Feature for probing functions

Introduces a single-flight call to a function's health
endpoint to verify that it is registered with an Istio
sidecar (Envoy) before letting the invocation through.

Results are cached for 5 seconds, before a probe is
required again.

Tested without Istio, with probe_functions environment
variable set to true, I saw a probe execute in the logs.

Fixes: #1721 for Istio users.

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

gateway/handlers/probe_handler.go

@@ -0,0 +1,45 @@
+package handlers
+
+import (
+	"fmt"
+	"net/http"
+
+	"golang.org/x/sync/singleflight"
+
+	"github.com/openfaas/faas/gateway/pkg/middleware"
+	"github.com/openfaas/faas/gateway/probing"
+)
+
+func MakeProbeHandler(prober probing.FunctionProber, cache probing.ProbeCacher, resolver middleware.BaseURLResolver, next http.HandlerFunc, defaultNamespace string) http.HandlerFunc {
+
+	group := singleflight.Group{}
+
+	return func(w http.ResponseWriter, r *http.Request) {
+		functionName, namespace := middleware.GetNamespace(defaultNamespace, middleware.GetServiceName(r.URL.String()))
+
+		key := fmt.Sprintf("Probe-%s.%s", functionName, namespace)
+		res, _, _ := group.Do(key, func() (interface{}, error) {
+
+			cached, hit := cache.Get(functionName, namespace)
+			var probeResult probing.FunctionProbeResult
+			if hit && cached != nil && cached.Available {
+				probeResult = *cached
+			} else {
+				probeResult = prober.Probe(functionName, namespace)
+				cache.Set(functionName, namespace, &probeResult)
+			}
+
+			return probeResult, nil
+		})
+
+		fnRes := res.(probing.FunctionProbeResult)
+
+		if !fnRes.Available {
+			http.Error(w, fmt.Sprintf("unable to probe function endpoint %s", fnRes.Error),
+				http.StatusServiceUnavailable)
+			return
+		}
+
+		next(w, r)
+	}
+}