Upsert method - inserts new or updates existing element

Upsert method uses callback which return value is inserted
into a map. Callback is given 3 arguments:
  - exists - whether given key already exists in map
  - valueInMap - existing value in map
  - valueBeingAdded - value which was passed to Upsert method

Return value of a callback is used to save new value in map.

This approach allows map users to do non-trivial transformations
of value stored in map, while still protected by a lock

It is somewhat ovelaps with orcaman#29, but more generic and allows
more use cases

concurrent_map.go

@@ -50,6 +50,23 @@ func (m *ConcurrentMap) Set(key string, value interface{}) {
 	shard.Unlock()
 }
 
+// Callback to return new element to be inserted into the map
+// It is called while lock is held, therefore it MUST NOT
+// try to access other keys in same map, as it can lead to deadlock since
+// Go sync.RWLock is not reentrant
+type UpsertCb func(exist bool, valueInMap interface{}, newValue interface{}) interface{}
+
+// Insert or Update - updates existing element or inserts a new one using UpsertCb
+func (m *ConcurrentMap) Upsert(key string, value interface{}, cb UpsertCb) (res interface{}) {
+	shard := m.GetShard(key)
+	shard.Lock()
+	v, ok := shard.items[key]
+	res = cb(ok, v, value)
+	shard.items[key] = res
+	shard.Unlock()
+	return res
+}
+
 // Sets the given value under the specified key if no value was associated with it.
 func (m *ConcurrentMap) SetIfAbsent(key string, value interface{}) bool {
 	// Get map shard.

concurrent_map_test.go

@@ -327,3 +327,56 @@ func TestFnv32(t *testing.T) {
 		t.Errorf("Bundled fnv32 produced %d, expected result from hash/fnv32 is %d", fnv32(key), hasher.Sum32())
 	}
 }
+
+func TestUpsert(t *testing.T) {
+	dolphin := Animal{"dolphin"}
+	whale := Animal{"whale"}
+	tiger := Animal{"tiger"}
+	lion := Animal{"lion"}
+
+	cb := func(exists bool, valueInMap interface{}, newValue interface{}) interface{} {
+		nv := newValue.(Animal)
+		if !exists {
+			return []Animal{nv}
+		}
+		res := valueInMap.([]Animal)
+		return append(res, nv)
+	}
+
+	m := New()
+	m.Set("marine", []Animal{dolphin})
+	m.Upsert("marine", whale, cb)
+	m.Upsert("predator", tiger, cb)
+	m.Upsert("predator", lion, cb)
+
+	if m.Count() != 2 {
+		t.Error("map should contain exactly two elements.")
+	}
+
+	compare := func(a, b []Animal) bool {
+		if a == nil || b == nil {
+			return false
+		}
+
+		if len(a) != len(b) {
+			return false
+		}
+
+		for i, v := range a {
+			if v != b[i] {
+				return false
+			}
+		}
+		return true
+	}
+
+	marineAnimals, ok := m.Get("marine")
+	if !ok || !compare(marineAnimals.([]Animal), []Animal{dolphin, whale}) {
+		t.Error("Set, then Upsert failed")
+	}
+
+	predators, ok := m.Get("predator")
+	if !ok || !compare(predators.([]Animal), []Animal{tiger, lion}) {
+		t.Error("Upsert, then Upsert failed")
+	}
+}