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.
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: