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/providers/mullvad/servers.go

@@ -0,0 +1,68 @@
+// Package mullvad contains code to obtain the server information
+// for the Mullvad provider.
+package mullvad
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net/http"
+	"strconv"
+
+	"github.com/qdm12/gluetun/internal/models"
+)
+
+var ErrNotEnoughServers = errors.New("not enough servers found")
+
+func GetServers(ctx context.Context, client *http.Client, minServers int) (
+	servers []models.MullvadServer, err error) {
+	data, err := fetchAPI(ctx, client)
+	if err != nil {
+		return nil, err
+	}
+
+	hts := make(hostToServer)
+	for _, serverData := range data {
+		if err := hts.add(serverData); err != nil {
+			return nil, err
+		}
+	}
+
+	if len(hts) < minServers {
+		return nil, fmt.Errorf("%w: %d and expected at least %d",
+			ErrNotEnoughServers, len(hts), minServers)
+	}
+
+	servers = hts.toServersSlice()
+
+	servers = groupByProperties(servers)
+
+	sortServers(servers)
+
+	return servers, nil
+}
+
+// TODO group by hostname so remove this.
+func groupByProperties(serversByHost []models.MullvadServer) (serversByProps []models.MullvadServer) {
+	propsToServer := make(map[string]models.MullvadServer, len(serversByHost))
+	for _, server := range serversByHost {
+		key := server.Country + server.City + server.ISP + strconv.FormatBool(server.Owned)
+		serverByProps, ok := propsToServer[key]
+		if !ok {
+			serverByProps.Country = server.Country
+			serverByProps.City = server.City
+			serverByProps.ISP = server.ISP
+			serverByProps.Owned = server.Owned
+		}
+		serverByProps.IPs = append(serverByProps.IPs, server.IPs...)
+		serverByProps.IPsV6 = append(serverByProps.IPsV6, server.IPsV6...)
+		propsToServer[key] = serverByProps
+	}
+
+	serversByProps = make([]models.MullvadServer, 0, len(propsToServer))
+	for _, serverByProp := range propsToServer {
+		serversByProps = append(serversByProps, serverByProp)
+	}
+
+	return serversByProps
+}