The Race Detector

One possible output:

last fetched: https://example.com/b
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The program compiles, passes go vet, and prints a sensible line. It also contains a data race, as defined in Data Races: two goroutines write lastURL, and nothing orders the two writes. Careful reading finds this race. A real program, however, has thousands of lines and many goroutines, and careful reading cannot check that much code reliably.

Go ships a tool that finds such races while the program runs