test(01-02): add failing tests for capture package

- TestListInterfaces: verifies net.Interfaces() returns at least 1 interface
- TestListInterfacesFormat: verifies interface Name is non-empty
- TestIsPermissionError: table-driven positive cases for perm error detection
- TestIsPermissionErrorNegative: non-perm errors not flagged
- TestPermissionErrorMsg: verifies sudo + interface name + Linux setcap hint

capture/capture_test.go

@@ -0,0 +1,94 @@
+package capture
+
+import (
+	"errors"
+	"runtime"
+	"strings"
+	"testing"
+)
+
+// TestListInterfaces verifies that net.Interfaces() returns at least one interface.
+// The loopback interface (lo on Linux, lo0 on macOS) always exists.
+func TestListInterfaces(t *testing.T) {
+	ifaces, err := ListInterfaces()
+	if err != nil {
+		t.Fatalf("ListInterfaces() error: %v", err)
+	}
+	if len(ifaces) == 0 {
+		t.Fatal("ListInterfaces() returned 0 interfaces; expected at least 1 (loopback)")
+	}
+	t.Logf("Found %d interfaces", len(ifaces))
+}
+
+// TestListInterfacesFormat verifies that each returned interface has a non-empty Name.
+func TestListInterfacesFormat(t *testing.T) {
+	ifaces, err := ListInterfaces()
+	if err != nil {
+		t.Fatalf("ListInterfaces() error: %v", err)
+	}
+	for i, iface := range ifaces {
+		if iface.Name == "" {
+			t.Errorf("Interface[%d] has empty Name", i)
+		}
+	}
+}
+
+// TestIsPermissionError checks positive cases for isPermissionError.
+func TestIsPermissionError(t *testing.T) {
+	positives := []string{
+		"permission denied",
+		"Permission Denied",
+		"PERMISSION DENIED",
+		"operation not permitted",
+		"Operation Not Permitted",
+		"pcap_create: foo",
+		"pcap_create: operation not permitted",
+	}
+	for _, msg := range positives {
+		err := errors.New(msg)
+		if !isPermissionError(err) {
+			t.Errorf("isPermissionError(%q) = false; want true", msg)
+		}
+	}
+}
+
+// TestIsPermissionErrorNegative checks that non-permission errors are not flagged.
+func TestIsPermissionErrorNegative(t *testing.T) {
+	negatives := []string{
+		"interface not found",
+		"no such device",
+		"network is unreachable",
+		"device busy",
+	}
+	for _, msg := range negatives {
+		err := errors.New(msg)
+		if isPermissionError(err) {
+			t.Errorf("isPermissionError(%q) = true; want false", msg)
+		}
+	}
+}
+
+// TestPermissionErrorMsg verifies the platform-specific error message.
+func TestPermissionErrorMsg(t *testing.T) {
+	iface := "eth0"
+	err := permissionErrorMsg(iface)
+	if err == nil {
+		t.Fatal("permissionErrorMsg() returned nil; want an error")
+	}
+	msg := err.Error()
+
+	// All platforms should mention sudo and the interface name.
+	if !strings.Contains(msg, "sudo") {
+		t.Errorf("permissionErrorMsg(%q) message missing 'sudo': %s", iface, msg)
+	}
+	if !strings.Contains(msg, iface) {
+		t.Errorf("permissionErrorMsg(%q) message missing interface name: %s", iface, msg)
+	}
+
+	// Linux-specific: must mention setcap.
+	if runtime.GOOS == "linux" {
+		if !strings.Contains(msg, "setcap cap_net_raw+ep") {
+			t.Errorf("permissionErrorMsg on linux missing 'setcap cap_net_raw+ep': %s", msg)
+		}
+	}
+}