snet: fix UDP port selection on Windows (#4656)

SNET is checking the wrong error code when probing for available UDP ports.
Windows socket error codes are completely different from Linux, thus different OSes must be distinguished.

pkg/snet/BUILD.bazel

@@ -13,6 +13,8 @@ go_library(
         "router.go",
         "scmp.go",
         "snet.go",
+        "sock_error_posix.go",
+        "sock_error_windows.go",
         "svcaddr.go",
         "udpaddr.go",
         "writer.go",
@@ -37,7 +39,12 @@ go_library(
         "//private/topology:go_default_library",
         "//private/topology/underlay:go_default_library",
         "@com_github_gopacket_gopacket//:go_default_library",
-    ],
+    ] + select({
+        "@io_bazel_rules_go//go/platform:windows": [
+            "@org_golang_x_sys//windows:go_default_library",
+        ],
+        "//conditions:default": [],
+    }),
 )
 
 go_test(

pkg/snet/snet.go

@@ -38,10 +38,8 @@ package snet
 
 import (
 	"context"
-	"errors"
 	"net"
 	"net/netip"
-	"syscall"
 
 	"github.com/scionproto/scion/pkg/addr"
 	"github.com/scionproto/scion/pkg/log"
@@ -195,7 +193,7 @@ func listenUDPRange(addr *net.UDPAddr, start, end uint16) (*net.UDPConn, error)
 	// by longer-lived applications, e.g., server applications.
 	//
 	// Ideally we would only take a standard ephemeral range, e.g., 32768-65535,
-	// Unfortunately, this range was ocuppied by the old dispatcher.
+	// Unfortunately, this range was occupied by the old dispatcher.
 	// The default range for the dispatched ports is 31000-32767.
 	// By configuration other port ranges may be defined and restricting to the default
 	// range for applications may cause problems.
@@ -213,12 +211,12 @@ func listenUDPRange(addr *net.UDPAddr, start, end uint16) (*net.UDPConn, error)
 		if err == nil {
 			return pconn, nil
 		}
-		if errors.Is(err, syscall.EADDRINUSE) {
+		if errorIsAddrUnavailable(err) {
 			continue
 		}
 		return nil, err
 	}
-	return nil, serrors.Wrap("binding to port range", syscall.EADDRINUSE,
+	return nil, serrors.Wrap("binding to port range", ErrAddrInUse,
 		"start", restrictedStart, "end", end)
 
 }

pkg/snet/sock_error_posix.go

@@ -0,0 +1,32 @@
+// Copyright 2024 OVGU Magdeburg
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//go:build !windows
+
+package snet
+
+import (
+	"errors"
+	"syscall"
+)
+
+const (
+	ErrAddrInUse = syscall.EADDRINUSE
+)
+
+// errorIsAddrUnavailable checks whether the error returned from a syscall to
+// bind indicates that the requested address is not available.
+func errorIsAddrUnavailable(err error) bool {
+	return errors.Is(err, syscall.EADDRINUSE)
+}

pkg/snet/sock_error_windows.go

@@ -0,0 +1,36 @@
+// Copyright 2024 OVGU Magdeburg
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//go:build windows
+
+package snet
+
+import (
+	"errors"
+
+	"golang.org/x/sys/windows"
+)
+
+const (
+	ErrAddrInUse = windows.WSAEADDRINUSE
+)
+
+// errorIsAddrUnavailable checks whether the error returned from a syscall to
+// bind indicates that the requested address is not available.
+func errorIsAddrUnavailable(err error) bool {
+	// WSAEADDRINUSE is returned if another socket is bound to the same address.
+	// WSAEACCES is returned if another process is bound to the same address
+	// with exclusive access.
+	return errors.Is(err, windows.WSAEADDRINUSE) || errors.Is(err, windows.WSAEACCES)
+}