A Go map thread safety problem while reading a cache DIY book - multithreading

Question

I am reading a book which teaches me how to write a simple cache like redis.
With a goal to implement a distribute hash, the project must have key migrate, which needs an iterator. And I think there may be some problems.
His book about iterating a map, but while the iteration, the hold of read lock not continuously. The reason is trying not to effect main cache process. I believe there must be a thread safety problem because the main cache thread is still writing to map. I wrote a demo, but not sure.
//book code
type inMemoryScanner struct {
pair
pair Chan *pair
closeCh chan struct{}
}
func (c *inMemoryCache) NewScanner() Scanner {
pairCh := make(chan *pair)
closeCh := make(chan struct{})
go func() {
defer close(pairCh)
c.mutex.RLock()
//the c.c is book's map
for k, v := range c.c {
c.mutex.RUnlock()
select {
case <-closeCh:
return
case pairCh <- &pair{k, v}:
}
c.mutex.RLock()
}
c.mutex.RUnLock()
}
return &inMemoryScanner{pair{}, pairCh, closeCh}
}
//my demo
func main() {
testMap := make(map[string]string)
mutex := sync.RWMutex{}
for i := 0; i < 64; i ++ {
mutex.Lock()
testMap[uuid.New().String()] = uuid.New().String()
mutex.Unlock()
fmt.Println("Write")
}
go func() {
for {
mutex.Lock()
testMap[uuid.New().String()] = uuid.New().String()
time.Sleep(100 * time.Millisecond)
mutex.Unlock()
fmt.Println("Write")
}
} ()
for k, v := range testMap {
mutex.RLock()
fmt.Println("k" + k + "v" + v)
mutex.RUnlock()
time.Sleep(100 * time.Millisecond)
}
}
In my demo, the 'Write' and the map's result amount not equal! And I believe, In an reality project, the rebalance can't be once, there must be continuous background work, doesn't it?

Answer 1

You have a data race. Your results are undefined.
Simplifying your code so that it compiles and runs,
package main
import (
"sync"
"time"
)
func main() {
testMap := make(map[string]string)
mutex := sync.RWMutex{}
for i := 0; i < 64; i++ {
mutex.Lock()
now := time.Now().String()
testMap[now] = now
mutex.Unlock()
}
go func() {
for {
mutex.Lock()
now := time.Now().String()
testMap[now] = now
time.Sleep(100 * time.Millisecond)
mutex.Unlock()
}
}()
for k, v := range testMap {
mutex.RLock()
_, _ = k, v
mutex.RUnlock()
time.Sleep(100 * time.Millisecond)
}
}
Output:
$ go run -race racer.go
==================
WARNING: DATA RACE
Read at 0x00c00008c060 by main goroutine:
runtime.mapiternext()
/home/peter/go/src/runtime/map.go:844 +0x0
main.main()
/home/peter/gopath/src/racer.go:26 +0x217
Previous write at 0x00c00008c060 by goroutine 5:
runtime.mapassign_faststr()
/home/peter/go/src/runtime/map_faststr.go:202 +0x0
main.main.func1()
/home/peter/gopath/src/racer.go:21 +0x9b
Goroutine 5 (running) created at:
main.main()
/home/peter/gopath/src/racer.go:17 +0x17b
==================
==================
WARNING: DATA RACE
Read at 0x00c0000a6638 by main goroutine:
main.main()
/home/peter/gopath/src/racer.go:26 +0x1d0
Previous write at 0x00c0000a6638 by goroutine 5:
main.main.func1()
/home/peter/gopath/src/racer.go:21 +0xb0
Goroutine 5 (running) created at:
main.main()
/home/peter/gopath/src/racer.go:17 +0x17b
==================
fatal error: concurrent map iteration and map write
goroutine 1 [running]:
runtime.throw(0x4b1eb7, 0x26)
/home/peter/go/src/runtime/panic.go:617 +0x72 fp=0xc000059e48 sp=0xc000059e18 pc=0x44d722
runtime.mapiternext(0xc000059f28)
/home/peter/go/src/runtime/map.go:851 +0x55e fp=0xc000059ed0 sp=0xc000059e48 pc=0x434c2e
main.main()
/home/peter/gopath/src/racer.go:26 +0x218 fp=0xc000059f98 sp=0xc000059ed0 pc=0x48a1f8
runtime.main()
/home/peter/go/src/runtime/proc.go:200 +0x20c fp=0xc000059fe0 sp=0xc000059f98 pc=0x44f06c
runtime.goexit()
/home/peter/go/src/runtime/asm_amd64.s:1337 +0x1 fp=0xc000059fe8 sp=0xc000059fe0 pc=0x475751
goroutine 4 [sleep]:
runtime.goparkunlock(...)
/home/peter/go/src/runtime/proc.go:307
time.Sleep(0x5f5e100)
/home/peter/go/src/runtime/time.go:105 +0x159
main.main.func1(0xc00001c280, 0xc00008c060)
/home/peter/gopath/src/racer.go:22 +0x3e
created by main.main
/home/peter/gopath/src/racer.go:17 +0x17c
exit status 2
$
You are not locking the map reads,
for k, v := range testMap {
mutex.RLock()
_, _ = k, v
mutex.RUnlock()
time.Sleep(100 * time.Millisecond)
}
for k, v := range testMap { ... } reads the map. k, v are local variables.
You need to lock the map reads,
mutex.RLock()
for k, v := range testMap {
_, _ = k, v
time.Sleep(100 * time.Millisecond)
}
mutex.RUnlock()
Go: Data Race Detector
The Go Blog: Introducing the Go Race Detector
GopherCon 2016: Keith Randall - Inside the Map Implementation