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.go

@@ -0,0 +1,63 @@
+// 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 (
+	"sync"
+	"time"
+)
+
+// FunctionMeta holds the last refresh and any other
+// meta-data needed for caching.
+type FunctionMeta struct {
+	LastRefresh          time.Time
+	ServiceQueryResponse ServiceQueryResponse
+}
+
+// Expired find out whether the cache item has expired with
+// the given expiry duration from when it was stored.
+func (fm *FunctionMeta) Expired(expiry time.Duration) bool {
+	return time.Now().After(fm.LastRefresh.Add(expiry))
+}
+
+// FunctionCache provides a cache of Function replica counts
+type FunctionCache struct {
+	Cache  map[string]*FunctionMeta
+	Expiry time.Duration
+	Sync   sync.Mutex
+}
+
+// Set replica count for functionName
+func (fc *FunctionCache) Set(functionName string, serviceQueryResponse ServiceQueryResponse) {
+	fc.Sync.Lock()
+	defer fc.Sync.Unlock()
+
+	if _, exists := fc.Cache[functionName]; !exists {
+		fc.Cache[functionName] = &FunctionMeta{}
+	}
+
+	entry := fc.Cache[functionName]
+	entry.LastRefresh = time.Now()
+	entry.ServiceQueryResponse = serviceQueryResponse
+
+}
+
+// Get replica count for functionName
+func (fc *FunctionCache) Get(functionName string) (ServiceQueryResponse, bool) {
+
+	fc.Sync.Lock()
+	defer fc.Sync.Unlock()
+
+	replicas := ServiceQueryResponse{
+		AvailableReplicas: 0,
+	}
+
+	hit := false
+	if val, exists := fc.Cache[functionName]; exists {
+		replicas = val.ServiceQueryResponse
+		hit = !val.Expired(fc.Expiry)
+	}
+
+	return replicas, hit
+}