Add health check endpoint so uptime tools can watch scrapers

Author: mjarkk

README.md

@@ -161,3 +161,20 @@ You can do that with the following shell variable:
 ```sh
 export RTCV_SCRAPER_CLIENT_ENV='{}'
 ```
+
+## Health check service
+
+For monitoring the health of a scraper you can start a small web service that will only return a 200 if the scraper is running.
+
+```sh
+export RTCV_SCRAPER_CLIENT_HEALTH_CHECK_PORT=2000
+rtcv_scraper_client ..
+```
+
+You should now be able to check if the scraper is running by executing the following command
+
+```sh
+curl -s -D - http://localhost:2000
+# HTTP/1.1 200 OK
+# ...
+```

health_check.go

@@ -0,0 +1,31 @@
+package main
+
+import (
+	"fmt"
+	"log"
+	"net"
+
+	"github.com/valyala/fasthttp"
+)
+
+func startHealthCheckServer(port string) {
+	requestHandler := func(ctx *fasthttp.RequestCtx) {
+		ctx.Response.AppendBody([]byte("true"))
+		ctx.Response.Header.Set("Content-Type", "application/json")
+	}
+
+	s := &fasthttp.Server{Handler: requestHandler}
+
+	address := "0.0.0.0:" + port
+	l, err := net.Listen("tcp4", address)
+	if err != nil {
+		log.Fatalf("Error, unable to start health check service at \"%s\" error %s", address, err.Error())
+	}
+
+	fmt.Println("running health check service at", address)
+
+	err = s.Serve(l)
+	if err != nil {
+		log.Fatal("Error in health check service Serve: " + err.Error())
+	}
+}

main.go

@@ -155,6 +155,11 @@ func main() {
 
 	useAddress := startWebserver(env, api, loginUsers)
 
+	healthCheckPort := os.Getenv("RTCV_SCRAPER_CLIENT_HEALTH_CHECK_PORT")
+	if healthCheckPort != "" {
+		go startHealthCheckServer(healthCheckPort)
+	}
+
 	fmt.Println("running scraper..")
 
 	if len(os.Args) <= 1 {

webserver.go

@@ -2,9 +2,9 @@ package main
 
 import (
 	"encoding/json"
-	"fmt"
 	"log"
 	"net"
+	"strconv"
 	"strings"
 	"time"
 
@@ -122,7 +122,7 @@ func startWebserver(env Env, api *API, loginUsers []EnvUser) string {
 			log.Fatal("Could not find a free port to start the webserver")
 		}
 
-		address := fmt.Sprintf("127.0.0.1:%d", portAttempt)
+		address := "127.0.0.1:" + strconv.Itoa(portAttempt)
 
 		l, err := net.Listen("tcp4", address)
 		if err != nil {