Topic - libyang接口使用

libyang学习：主要是为了学习如何将yang模型data path 转化为schema path。

背景：最近在做一个yang的gtest配置下发测试工具时，遇到yang模型xpath带choice case时（参见文章后面的yang模型）：
data path不带choice case节点，如：/acme-system:system/food/tea
schema path是带的，如：/acme-system:system/food/snack/late-bight/tea
导致业务基于schema path注册回调函数时，基于data path数据注入时找不到回调函数，所以需要做一次转换，以便能够给予path找到回调函数。

思路：从data path获取 lysc_node，再从 lysc_node 获取schema path。
```c

LY_ERR ret = LY_SUCCESS;
    struct ly_ctx *ctx = NULL;
    const struct lys_module *module;

// 初始化libyang上下文
    ret = ly_ctx_new("/home/jungle/code/libyang/tests/modules/yang", 0, &ctx);
    if (!ctx) {
        fprintf(stderr, "Failed to create context.\n");
        return 1;
    }

// 加载YANG模块
    // 假设YANG模块文件名为 "acme-system.yang"
    module = ly_ctx_load_module(ctx, "acme-system", NULL, NULL);
    if (!module) {
        fprintf(stderr, "Failed to load module.\n");
        ly_ctx_destroy(ctx);
        return 1;
    }
	
	// 打印整个yang模型
	char *str = NULL;
	lys_print_mem(&str, module, LYS_OUT_TREE, 0);
	printf("\r\nyang module:\r\n%s\r\n", str);
	free(str);

/* using lysc tree */
	ly_ctx_set_options(ctx, LY_CTX_SET_PRIV_PARSED);

// 初始化输出
    char *printed;
	struct ly_out *out;
    ly_out_new_memory(&printed, 0, &out);

// 从data path获取 lysc_node
	const struct lysc_node *node_lysc = lys_find_path(ctx, NULL, "/acme-system:system/food/tea", 0);
	lys_print_node(out, node_lysc, LYS_OUT_TREE, 0, LYS_PRINT_NO_SUBSTMT);
	int len = ly_out_printed(out);
	printf("\r\nlys_find_path:\r\n%s\r\n", printed);
	
	// 根据lysc_node 转 schema path
	char path[4078] = {0};
	lysc_path(node_lysc, LYSC_PATH_LOG, path, 4078);
	printf("\r\nschema path=%s\r\n", path);

// 从schema path获取 lysc_node
	node_lysc = find_schema_path(ctx, "/acme-system:system/food/snack/late-bight/tea");
	if (node_lysc) {
		lys_print_node(out, node_lysc, LYS_OUT_TREE, 0, LYS_PRINT_NO_SUBSTMT);
		int len = ly_out_printed(out);
		printf("\r\nfind_schema_path:\r\n%s\r\n", printed);
	}

ly_out_reset(out);

// 创建树
	struct lyd_node *data = NULL;
    //lyd_new_path(NULL, ctx, "/acme-system:system/host-name", "huawei", 0, &data);
    lyd_new_path(NULL, ctx, "/acme-system:system/food/tea", "longjungcha", 0, &data);
	LY_ERR _r = lyd_print_mem(&str, data, LYD_JSON, LYS_PRINT_SHRINK);
	printf("\r\nLYD_JSON:\r\n%s\r\n", str);
	free(str);

// 打印data path
	char *path_ =  lyd_path(data, LYD_PATH_STD, NULL, 0);
	printf("\r\nLYD_PATH_STD:\r\n%s\r\n", path_);
	free(path_);

lyd_free_siblings(data);
    ly_ctx_destroy(ctx);
```
运行结果如下：
```c
yang module:
module: acme-system
  +--rw system
     +--rw host-name?       string
     +--rw domain-search*   string
     +--rw login
     |  +--rw message?   string
     |  +--rw user* [name]
     |     +--rw name    string
     +--rw food
        +--rw (snack)?
           +--:(sport-arena)
           |  +--rw pretzel?   string
           +--:(late-bight)
              +--rw tea?   string

lys_find_path:
module: acme-system
  +--rw system
     +--rw food
        +--rw (snack)?
           +--:(late-bight)
              +--rw tea?   string

schema path=/acme-system:system/food/snack/late-bight/tea

find_schema_path:
module: acme-system
  +--rw system
     +--rw food
        +--rw (snack)?
           +--:(late-bight)
              +--rw tea?   string
module: acme-system
  +--rw system
     +--rw food
        +--rw (snack)?
           +--:(late-bight)
              +--rw tea?   string

LYD_JSON:
{"acme-system:system":{"food":{"tea":"longjungcha"}}}

LYD_PATH_STD:
/acme-system:system
```

最后附上yang模型：
```xml
module acme-system {
 namespace "http://acme.example.com/system";
 prefix "acme";
 organization "ACME Inc.";
 contact "joe@acme.example.com";
 description
 "The module for entities implementing the ACME system.";
 revision 2007-06-09 {
  description "Initial revision.";
 }
 container system {
  leaf host-name {
   type string;
   description "Hostname for this system";
  }
  leaf-list domain-search {
   type string;
   description "List of domain names to search";
  }
  container login {
   leaf message {
    type string;
    description
    "Message given at start of login session";
   }
   list user {
    key "name";
    leaf name {
     type string;
    }
   }
  }
  container food {
    choice snack {
        case sport-arena {
            leaf pretzel {
                type string;
            }
        }
        case late-bight {
            leaf tea {
                type string;
            }
        }
    }
  }
 }
}
```

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