Add local resolver for system containers

System containers can now be proxied to the localhost or to
all adapters using docker-compose.

Tested with NATS and Prometheus to 127.0.0.1 in multipass
and with the gateway to 0.0.0.0.

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

pkg/local_resolver.go

@@ -0,0 +1,106 @@
+package pkg
+
+import (
+	"io/ioutil"
+	"log"
+	"os"
+	"strings"
+	"sync"
+	"time"
+)
+
+type Resolver interface {
+	Start()
+	Get(upstream string, got chan<- string, timeout time.Duration)
+}
+
+type LocalResolver struct {
+	Path  string
+	Map   map[string]string
+	Mutex *sync.RWMutex
+}
+
+func NewLocalResolver(path string) Resolver {
+	return &LocalResolver{
+		Path:  path,
+		Mutex: &sync.RWMutex{},
+		Map:   make(map[string]string),
+	}
+}
+
+func (l *LocalResolver) Start() {
+	var lastStat os.FileInfo
+
+	for {
+		rebuild := false
+		if info, err := os.Stat(l.Path); err == nil {
+			if lastStat == nil {
+				rebuild = true
+			} else {
+				if !lastStat.ModTime().Equal(info.ModTime()) {
+					rebuild = true
+				}
+			}
+			lastStat = info
+		}
+
+		if rebuild {
+			log.Printf("Resolver rebuilding map")
+			l.rebuild()
+		}
+		time.Sleep(time.Second * 3)
+	}
+}
+
+func (l *LocalResolver) rebuild() {
+	l.Mutex.Lock()
+	defer l.Mutex.Unlock()
+
+	fileData, fileErr := ioutil.ReadFile(l.Path)
+	if fileErr != nil {
+		log.Printf("resolver rebuild error: %s", fileErr.Error())
+		return
+	}
+
+	lines := strings.Split(string(fileData), "\n")
+
+	for _, line := range lines {
+		index := strings.Index(line, "\t")
+
+		if len(line) > 0 && index > -1 {
+			ip := line[:index]
+			host := line[index+1:]
+			log.Printf("Resolver: %q=%q", host, ip)
+			l.Map[host] = ip
+		}
+	}
+}
+
+// Get resolve an entry
+func (l *LocalResolver) Get(upstream string, got chan<- string, timeout time.Duration) {
+	start := time.Now()
+	for {
+		if val := l.get(upstream); len(val) > 0 {
+			got <- val
+			break
+		}
+
+		if time.Now().After(start.Add(timeout)) {
+			log.Printf("Timed out after %s getting host %q", timeout.String(), upstream)
+			break
+		}
+
+		time.Sleep(time.Millisecond * 250)
+	}
+}
+
+func (l *LocalResolver) get(upstream string) string {
+	l.Mutex.RLock()
+	defer l.Mutex.RUnlock()
+
+	if val, ok := l.Map[upstream]; ok {
+		return val
+	}
+
+	return ""
+}