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priority_mutex.go
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/
priority_mutex.go
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// Copyright 2020 Coinbase, Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package utils
import (
"sync"
)
// PriorityMutex is a special type of mutex
// that allows callers to request priority
// over other callers. This can be useful
// if there is a "hot path" in an application
// that requires lock access.
//
// WARNING: It is possible to cause lock starvation
// if not careful (i.e. only high priority callers
// ever do work).
type PriorityMutex struct {
high []chan struct{}
low []chan struct{}
mutex sync.Mutex
lock bool
}
// Lock attempts to acquire either a high or low
// priority mutex. When priority is true, a lock
// will be granted before other low priority callers.
func (m *PriorityMutex) Lock(priority bool) {
c := m.lockInternal(priority)
if c != nil {
<-c
}
}
func (m *PriorityMutex) lockInternal(priority bool) <-chan struct{} {
m.mutex.Lock()
defer m.mutex.Unlock()
if !m.lock {
m.lock = true
return nil
}
c := make(chan struct{})
if priority {
m.high = append(m.high, c)
} else {
m.low = append(m.low, c)
}
return c
}
// Unlock selects the next highest priority lock
// to grant. If there are no locks to grant, it
// sets the value of m.lock to false.
func (m *PriorityMutex) Unlock() {
m.mutex.Lock()
defer m.mutex.Unlock()
if len(m.high) > 0 {
c := m.high[0]
m.high = m.high[1:]
close(c)
return
}
if len(m.low) > 0 {
c := m.low[0]
m.low = m.low[1:]
close(c)
return
}
// We only set m.lock to false when there are
// no items to unlock because it could cause
// lock contention for the next lock to fetch it.
m.lock = false
}