android OTA升级（VAB分区）

android OTA升级（VAB分区）

升级开始前，升级程序update_engine将要升级的slot标记为unsuccessful，调用boot_ctl的SetSlotAsUnbootable()

hardware/interfaces/boot/1.1/default/boot_control/libboot_control.cpp

316  bool BootControl::SetSlotAsUnbootable(unsigned int slot) {
317    if (slot >= kMaxNumSlots || slot >= num_slots_) {
318      // Invalid slot number.
319      return false;
320    }
321  
322    bootloader_control bootctrl;
323    if (!LoadBootloaderControl(misc_device_, &bootctrl)) return false;
324  
325    // The only way to mark a slot as unbootable, regardless of the priority is to
326    // set the tries_remaining to 0.
327    bootctrl.slot_info[slot].successful_boot = 0;
328    bootctrl.slot_info[slot].tries_remaining = 0;
329    return UpdateAndSaveBootloaderControl(misc_device_, &bootctrl);
330  }