Watchtower HTTP API based updates (#432)

* Added HTTP API trigger to update running images

* Adds HTTP API authentication token parameter and handling

* Exposes port 8080 in Dockerfile to allow inter-container update triggering via HTTP API

* Fixes codacy issue

* Adds API usage doc

* Fix grammar

* Moves api logic to a package of its own

* Makes WT exit if token has not been set in HTTP API mode

* Adds lock to prevent concurrent updates when in HTTP API mode

Co-authored-by: Simon Aronsson <simme@arcticbit.se>

pkg/api/api.go

@@ -0,0 +1,61 @@
+package api
+
+import (
+	"os"
+	"net/http"
+	"errors"
+	log "github.com/sirupsen/logrus"
+	"io"
+)
+
+var (
+	lock         chan bool
+)
+
+func init() {
+	lock = make(chan bool, 1)
+	lock <- true
+}
+
+func SetupHTTPUpdates(apiToken string, updateFunction func()) error {
+	if apiToken == "" {
+		return errors.New("API token is empty or has not been set. Not starting API.")
+	}
+	
+	log.Println("Watchtower HTTP API started.")
+
+	http.HandleFunc("/v1/update", func(w http.ResponseWriter, r *http.Request){
+		log.Info("Updates triggered by HTTP API request.")
+		
+		_, err := io.Copy(os.Stdout, r.Body)
+		if err != nil {
+			log.Println(err)
+			return
+		}
+
+		if r.Header.Get("Token") != apiToken {
+			log.Println("Invalid token. Not updating.")
+			return
+		}
+
+		log.Println("Valid token found. Attempting to update.")
+		
+		select {
+			case chanValue := <- lock:
+				defer func() { lock <- chanValue }()
+				updateFunction()
+			default:
+				log.Debug("Skipped. Another update already running.")
+		}
+
+		
+	})
+	
+	return nil
+}
+
+func WaitForHTTPUpdates() error {
+	log.Fatal(http.ListenAndServe(":8080", nil))
+	os.Exit(0)
+	return nil
+}