Extract scaling from zero

- extracting this package means it can be used in other components
such as the asynchronous nats-queue-worker which may need to
invoke functions which are scaled down to zero replicas.

Ref: https://github.com/openfaas/nats-queue-worker/issues/32

Tested on Docker Swarm for scaling up, already scaled and not
found error.

Signed-off-by: Alex Ellis (VMware) <alexellis2@gmail.com>

gateway/scaling/function_cache_test.go

@@ -0,0 +1,115 @@
+// Copyright (c) OpenFaaS Author(s). All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+package scaling
+
+import (
+	"testing"
+	"time"
+)
+
+func Test_LastRefreshSet(t *testing.T) {
+	before := time.Now()
+
+	fnName := "echo"
+
+	cache := FunctionCache{
+		Cache:  make(map[string]*FunctionMeta),
+		Expiry: time.Millisecond * 1,
+	}
+
+	if cache.Cache == nil {
+		t.Errorf("Expected cache map to be initialized")
+		t.Fail()
+	}
+
+	cache.Set(fnName, ServiceQueryResponse{AvailableReplicas: 1})
+
+	if _, exists := cache.Cache[fnName]; !exists {
+		t.Errorf("Expected entry to exist after setting %s", fnName)
+		t.Fail()
+	}
+
+	if cache.Cache[fnName].LastRefresh.Before(before) {
+		t.Errorf("Expected LastRefresh for function to have been after start of test")
+		t.Fail()
+	}
+}
+
+func Test_CacheExpiresIn1MS(t *testing.T) {
+	fnName := "echo"
+
+	cache := FunctionCache{
+		Cache:  make(map[string]*FunctionMeta),
+		Expiry: time.Millisecond * 1,
+	}
+
+	cache.Set(fnName, ServiceQueryResponse{AvailableReplicas: 1})
+	time.Sleep(time.Millisecond * 2)
+
+	_, hit := cache.Get(fnName)
+
+	wantHit := false
+
+	if hit != wantHit {
+		t.Errorf("hit, want: %v, got %v", wantHit, hit)
+	}
+}
+
+func Test_CacheGivesHitWithLongExpiry(t *testing.T) {
+	fnName := "echo"
+
+	cache := FunctionCache{
+		Cache:  make(map[string]*FunctionMeta),
+		Expiry: time.Millisecond * 500,
+	}
+
+	cache.Set(fnName, ServiceQueryResponse{AvailableReplicas: 1})
+
+	_, hit := cache.Get(fnName)
+	wantHit := true
+
+	if hit != wantHit {
+		t.Errorf("hit, want: %v, got %v", wantHit, hit)
+	}
+}
+
+func Test_CacheFunctionExists(t *testing.T) {
+	fnName := "echo"
+
+	cache := FunctionCache{
+		Cache:  make(map[string]*FunctionMeta),
+		Expiry: time.Millisecond * 10,
+	}
+
+	cache.Set(fnName, ServiceQueryResponse{AvailableReplicas: 1})
+	time.Sleep(time.Millisecond * 2)
+
+	_, hit := cache.Get(fnName)
+
+	wantHit := true
+
+	if hit != wantHit {
+		t.Errorf("hit, want: %v, got %v", wantHit, hit)
+	}
+}
+func Test_CacheFunctionNotExist(t *testing.T) {
+	fnName := "echo"
+	testName := "burt"
+
+	cache := FunctionCache{
+		Cache:  make(map[string]*FunctionMeta),
+		Expiry: time.Millisecond * 10,
+	}
+
+	cache.Set(fnName, ServiceQueryResponse{AvailableReplicas: 1})
+	time.Sleep(time.Millisecond * 2)
+
+	_, hit := cache.Get(testName)
+
+	wantHit := false
+
+	if hit != wantHit {
+		t.Errorf("hit, want: %v, got %v", wantHit, hit)
+	}
+}