Nothing in Go is thread-safe, unless explicitly documented otherwise. Some examples of explicitly-documented-otherwise stuff are in package sync and package syn

Nothing in Go is thread-safe, unless explicitly documented otherwise. Some examples of explicitly-documented-otherwise stuff are in package sync and package sync/atomic. cycleMap is definitely not thread-safe. The authors knew this, to some extent, because they synchronized writes via an adjacent mutex. But they didn't synchronize reads thru the same mutex, which is the issue.