Merge pull request #244 from coinbase/patrick/just-priority-lock

[utils] Implement PriorityMutex

utils/priority_mutex.go

@@ -0,0 +1,86 @@
+// 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{}
+
+	m sync.Mutex
+	l 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) {
+	m.m.Lock()
+
+	if !m.l {
+		m.l = true
+		m.m.Unlock()
+		return
+	}
+
+	c := make(chan struct{})
+	if priority {
+		m.high = append(m.high, c)
+	} else {
+		m.low = append(m.low, c)
+	}
+
+	m.m.Unlock()
+	<-c
+}
+
+// Unlock selects the next highest priority lock
+// to grant. If there are no locks to grant, it
+// sets the value of m.l to false.
+func (m *PriorityMutex) Unlock() {
+	m.m.Lock()
+	defer m.m.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.l to false when there are
+	// no items to unlock because it could cause
+	// lock contention for the next lock to fetch it.
+	m.l = false
+}

utils/priority_mutex_test.go

@@ -0,0 +1,73 @@
+// 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 (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"golang.org/x/sync/errgroup"
+)
+
+func TestPriorityMutex(t *testing.T) {
+	arr := []bool{}
+	expected := make([]bool, 60)
+	l := new(PriorityMutex)
+	g, _ := errgroup.WithContext(context.Background())
+
+	// Lock while adding all locks
+	l.Lock(true)
+
+	// Add a bunch of low prio items
+	for i := 0; i < 50; i++ {
+		expected[i+10] = false
+		g.Go(func() error {
+			l.Lock(false)
+			arr = append(arr, false)
+			l.Unlock()
+			return nil
+		})
+	}
+
+	// Add a few high prio items
+	for i := 0; i < 10; i++ {
+		expected[i] = true
+		g.Go(func() error {
+			l.Lock(true)
+			arr = append(arr, true)
+			l.Unlock()
+			return nil
+		})
+	}
+
+	// Wait for all goroutines to ask for lock
+	time.Sleep(1 * time.Second)
+
+	// Ensure number of expected locks is correct
+	assert.Len(t, l.high, 10)
+	assert.Len(t, l.low, 50)
+
+	l.Unlock()
+	assert.NoError(t, g.Wait())
+
+	// Check results array to ensure all of the high priority items processed first,
+	// followed by all of the low priority items.
+	assert.Equal(t, expected, arr)
+
+	// Ensure lock is no longer occupied
+	assert.False(t, l.l)
+}