Maintenance: refactor servers updater code

- Require at least 80% of number of servers now to pass
- Each provider is in its own package with a common structure
- Unzip package with unzipper interface
- Openvpn package with extraction and download functions

internal/updater/openvpn/extract.go

@@ -0,0 +1,66 @@
+package openvpn
+
+import (
+	"errors"
+	"fmt"
+	"net"
+	"strings"
+)
+
+var (
+	ErrNoRemoteHost = errors.New("remote host not found")
+	ErrNoRemoteIP   = errors.New("remote IP not found")
+)
+
+func ExtractHost(b []byte) (host, warning string, err error) {
+	const (
+		rejectIP     = true
+		rejectDomain = false
+	)
+	hosts := extractRemoteHosts(b, rejectIP, rejectDomain)
+	if len(hosts) == 0 {
+		return "", "", ErrNoRemoteHost
+	} else if len(hosts) > 1 {
+		warning = fmt.Sprintf(
+			"only using the first host %q and discarding %d other hosts",
+			hosts[0], len(hosts)-1)
+	}
+	return hosts[0], warning, nil
+}
+
+func ExtractIP(b []byte) (ip net.IP, warning string, err error) {
+	const (
+		rejectIP     = false
+		rejectDomain = true
+	)
+	ips := extractRemoteHosts(b, rejectIP, rejectDomain)
+	if len(ips) == 0 {
+		return nil, "", ErrNoRemoteIP
+	} else if len(ips) > 1 {
+		warning = fmt.Sprintf(
+			"only using the first IP address %s and discarding %d other hosts",
+			ips[0], len(ips)-1)
+	}
+	return net.ParseIP(ips[0]), warning, nil
+}
+
+func extractRemoteHosts(content []byte, rejectIP, rejectDomain bool) (hosts []string) {
+	lines := strings.Split(string(content), "\n")
+	for _, line := range lines {
+		if !strings.HasPrefix(line, "remote ") {
+			continue
+		}
+		fields := strings.Fields(line)
+		if len(fields) == 1 || len(fields[1]) == 0 {
+			continue
+		}
+		host := fields[1]
+		parsedIP := net.ParseIP(host)
+		if (rejectIP && parsedIP != nil) ||
+			(rejectDomain && parsedIP == nil) {
+			continue
+		}
+		hosts = append(hosts, host)
+	}
+	return hosts
+}