			return fmt.Errorf("failed to create walker: %w", err)
 		}
 
-		// close the files channel when we're done walking the file system
-		defer close(f.filesCh)
-
 		// if no cache has been configured, or we are processing from stdin, we invoke the walker directly
 		if f.NoCache || f.Stdin {
 			return walker.Walk(ctx, func(file *walk.File, err error) error {