Maint: rework publicip package

- Use loopstate package
- Loop interface composition
- Return concrete struct from constructors
- Split into more files
- Add publicip/state package

internal/publicip/fetch.go

@@ -0,0 +1,72 @@
+// Package publicip defines interfaces to get your public IP address.
+package publicip
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"math/rand"
+	"net"
+	"net/http"
+	"strings"
+)
+
+type Fetcher interface {
+	FetchPublicIP(ctx context.Context) (ip net.IP, err error)
+}
+
+type Fetch struct {
+	client   *http.Client
+	randIntn func(n int) int
+}
+
+func NewFetch(client *http.Client) *Fetch {
+	return &Fetch{
+		client:   client,
+		randIntn: rand.Intn,
+	}
+}
+
+var ErrParseIP = errors.New("cannot parse IP address")
+
+func (f *Fetch) FetchPublicIP(ctx context.Context) (ip net.IP, err error) {
+	urls := []string{
+		"https://ifconfig.me/ip",
+		"http://ip1.dynupdate.no-ip.com:8245",
+		"http://ip1.dynupdate.no-ip.com",
+		"https://api.ipify.org",
+		"https://diagnostic.opendns.com/myip",
+		"https://domains.google.com/checkip",
+		"https://ifconfig.io/ip",
+		"https://ipinfo.io/ip",
+	}
+	url := urls[f.randIntn(len(urls))]
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return nil, err
+	}
+
+	response, err := f.client.Do(req)
+	if err != nil {
+		return nil, err
+	}
+	defer response.Body.Close()
+
+	if response.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("%w from %s: %s", ErrBadStatusCode, url, response.Status)
+	}
+
+	content, err := io.ReadAll(response.Body)
+	if err != nil {
+		return nil, fmt.Errorf("%w: %s", ErrCannotReadBody, err)
+	}
+
+	s := strings.ReplaceAll(string(content), "\n", "")
+	ip = net.ParseIP(s)
+	if ip == nil {
+		return nil, fmt.Errorf("%w: %s", ErrParseIP, s)
+	}
+	return ip, nil
+}