Add levelmu, a package implementing priority mutex with levels

util/levelmu/levelmu.go

@@ -0,0 +1,39 @@
+package levelmu
+
+import (
+	"sync"
+)
+
+type LevelMutex struct {
+	mus []sync.Mutex
+	// Protected by the very last mutex.
+	lastLevel int
+}
+
+func (lm *LevelMutex) Init(levels int) {
+	if lm.mus != nil {
+		panic("level mutex already initialized")
+	}
+	lm.mus = make([]sync.Mutex, levels)
+}
+
+func (lm *LevelMutex) Lock() {
+	lm.LevelLock(0)
+}
+
+func (lm *LevelMutex) Unlock() {
+	stopLevel := lm.lastLevel
+	for i := len(lm.mus) - 1; i >= stopLevel; i-- {
+		lm.mus[i].Unlock()
+	}
+}
+
+func (lm *LevelMutex) LevelLock(level int) {
+	if level >= len(lm.mus) {
+		panic("lock level exceeds configured level count")
+	}
+	for l := level; l < len(lm.mus); l++ {
+		lm.mus[l].Lock()
+	}
+	lm.lastLevel = level
+}