How should I avoid - literal copies lock value from

[Kasun Vithanage]

I'm implementing some thread safe data structures

type Set struct {
   m   map[string]int
   mux sync.Mutex
}

func New() *Set {
   return &Set{m: make(map[string]int)}
}

In my Diff function i need to pass an array of sets to do a diff on another set

func (set *Set) DiffS(sets []Set) *Set {
   set.mux.Lock()
   defer set.mux.Unlock()
   dup := duplicateMap(set.m)

   for i := 0; i < len(sets); i++ {
      for _, key := range sets[i].Elems() {
         delete(dup, key)
      }
   }

   return &Set{m: dup}
}

Code works fine, i ve written a unit test like this

func TestSet_Diff(t *testing.T) {
   set1 := New()
   set2 := New()
   set3 := New()

   set1.Add([]string{"a", "b", "c", "d"})
   set2.Add([]string{"c"})
   set3.Add([]string{"a", "b"})

   el := set1.Diff([]Set{*set2, *set3})
   testsuite.ContainsElements(t, []string{"d"}, el)
}

This Unit Test passes fine

But when i run the vet command with

go vet ./...

I get the following error

internal\types\set\set_test.go:41: literal copies lock value from *set2: set.Set contains sync.Mutex
internal\types\set\set_test.go:41: literal copies lock value from *set3: set.Set contains sync.Mutex

What is the ideal solution for this? 

[Kasun Vithanage]

Error is gone when used 

mux *sync.Mutex

[Dave Cheney]

Pass a pointer, *Set into your Diff method.

[Kasun Vithanage]

I want an slice of sets

[jake...@gmail.com]

I don't see anything fundamentally wrong with using a *sync.Mutex as long as users always use New(). It allows Set to be passed by value, which is nice.





But if you keep the mutex as a non pointer, then you probably want to pass a slice of set pointers:

func (set *Set) Diff(sets []*Set) *Set

This avoids copying the mutex when you create your slice, so the test line becomes:

set1.Diff([]*Set{set2, set3})

Of course your original Diff code is technically ok. The problem is in creating a []Set from individual Set objects without violating the mutex copy rules is difficult, and would require a bunch of code.