I'm logging every second the length of a map; I don't care if I have the "exact" value / race conditions (off by one is ok). I'm interested to know if this could cause a panic and if I have to enclose len() with some .RLock()/Unlock() or not.
I'm asking because concurrent reads/writes in a map will cause a panic (Go detects that) but I don't know if reading the length counts as a "read". I have tried with a test program but cannot produce a crash but I'd rather have an exact answer, at least for the sake of it.
If it matters I'm interested in both len for arrays and for maps. Thanks!
It is a race condition. The results are undefined. For example,
racer.go:
package main
func main() {
m := make(map[int]int)
l := 0
go func() {
for {
l = len(m)
}
}()
for i := 0; i < 10000; i++ {
m[i] = i
}
}
Output:
$ go run -race racer.go
==================
WARNING: DATA RACE
Read at 0x00c00008e000 by goroutine 5:
main.main.func1()
/home/peter/gopath/src/racer.go:8 +0x5f
Previous write at 0x00c00008e000 by main goroutine:
runtime.mapassign_fast64()
/home/peter/go/src/runtime/map_fast64.go:92 +0x0
main.main()
/home/peter/gopath/src/racer.go:12 +0xba
Goroutine 5 (running) created at:
main.main()
/home/peter/gopath/src/racer.go:6 +0x92
==================
Found 1 data race(s)
exit status 66
$
References: