go runtime_error ai_generated true

fatal error: concurrent map iteration and map write

ID: go/concurrent-map-iteration-write

Also available as: JSON · Markdown · 中文
80%Fix Rate
90%Confidence
0Evidence
2024-06-19First Seen

Version Compatibility

VersionStatusIntroducedDeprecatedNotes
1.19 active — — —
1.21 active — — —
1.22 active — — —

Root Cause

A range loop over a map runs concurrently with a write to the same map in another goroutine. The runtime detects the iterator/writer conflict and aborts.

generic

中文

对 map 的 range 迭代与另一个 goroutine 对同一 map 的写入并发执行。运行时检测到迭代器与写入冲突后终止程序。

Workarounds

  1. 95% success 
    Hold a read lock for the whole iteration and a write lock for mutations:

s.mu.RLock()
for k, v := range s.m {
    _ = k; _ = v
}
s.mu.RUnlock()
  2. 90% success 
    Snapshot keys under lock, then iterate the snapshot:

s.mu.RLock()
keys := make([]string, 0, len(s.m))
for k := range s.m { keys = append(keys, k) }
s.mu.RUnlock()
for _, k := range keys { _ = k }

Dead Ends

Common approaches that don't work:

  1. 90% fail

    Assigning a map copies only the header, not the contents; both variables share the same underlying map, so writes still race with iteration.

  2. 88% fail

    Sleeping widens or shifts the race window but does not synchronize; the fatal error still occurs under load.