[client] Add non default socket file discovery (#5425)

- Automatic Unix daemon address discovery: if the default socket is missing, the client can find and use a single available socket.
- Client startup now resolves daemon addresses more robustly while preserving non-Unix behavior.

client/internal/daemonaddr/resolve.go

@@ -0,0 +1,60 @@
+//go:build !windows && !ios && !android
+
+package daemonaddr
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+
+	log "github.com/sirupsen/logrus"
+)
+
+var scanDir = "/var/run/netbird"
+
+// setScanDir overrides the scan directory (used by tests).
+func setScanDir(dir string) {
+	scanDir = dir
+}
+
+// ResolveUnixDaemonAddr checks whether the default Unix socket exists and, if not,
+// scans /var/run/netbird/ for a single .sock file to use instead. This handles the
+// mismatch between the netbird@.service template (which places the socket under
+// /var/run/netbird/<instance>.sock) and the CLI default (/var/run/netbird.sock).
+func ResolveUnixDaemonAddr(addr string) string {
+	if !strings.HasPrefix(addr, "unix://") {
+		return addr
+	}
+
+	sockPath := strings.TrimPrefix(addr, "unix://")
+	if _, err := os.Stat(sockPath); err == nil {
+		return addr
+	}
+
+	entries, err := os.ReadDir(scanDir)
+	if err != nil {
+		return addr
+	}
+
+	var found []string
+	for _, e := range entries {
+		if e.IsDir() {
+			continue
+		}
+		if strings.HasSuffix(e.Name(), ".sock") {
+			found = append(found, filepath.Join(scanDir, e.Name()))
+		}
+	}
+
+	switch len(found) {
+	case 1:
+		resolved := "unix://" + found[0]
+		log.Infof("Default daemon socket not found, using discovered socket: %s", resolved)
+		return resolved
+	case 0:
+		return addr
+	default:
+		log.Warnf("Default daemon socket not found and multiple sockets discovered in %s; pass --daemon-addr explicitly", scanDir)
+		return addr
+	}
+}

client/internal/daemonaddr/resolve_stub.go

@@ -0,0 +1,8 @@
+//go:build windows || ios || android
+
+package daemonaddr
+
+// ResolveUnixDaemonAddr is a no-op on platforms that don't use Unix sockets.
+func ResolveUnixDaemonAddr(addr string) string {
+	return addr
+}

client/internal/daemonaddr/resolve_test.go

@@ -0,0 +1,121 @@
+//go:build !windows && !ios && !android
+
+package daemonaddr
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+// createSockFile creates a regular file with a .sock extension.
+// ResolveUnixDaemonAddr uses os.Stat (not net.Dial), so a regular file is
+// sufficient and avoids Unix socket path-length limits on macOS.
+func createSockFile(t *testing.T, path string) {
+	t.Helper()
+	if err := os.WriteFile(path, nil, 0o600); err != nil {
+		t.Fatalf("failed to create test sock file at %s: %v", path, err)
+	}
+}
+
+func TestResolveUnixDaemonAddr_DefaultExists(t *testing.T) {
+	tmp := t.TempDir()
+	sock := filepath.Join(tmp, "netbird.sock")
+	createSockFile(t, sock)
+
+	addr := "unix://" + sock
+	got := ResolveUnixDaemonAddr(addr)
+	if got != addr {
+		t.Errorf("expected %s, got %s", addr, got)
+	}
+}
+
+func TestResolveUnixDaemonAddr_SingleDiscovered(t *testing.T) {
+	tmp := t.TempDir()
+
+	// Default socket does not exist
+	defaultAddr := "unix://" + filepath.Join(tmp, "netbird.sock")
+
+	// Create a scan dir with one socket
+	sd := filepath.Join(tmp, "netbird")
+	if err := os.MkdirAll(sd, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	instanceSock := filepath.Join(sd, "main.sock")
+	createSockFile(t, instanceSock)
+
+	origScanDir := scanDir
+	setScanDir(sd)
+	t.Cleanup(func() { setScanDir(origScanDir) })
+
+	got := ResolveUnixDaemonAddr(defaultAddr)
+	expected := "unix://" + instanceSock
+	if got != expected {
+		t.Errorf("expected %s, got %s", expected, got)
+	}
+}
+
+func TestResolveUnixDaemonAddr_MultipleDiscovered(t *testing.T) {
+	tmp := t.TempDir()
+
+	defaultAddr := "unix://" + filepath.Join(tmp, "netbird.sock")
+
+	sd := filepath.Join(tmp, "netbird")
+	if err := os.MkdirAll(sd, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	createSockFile(t, filepath.Join(sd, "main.sock"))
+	createSockFile(t, filepath.Join(sd, "other.sock"))
+
+	origScanDir := scanDir
+	setScanDir(sd)
+	t.Cleanup(func() { setScanDir(origScanDir) })
+
+	got := ResolveUnixDaemonAddr(defaultAddr)
+	if got != defaultAddr {
+		t.Errorf("expected original %s, got %s", defaultAddr, got)
+	}
+}
+
+func TestResolveUnixDaemonAddr_NoSocketsFound(t *testing.T) {
+	tmp := t.TempDir()
+
+	defaultAddr := "unix://" + filepath.Join(tmp, "netbird.sock")
+
+	sd := filepath.Join(tmp, "netbird")
+	if err := os.MkdirAll(sd, 0o755); err != nil {
+		t.Fatal(err)
+	}
+
+	origScanDir := scanDir
+	setScanDir(sd)
+	t.Cleanup(func() { setScanDir(origScanDir) })
+
+	got := ResolveUnixDaemonAddr(defaultAddr)
+	if got != defaultAddr {
+		t.Errorf("expected original %s, got %s", defaultAddr, got)
+	}
+}
+
+func TestResolveUnixDaemonAddr_NonUnixAddr(t *testing.T) {
+	addr := "tcp://127.0.0.1:41731"
+	got := ResolveUnixDaemonAddr(addr)
+	if got != addr {
+		t.Errorf("expected %s, got %s", addr, got)
+	}
+}
+
+func TestResolveUnixDaemonAddr_ScanDirMissing(t *testing.T) {
+	tmp := t.TempDir()
+
+	defaultAddr := "unix://" + filepath.Join(tmp, "netbird.sock")
+
+	origScanDir := scanDir
+	setScanDir(filepath.Join(tmp, "nonexistent"))
+	t.Cleanup(func() { setScanDir(origScanDir) })
+
+	got := ResolveUnixDaemonAddr(defaultAddr)
+	if got != defaultAddr {
+		t.Errorf("expected original %s, got %s", defaultAddr, got)
+	}
+}