[management, reverse proxy] Add reverse proxy feature (#5291)

* implement reverse proxy


---------

Co-authored-by: Alisdair MacLeod <git@alisdairmacleod.co.uk>
Co-authored-by: mlsmaycon <mlsmaycon@gmail.com>
Co-authored-by: Eduard Gert <kontakt@eduardgert.de>
Co-authored-by: Viktor Liu <viktor@netbird.io>
Co-authored-by: Diego Noguês <diego.sure@gmail.com>
Co-authored-by: Diego Noguês <49420+diegocn@users.noreply.github.com>
Co-authored-by: Bethuel Mmbaga <bethuelmbaga12@gmail.com>
Co-authored-by: Zoltan Papp <zoltan.pmail@gmail.com>
Co-authored-by: Ashley Mensah <ashleyamo982@gmail.com>

proxy/internal/flock/flock_test.go

@@ -0,0 +1,79 @@
+//go:build unix
+
+package flock
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestLockUnlock(t *testing.T) {
+	lockPath := filepath.Join(t.TempDir(), "test.lock")
+
+	f, err := Lock(context.Background(), lockPath)
+	require.NoError(t, err)
+	require.NotNil(t, f)
+
+	_, err = os.Stat(lockPath)
+	assert.NoError(t, err, "lock file should exist")
+
+	err = Unlock(f)
+	assert.NoError(t, err)
+}
+
+func TestUnlockNil(t *testing.T) {
+	err := Unlock(nil)
+	assert.NoError(t, err, "unlocking nil should be a no-op")
+}
+
+func TestLockRespectsContext(t *testing.T) {
+	lockPath := filepath.Join(t.TempDir(), "test.lock")
+
+	f1, err := Lock(context.Background(), lockPath)
+	require.NoError(t, err)
+	defer func() { require.NoError(t, Unlock(f1)) }()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 300*time.Millisecond)
+	defer cancel()
+
+	_, err = Lock(ctx, lockPath)
+	assert.ErrorIs(t, err, context.DeadlineExceeded)
+}
+
+func TestLockBlocks(t *testing.T) {
+	lockPath := filepath.Join(t.TempDir(), "test.lock")
+
+	f1, err := Lock(context.Background(), lockPath)
+	require.NoError(t, err)
+
+	var wg sync.WaitGroup
+	wg.Add(1)
+
+	start := time.Now()
+	var elapsed time.Duration
+
+	go func() {
+		defer wg.Done()
+		f2, err := Lock(context.Background(), lockPath)
+		elapsed = time.Since(start)
+		assert.NoError(t, err)
+		if f2 != nil {
+			assert.NoError(t, Unlock(f2))
+		}
+	}()
+
+	// Hold the lock for 200ms, then release.
+	time.Sleep(200 * time.Millisecond)
+	require.NoError(t, Unlock(f1))
+
+	wg.Wait()
+	assert.GreaterOrEqual(t, elapsed, 150*time.Millisecond,
+		"Lock should have blocked for at least ~200ms")
+}