在分析android 8.0 property的时候，发现有2个后缀为recovery_id的变量，前者在init初始化property时，通过计算recovery分区得出，后者在生成recovery.img阶段计算得出。但似乎没什么地方在用，仅有的比较逻辑在2015.6月被revert掉了（https://gitlab.tubit.tu-berlin.de/justus.beyer/streamagame_platform_frameworks_base/commit/c97b92e3460e56bbec5ac954873d7eea978bd408）。

  • ro.recovery_id
  • ro.expect.recovery_id

ro.recovery_id

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    void load_recovery_id_prop() {
        std::unique_ptr<fstab, decltype(&fs_mgr_free_fstab)> fstab(fs_mgr_read_fstab_default(),
                                                                   fs_mgr_free_fstab);
        if (!fstab) {
            PLOG(ERROR) << "unable to read default fstab";
            return;
        }

        fstab_rec* rec = fs_mgr_get_entry_for_mount_point(fstab.get(), RECOVERY_MOUNT_POINT);
        if (rec == NULL) {
            LOG(ERROR) << "/recovery not specified in fstab";
            return;
        }

        int fd = open(rec->blk_device, O_RDONLY);
        if (fd == -1) {
            PLOG(ERROR) << "error opening block device " << rec->blk_device;
            return;
        }

        boot_img_hdr hdr;
        if (android::base::ReadFully(fd, &hdr, sizeof(hdr))) {
            std::string hex = bytes_to_hex(reinterpret_cast<uint8_t*>(hdr.id), sizeof(hdr.id));
            property_set("ro.recovery_id", hex.c_str());
        } else {
            PLOG(ERROR) << "error reading /recovery";
        }

        close(fd);
    }

    void load_system_props() {
        load_properties_from_file("/system/build.prop", NULL);
        load_properties_from_file("/odm/build.prop", NULL);
        load_properties_from_file("/vendor/build.prop", NULL);
        load_properties_from_file("/factory/factory.prop", "ro.*");
        load_recovery_id_prop();
    }

ro.expect.recovery_id

生成recovery镜像的时候，通过命令得到recovery.id文件，进而将文件内容写到property里面

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RECOVERYIMAGE_ID_FILE := $(PRODUCT_OUT)/recovery.id

... ...

$(hide) $(MKBOOTIMG) $(INTERNAL_RECOVERYIMAGE_ARGS) $(INTERNAL_MKBOOTIMG_VERSION_ARGS) $(BOARD_MKBOOTIMG_ARGS) --output $(1) --id > $(RECOVERYIMAGE_ID_FILE))

... ...

$(hide) echo ro.expect.recovery_id=`cat $(RECOVERYIMAGE_ID_FILE)` >> $@