Feat: OPENVPN_TARGET_IP overrides IP

- Check target IP matches a server for Wireguard since we need the public key
- Streamline connection picking for all providers

internal/provider/utils/pick.go

@@ -1,12 +1,51 @@
 package utils
 
 import (
+	"errors"
+	"fmt"
 	"math/rand"
+	"net"
 
+	"github.com/qdm12/gluetun/internal/configuration"
+	"github.com/qdm12/gluetun/internal/constants"
 	"github.com/qdm12/gluetun/internal/models"
 )
 
-func PickRandomConnection(connections []models.Connection,
+// PickConnection picks a connection from a pool of connections.
+// If the VPN protocol is Wireguard and the target IP is set,
+// it finds the connection corresponding to this target IP.
+// Otherwise, it picks a random connection from the pool of connections
+// and sets the target IP address as the IP if this one is set.
+func PickConnection(connections []models.Connection,
+	selection configuration.ServerSelection, randSource rand.Source) (
+	connection models.Connection, err error) {
+	if selection.TargetIP != nil && selection.VPN == constants.Wireguard {
+		// we need the right public key
+		return getTargetIPConnection(connections, selection.TargetIP)
+	}
+
+	connection = pickRandomConnection(connections, randSource)
+	if selection.TargetIP != nil {
+		connection.IP = selection.TargetIP
+	}
+
+	return connection, nil
+}
+
+func pickRandomConnection(connections []models.Connection,
 	source rand.Source) models.Connection {
 	return connections[rand.New(source).Intn(len(connections))] //nolint:gosec
 }
+
+var errTargetIPNotFound = errors.New("target IP address not found")
+
+func getTargetIPConnection(connections []models.Connection,
+	targetIP net.IP) (connection models.Connection, err error) {
+	for _, connection := range connections {
+		if targetIP.Equal(connection.IP) {
+			return connection, nil
+		}
+	}
+	return connection, fmt.Errorf("%w: in %d filtered connections",
+		errTargetIPNotFound, len(connections))
+}