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Bugfix // Fix race condition in Pop & PopN operation of ring buffer #177

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Bugfix // Fix race condition in Pop & PopN operation of ring buffer #177

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a4d658e
reproduce race condition
mapogolions Jan 6, 2025
0d87bc3
double-checked locking to fix race condition
mapogolions Jan 6, 2025
a07464a
lock immediately
mapogolions Jan 7, 2025
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10 changes: 6 additions & 4 deletions ringbuffer/ringbuffer.go
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Original file line number Diff line number Diff line change
Expand Up @@ -56,11 +56,12 @@ func (rb *RingBuffer[T]) Len() int64 {
}

func (rb *RingBuffer[T]) Pop() (T, bool) {
if rb.Len() == 0 {
rb.mu.Lock()
if rb.len == 0 {
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@tprifti tprifti Jan 6, 2025 •
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we check for the ringbuffer length twice here, 1st time using rb.Len(), and second time using rb.len

func (rb *RingBuffer[T]) Pop() (T, bool) {
	rb.mu.Lock() // lock here to avoid reading length 2 times
	if rb.Len() == 0 {
		rb.mu.Unlock()
		var t T
		return t, false
	}
	rb.content.head = (rb.content.head + 1) % rb.content.mod
	item := rb.content.items[rb.content.head]
	var t T
	rb.content.items[rb.content.head] = t
	atomic.AddInt64(&rb.len, -1)
	rb.mu.Unlock()
	return item, true
}

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@mapogolions mapogolions Jan 6, 2025 •
edited
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Yes, it's called double-checked locking pattern. There are usually two options we can use:

  1. Lock immediately:
rb.mu.Lock()
if rb.len == 0 {
	rb.mu.Unlock()
	var t T
	return t, false
}
  1. Use double-checked locking:
if rb.Len() == 0 { // As far as I understand, we read atomically to prevent tearing read of int64.
	var t T
	return t, false
}
rb.mu.Lock()
if rb.len == 0 {
	rb.mu.Unlock()
	var t T
	return t, false
}

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Personally I would prefer the first option, since it's bit more readable and we do not need to check the rb length twice.

Good job for detecting the issue!

rb.mu.Unlock()
var t T
return t, false
}
rb.mu.Lock()
rb.content.head = (rb.content.head + 1) % rb.content.mod
item := rb.content.items[rb.content.head]
var t T
Expand All @@ -71,10 +72,11 @@ func (rb *RingBuffer[T]) Pop() (T, bool) {
}

func (rb *RingBuffer[T]) PopN(n int64) ([]T, bool) {
if rb.Len() == 0 {
rb.mu.Lock()
if rb.len == 0 {
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@tprifti tprifti Jan 6, 2025 •
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same thing here:

func (rb *RingBuffer[T]) PopN(n int64) ([]T, bool) {
	rb.mu.Lock()
	if rb.Len() == 0 {
		rb.mu.Unlock()
		return nil, false
	}
	content := rb.content
	if n >= rb.len {
		n = rb.len
	}
	atomic.AddInt64(&rb.len, -n)

	items := make([]T, n)
	for i := int64(0); i < n; i++ {
		pos := (content.head + 1 + i) % content.mod
		items[i] = content.items[pos]
		var t T
		content.items[pos] = t
	}
	content.head = (content.head + n) % content.mod

	rb.mu.Unlock()
	return items, true
}

rb.mu.Unlock()
return nil, false
}
rb.mu.Lock()
content := rb.content

if n >= rb.len {
Expand Down
56 changes: 56 additions & 0 deletions ringbuffer/ringbuffer_test.go
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Original file line number Diff line number Diff line change
@@ -1,6 +1,8 @@
package ringbuffer

import (
"sync"
"sync/atomic"
"testing"
)

Expand Down Expand Up @@ -37,3 +39,57 @@ func TestPushPopN(t *testing.T) {
}
}
}

func TestPopThreadSafety(t *testing.T) {
t.Run("Pop should be thread-safe", func(t *testing.T) {
testCase := func() {
rb := New[int](4)
rb.Push(1)
wg := sync.WaitGroup{}
for i := 0; i < 2; i++ {
wg.Add(1)
go func() {
defer wg.Done()
rb.Pop()
}()
}
wg.Wait()
if rb.Len() == -1 {
t.Fatal("item popped twice")
}
}

// Increase the number of iterations to raise the likelihood of reproducing the race condition
for i := 0; i < 100_000; i++ {
testCase()
}
})

t.Run("PopN should be thread-safe", func(t *testing.T) {
testCase := func() {
rb := New[int](4)
rb.Push(1)
counter := atomic.Int32{}
wg := sync.WaitGroup{}
for i := 0; i < 2; i++ {
wg.Add(1)
go func() {
defer wg.Done()
_, ok := rb.PopN(1)
if ok {
counter.Add(1)
}
}()
}
wg.Wait()
if counter.Load() > 1 {
t.Fatal("false positive item removal")
}
}

// Increase the number of iterations to raise the likelihood of reproducing the race condition
for i := 0; i < 100_000; i++ {
testCase()
}
})
}