Extract a caching function_query type

This type abstracts the function_query type and introduces an
interface for testing and substitution.

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

gateway/scaling/function_query.go

@@ -0,0 +1,60 @@
+// 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 "fmt"
+
+type CachedFunctionQuery struct {
+	cache            FunctionCacher
+	serviceQuery     ServiceQuery
+	emptyAnnotations map[string]string
+}
+
+func NewCachedFunctionQuery(cache FunctionCacher, serviceQuery ServiceQuery) FunctionQuery {
+	return &CachedFunctionQuery{
+		cache:            cache,
+		serviceQuery:     serviceQuery,
+		emptyAnnotations: map[string]string{},
+	}
+}
+
+func (c *CachedFunctionQuery) GetAnnotations(name string, namespace string) (annotations map[string]string, err error) {
+	res, err := c.Get(name, namespace)
+	if err != nil {
+		return c.emptyAnnotations, err
+	}
+
+	if res.Annotations == nil {
+		return c.emptyAnnotations, nil
+	}
+	return *res.Annotations, nil
+}
+
+func (c *CachedFunctionQuery) Get(fn string, ns string) (ServiceQueryResponse, error) {
+
+	query, hit := c.cache.Get(fn, ns)
+	if !hit {
+
+		// If there is a cache miss, then fetch the value from the provider API
+		queryResponse, err := c.serviceQuery.GetReplicas(fn, ns)
+		if err != nil {
+			return ServiceQueryResponse{}, err
+		}
+		c.cache.Set(fn, ns, queryResponse)
+	}
+
+	// At this point the value almost certainly must be present, so if not
+	// return an error.
+	query, hit = c.cache.Get(fn, ns)
+	if !hit {
+		return ServiceQueryResponse{}, fmt.Errorf("error with cache key: %s", fn+"."+ns)
+	}
+
+	return query, nil
+}
+
+type FunctionQuery interface {
+	Get(name string, namespace string) (ServiceQueryResponse, error)
+	GetAnnotations(name string, namespace string) (annotations map[string]string, err error)
+}