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/pia/servers.go

@@ -0,0 +1,90 @@
+// Package pia contains code to obtain the server information
+// for the Private Internet Access provider.
+package pia
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net/http"
+
+	"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.PIAServer, err error) {
+	data, err := fetchAPI(ctx, client)
+	if err != nil {
+		return nil, err
+	}
+
+	for _, region := range data.Regions {
+		// Deduplicate servers with the same common name
+		commonNameToProtocols := dedupByProtocol(region)
+
+		// newServers can support only UDP or both TCP and UDP
+		newServers := dataToServers(region.Servers.UDP, region.Name,
+			region.PortForward, commonNameToProtocols)
+		servers = append(servers, newServers...)
+
+		// tcpServers only support TCP as mixed servers were found above.
+		tcpServers := dataToServers(region.Servers.TCP, region.Name,
+			region.PortForward, commonNameToProtocols)
+		servers = append(servers, tcpServers...)
+	}
+
+	if len(servers) < minServers {
+		return nil, fmt.Errorf("%w: %d and expected at least %d",
+			ErrNotEnoughServers, len(servers), minServers)
+	}
+
+	sortServers(servers)
+
+	return servers, nil
+}
+
+type protocols struct {
+	tcp bool
+	udp bool
+}
+
+// Deduplicate servers with the same common name for different protocols.
+func dedupByProtocol(region regionData) (commonNameToProtocols map[string]protocols) {
+	commonNameToProtocols = make(map[string]protocols)
+	for _, udpServer := range region.Servers.UDP {
+		protocols := commonNameToProtocols[udpServer.CN]
+		protocols.udp = true
+		commonNameToProtocols[udpServer.CN] = protocols
+	}
+	for _, tcpServer := range region.Servers.TCP {
+		protocols := commonNameToProtocols[tcpServer.CN]
+		protocols.tcp = true
+		commonNameToProtocols[tcpServer.CN] = protocols
+	}
+	return commonNameToProtocols
+}
+
+func dataToServers(data []serverData, region string, portForward bool,
+	commonNameToProtocols map[string]protocols) (
+	servers []models.PIAServer) {
+	servers = make([]models.PIAServer, 0, len(data))
+	for _, serverData := range data {
+		proto, ok := commonNameToProtocols[serverData.CN]
+		if !ok {
+			continue // server already added
+		}
+		delete(commonNameToProtocols, serverData.CN)
+		server := models.PIAServer{
+			Region:      region,
+			ServerName:  serverData.CN,
+			TCP:         proto.tcp,
+			UDP:         proto.udp,
+			PortForward: portForward,
+			IP:          serverData.IP,
+		}
+		servers = append(servers, server)
+	}
+	return servers
+}