handle: configure package-global handle using ConfigureHandle

This commit introduces the funcion ConfigureHandle to replace the global package
handle with one configured using the given HandleOptions. It can only be called
once per process.

Signed-off-by: Timo Beckers <timo@incline.eu>

Author: ti-mo

handle_linux.go

@@ -2,6 +2,7 @@ package netlink
 
 import (
 	"fmt"
+	"sync"
 	"sync/atomic"
 	"time"
 
@@ -13,6 +14,37 @@ import (
 // Empty handle used by the netlink package methods
 var pkgHandle = &Handle{}
 
+var configMu sync.Mutex
+var configDone bool
+
+// ConfigureHandle configures the default, package-wide netlink handle used by
+// the netlink package's global functions like [LinkList] and [AddrList] with
+// the given opts. It does not affect any existing or future [Handle] returned
+// by the library.
+//
+// This function is not safe to call concurrently with any netlink operations
+// using the global package handle. Invoke it from init() functions only.
+//
+// Returns an error if called more than once per process.
+func ConfigureHandle(opts HandleOptions) error {
+	configMu.Lock()
+	defer configMu.Unlock()
+
+	if configDone {
+		return fmt.Errorf("netlink package handle already configured")
+	}
+
+	h, err := NewHandleWithOptions(opts)
+	if err != nil {
+		return fmt.Errorf("creating handle: %w", err)
+	}
+
+	configDone = true
+	pkgHandle = h
+
+	return nil
+}
+
 // HandleOptions defines the options for creating a netlink Handle, allowing the
 // caller to customize its behaviour.
 type HandleOptions struct {

handle_linux_test.go

@@ -3,6 +3,8 @@ package netlink
 import (
 	"testing"
 	"time"
+
+	"github.com/stretchr/testify/assert"
 )
 
 func TestSetGetSocketTimeout(t *testing.T) {
@@ -15,3 +17,19 @@ func TestSetGetSocketTimeout(t *testing.T) {
 		t.Fatalf("Unexpected socket timeout value: got=%v, expected=%v", val, timeout)
 	}
 }
+
+func TestConfigureHandle(t *testing.T) {
+	orig := pkgHandle
+	origDone := configDone
+	t.Cleanup(func() {
+		configMu.Lock()
+		defer configMu.Unlock()
+
+		pkgHandle = orig
+		configDone = origDone
+	})
+
+	assert.NoError(t, ConfigureHandle(HandleOptions{}))
+	assert.NotEqual(t, orig, pkgHandle)
+	assert.Error(t, ConfigureHandle(HandleOptions{}))
+}