Guideline - Copying Slices and Maps at the Edges

Receiving Slices and Maps

Remember that when a map or a slice is passed as a function parameter, if you store a reference to them, users can modify them.

Non-recommended approach:

func (d *Driver) SetTrips(trips []Trip) {
  d.trips = trips
}

trips := ...
d1.SetTrips(trips)

// Are you trying to modify d1.trips?
trips[0] = ...

Recommended approach:

func (d *Driver) SetTrips(trips []Trip) {
  d.trips = make([]Trip, len(trips))
  copy(d.trips, trips)
}

trips := ...
d1.SetTrips(trips)

// Here we modify trips[0], but it won't affect d1.trips
trips[0] = ...

Returning Slices or Maps

Similarly, be mindful of users modifying a map or slice that exposes internal state.

Non-recommended approach:

type Stats struct {
  mu sync.Mutex
  counters map[string]int
}

// Snapshot returns the current state
func (s *Stats) Snapshot() map[string]int {
  s.mu.Lock()
  defer s.mu.Unlock()

  return s.counters
}

// snapshot is no longer protected by the mutex
// so any access to snapshot will be subject to data race and affect stats.counters
snapshot := stats.Snapshot()

Recommended approach:

type Stats struct {
  mu sync.Mutex
  counters map[string]int
}

func (s *Stats) Snapshot() map[string]int {
  s.mu.Lock()
  defer s.mu.Unlock()

  result := make(map[string]int, len(s.counters))
  for k, v := range s.counters {
    result[k] = v
  }
  return result
}

// snapshot is now a copy
snapshot := stats.Snapshot()