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// 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()
}
}
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