chore: do not use sentinel errors when unneeded

- main reason being it's a burden to always define sentinel errors at global scope, wrap them with `%w` instead of using a string directly
- only use sentinel errors when it has to be checked using `errors.Is`
- replace all usage of these sentinel errors in `fmt.Errorf` with direct strings that were in the sentinel error
- exclude the sentinel error definition requirement from .golangci.yml
- update unit tests to use ContainersError instead of ErrorIs so it stays as a "not a change detector test" without requiring a sentinel error

internal/routing/default.go

@@ -1,15 +1,12 @@
 package routing
 
 import (
-	"errors"
 	"fmt"
 	"net/netip"
 
 	"github.com/qdm12/gluetun/internal/netlink"
 )
 
-var ErrRouteDefaultNotFound = errors.New("default route not found")
-
 type DefaultRoute struct {
 	NetInterface string
 	Gateway      netip.Addr
@@ -60,7 +57,7 @@ func (r *Routing) DefaultRoutes() (defaultRoutes []DefaultRoute, err error) {
 	}
 
 	if len(defaultRoutes) == 0 {
-		return nil, fmt.Errorf("%w: in %d route(s)", ErrRouteDefaultNotFound, len(routes))
+		return nil, fmt.Errorf("default route not found: in %d route(s)", len(routes))
 	}
 
 	return defaultRoutes, nil

internal/routing/errors.go

@@ -1,7 +0,0 @@
-package routing
-
-import (
-	"errors"
-)
-
-var ErrLinkDefaultNotFound = errors.New("default link not found")

internal/routing/ip.go

@@ -1,7 +1,6 @@
 package routing
 
 import (
-	"errors"
 	"fmt"
 	"net"
 	"net/netip"
@@ -14,8 +13,6 @@ func ipIsPrivate(ip netip.Addr) bool {
 		ip.IsLinkLocalUnicast() || ip.IsLinkLocalMulticast()
 }
 
-var errInterfaceIPNotFound = errors.New("IP address not found for interface")
-
 func ipMatchesFamily(ip netip.Addr, family uint8) bool {
 	return (family == netlink.FamilyV4 && ip.Is4()) ||
 		(family == netlink.FamilyV6 && ip.Is6())
@@ -46,6 +43,6 @@ func (r *Routing) AssignedIP(interfaceName string, family uint8) (ip netip.Addr,
 
 		return ip, nil
 	}
-	return ip, fmt.Errorf("%w: interface %s in %d addresses",
-		errInterfaceIPNotFound, interfaceName, len(addresses))
+	return ip, fmt.Errorf("IP address not found for interface: interface %s in %d addresses",
+		interfaceName, len(addresses))
 }

internal/routing/local.go

@@ -1,19 +1,12 @@
 package routing
 
 import (
-	"errors"
 	"fmt"
 	"net/netip"
 
 	"github.com/qdm12/gluetun/internal/netlink"
 )
 
-var (
-	ErrLinkLocalNotFound     = errors.New("local link not found")
-	ErrSubnetDefaultNotFound = errors.New("default subnet not found")
-	ErrSubnetLocalNotFound   = errors.New("local subnet not found")
-)
-
 type LocalNetwork struct {
 	IPNet         netip.Prefix
 	InterfaceName string
@@ -38,7 +31,7 @@ func (r *Routing) LocalNetworks() (localNetworks []LocalNetwork, err error) {
 	}
 
 	if len(localLinks) == 0 {
-		return localNetworks, fmt.Errorf("%w: in %d links", ErrLinkLocalNotFound, len(links))
+		return localNetworks, fmt.Errorf("local link not found: in %d links", len(links))
 	}
 
 	routes, err := r.netLinker.RouteList(netlink.FamilyAll)
@@ -82,7 +75,7 @@ func (r *Routing) LocalNetworks() (localNetworks []LocalNetwork, err error) {
 	}
 
 	if len(localNetworks) == 0 {
-		return localNetworks, fmt.Errorf("%w: in %d routes", ErrSubnetLocalNotFound, len(routes))
+		return localNetworks, fmt.Errorf("local subnet not found: in %d routes", len(routes))
 	}
 
 	return localNetworks, nil

internal/routing/vpn.go

@@ -1,18 +1,12 @@
 package routing
 
 import (
-	"errors"
 	"fmt"
 	"net/netip"
 
 	"github.com/qdm12/gluetun/internal/netlink"
 )
 
-var (
-	ErrVPNLocalGatewayIPNotFound       = errors.New("VPN local gateway IP address not found")
-	ErrVPNLocalGatewayIPv6NotSupported = errors.New("VPN local gateway IPv6 address not supported")
-)
-
 func (r *Routing) VPNLocalGatewayIP(vpnIntf string) (ip netip.Addr, err error) {
 	vpnLink, err := r.netLinker.LinkByName(vpnIntf)
 	if err != nil {
@@ -36,7 +30,7 @@ func (r *Routing) VPNLocalGatewayIP(vpnIntf string) (ip netip.Addr, err error) {
 			route.Dst.Addr().Compare(route.Src.Addr()) == 0 &&
 			route.Table == tableLocal: // Wireguard
 			if route.Src.Addr().Is6() {
-				return netip.Addr{}, fmt.Errorf("%w: %s", ErrVPNLocalGatewayIPv6NotSupported, route.Src)
+				return netip.Addr{}, fmt.Errorf("VPN local gateway IPv6 address not supported: %s", route.Src)
 			}
 			bytes := route.Src.Addr().As4()
 			// force last byte to 1 to get the VPN gateway IP
@@ -45,11 +39,9 @@ func (r *Routing) VPNLocalGatewayIP(vpnIntf string) (ip netip.Addr, err error) {
 			return netip.AddrFrom4(bytes), nil
 		}
 	}
-	return ip, fmt.Errorf("%w: in %d routes", ErrVPNLocalGatewayIPNotFound, len(routes))
+	return ip, fmt.Errorf("VPN local gateway IP address not found: in %d routes", len(routes))
 }
 
-var ErrVPNRouteNotFound = errors.New("VPN route not found")
-
 // VPNRoutes returns the routes that are using the VPN interface, excluding local routes
 // and link-local multicast and unicast routes.
 func (r *Routing) VPNRoutes(vpnIntf string) (routes []netlink.Route, err error) {
@@ -80,8 +72,8 @@ func (r *Routing) VPNRoutes(vpnIntf string) (routes []netlink.Route, err error)
 	}
 
 	if len(routes) == 0 {
-		return nil, fmt.Errorf("%w: for interface %s in %d routes",
-			ErrVPNRouteNotFound, vpnIntf, len(allRoutes))
+		return nil, fmt.Errorf("VPN route not found: for interface %s in %d routes",
+			vpnIntf, len(allRoutes))
 	}
 
 	return routes, nil