Add ReferenceCountedDirectoryCloser

Sometimes you run into situations where you want to traverse directory
hierarchies in parallel. In those cases it's useful to have reference
counting on the directory objects, so that directories don't need to be
opened redundantly.

pkg/filesystem/directory.go

@@ -3,6 +3,7 @@ package filesystem
 import (
 	"io"
 	"os"
+	"sync/atomic"
 	"time"
 
 	"github.com/buildbarn/bb-storage/pkg/filesystem/path"
@@ -139,3 +140,43 @@ func NopDirectoryCloser(d Directory) DirectoryCloser {
 func (d nopDirectoryCloser) Close() error {
 	return nil
 }
+
+// ReferenceCountedDirectoryCloser is a decorator for DirectoryCloser that
+// adds reference counting. This makes it possible to duplicate it, and
+// call Close() on it multiple times.
+type ReferenceCountedDirectoryCloser struct {
+	DirectoryCloser
+	refcount atomic.Int32
+}
+
+// NewReferenceCountedDirectoryCloser creates a new
+// ReferenceCountedDirectoryCloser against which Close() can be called
+// exactly once.
+func NewReferenceCountedDirectoryCloser(directory DirectoryCloser) *ReferenceCountedDirectoryCloser {
+	d := &ReferenceCountedDirectoryCloser{
+		DirectoryCloser: directory,
+	}
+	d.refcount.Store(1)
+	return d
+}
+
+// Duplicate the ReferenceCountedDirectoryCloser by increasing its
+// reference count.
+func (d *ReferenceCountedDirectoryCloser) Duplicate() *ReferenceCountedDirectoryCloser {
+	if newRefcount := d.refcount.Add(1); newRefcount <= 1 {
+		panic("Invalid reference count")
+	}
+	return d
+}
+
+// Close the ReferenceCountedDirectoryCloser by decreasing its reference
+// count. If the reference count reaches zero, the underlying directory
+// is closed.
+func (d *ReferenceCountedDirectoryCloser) Close() error {
+	if newRefcount := d.refcount.Add(-1); newRefcount == 0 {
+		return d.DirectoryCloser.Close()
+	} else if newRefcount < 0 {
+		panic("Invalid reference count")
+	}
+	return nil
+}