1 files changed, 45 insertions, 0 deletions
diff --git a/lib/btrfsutil/nestedlock.go b/lib/btrfsutil/nestedlock.go
new file mode 100644
index 0000000..917b4cd
--- /dev/null
+++ b/lib/btrfsutil/nestedlock.go
@@ -0,0 +1,45 @@
+// Copyright (C) 2023  Luke Shumaker <lukeshu@lukeshu.com>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+package btrfsutil
+
+import (
+	"context"
+	"sync"
+)
+
+// A nestedMutex is like a sync.Mutex, but while it is locked by call
+// 'A', may be simultaneously locked by subsequent calls if the
+// subsequent calls use a Context descended from the one returned by
+// the 'A' call to .Lock().
+type nestedMutex struct {
+	inner sync.Mutex
+	depth int
+}
+
+type nestedMutexCtxKey struct{}
+
+// Lock locks the mutex.  It is invalid to use a Context returned from
+// Lock in a different goroutine than the one it was created in.  It
+// is invalid to use a Context returned from Lock after the mutex has
+// subsequently become unlocked.
+func (m *nestedMutex) Lock(ctx context.Context) context.Context {
+	if other, ok := ctx.Value(nestedMutexCtxKey{}).(*nestedMutex); ok && other == m {
+		m.depth++
+		return ctx
+	}
+	m.inner.Lock()
+	return context.WithValue(ctx, nestedMutexCtxKey{}, m)
+}
+
+// Unlock unlocks the mutex.  It is invalid to call Unlock if the
+// mutex is not already locked.  It is invalid to call Unlock from
+// multiple goroutines simultaneously.
+func (m *nestedMutex) Unlock() {
+	if m.depth > 0 {
+		m.depth--
+	} else {
+		m.inner.Unlock()
+	}
+}