Concurrency mutants

The opt-in concurrency operator (-operators default,concurrency) mimics real Go concurrency bugs by inverting their fixes (Tu et al., “Understanding real-world concurrency bugs in Go”, ASPLOS 2019): defer f() removed or run on the spot, go f() run inline, a send ch <- v removed, make(chan T) given a buffer of 1 and make(chan T, n) losing its buffer (or, for a size computed at run time, growing by one), a select case whose channel becomes nil so it never fires, atomic.AddT(&x, d) / atomic.StoreT(&x, v) made the plain x += d / x = v, and once.Do(f) made f(). Removing a close, a Lock / Unlock, a WaitGroup Add / Done / Wait or a cancel() is a call statement removed, which voidcall already does.

Their kills depend on scheduling. Build the test binary with the race detector (go test -c -race, the race attribute of mutation_test), so an introduced data race fails the test that hits it, and confirm kills with run -confirm-kills N. A mutant that deadlocks runs into the per-mutant timeout and counts as killed (TIMEOUT).