Rewrite project, add daily update of services list

The project was rewritten from Elixir to Go, primarily because:

- I don't write Elixir anymore and don't want to maintain a project in a
  language I no longer write
- I already write Go for other projects, including my day job, so it's
  a safer bet for a project that I want to maintain long term
- Go allows me to build portable executables that will make it easier
  for others to run farside on their own machines

The Go version of Farsside also has a built in task to fetch the latest
services{-full}.json file from the repo and ingest it, which makes
running a farside server a lot simpler.

It also automatically fetches the latest instance state from
https://farside.link unless configured as a primary farside node, which
will allow others to use farside without increasing traffic to all
instances that are queried by farside (just to the farside node itself).

db/cron.go

@@ -0,0 +1,139 @@
+package db
+
+import (
+	"encoding/json"
+	"fmt"
+	"io"
+	"log"
+	"net/http"
+	"os"
+	"strings"
+	"time"
+
+	"github.com/benbusby/farside/services"
+	"github.com/robfig/cron/v3"
+)
+
+const defaultPrimary = "https://farside.link/state"
+const defaultCFPrimary = "https://cf.farside.link/state"
+
+var LastUpdate time.Time
+
+func InitCronTasks() {
+	log.Println("Initializing cron tasks...")
+
+	cronDisabled := os.Getenv("FARSIDE_CRON")
+	if len(cronDisabled) == 0 || cronDisabled == "1" {
+		c := cron.New()
+		c.AddFunc("@every 10m", queryServiceInstances)
+		c.AddFunc("@daily", updateServiceList)
+		c.Start()
+	}
+
+	queryServiceInstances()
+}
+
+func updateServiceList() {
+	fileName := services.GetServicesFileName()
+	_, _ = services.FetchServicesFile(fileName)
+	services.InitializeServices()
+}
+
+func queryServiceInstances() {
+	log.Println("Starting instance queries...")
+
+	isPrimary := os.Getenv("FARSIDE_PRIMARY")
+	if len(isPrimary) == 0 || isPrimary != "1" {
+		remoteServices, err := fetchInstancesFromPrimary()
+		if err != nil {
+			log.Println("Unable to fetch instances from primary", err)
+		}
+
+		for _, service := range remoteServices {
+			SetInstances(service.Type, service.Instances)
+		}
+
+		return
+	}
+
+	for _, service := range services.ServiceList {
+		fmt.Printf("===== %s =====\n", service.Type)
+		var instances []string
+		for _, instance := range service.Instances {
+			testURL := strings.ReplaceAll(
+				service.TestURL,
+				"<%=query%>",
+				"current+weather")
+			available := queryServiceInstance(
+				instance,
+				testURL,
+			)
+
+			if available {
+				instances = append(instances, instance)
+			}
+		}
+
+		SetInstances(service.Type, instances)
+	}
+
+	LastUpdate = time.Now().UTC()
+}
+
+func fetchInstancesFromPrimary() ([]services.Service, error) {
+	primaryURL := defaultPrimary
+	useCF := os.Getenv("FARSIDE_CF_ENABLED")
+	if len(useCF) > 0 && useCF == "1" {
+		primaryURL = defaultCFPrimary
+	}
+
+	resp, err := http.Get(primaryURL)
+	if err != nil {
+		return nil, err
+	}
+
+	defer resp.Body.Close()
+
+	bodyBytes, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, err
+	}
+
+	var serviceList []services.Service
+	err = json.Unmarshal(bodyBytes, &serviceList)
+	return serviceList, err
+}
+
+func queryServiceInstance(instance, testURL string) bool {
+	testMode := os.Getenv("FARSIDE_TEST")
+	if len(testMode) > 0 && testMode == "1" {
+		return true
+	}
+
+	ua := "Mozilla/5.0 (compatible; Farside/1.0.0; +https://farside.link)"
+	url := instance + testURL
+
+	req, err := http.NewRequest(http.MethodGet, url, nil)
+	if err != nil {
+		fmt.Println("    [ERRO] Failed to create new http request!", err)
+		return false
+	}
+
+	req.Header.Set("User-Agent", ua)
+	client := &http.Client{
+		Timeout: 10 * time.Second,
+	}
+	resp, err := client.Do(req)
+
+	if err != nil {
+		fmt.Println("    [ERRO] Error fetching instance:", err)
+		return false
+	} else if resp.StatusCode != http.StatusOK {
+		fmt.Printf("    [WARN] Received non-200 status for %s\n", url)
+		return false
+	} else {
+		fmt.Printf("    [INFO] Received 200 status for %s\n", url)
+	}
+
+	return true
+}