file: linkstat to get link stat info

[systemcrash]

This augments rpcd to be able to inspect symbolic links, instead of simply returning the linked file info

See openwrt/luci#7300 for inspiration

Usage:

root@OpenWrt:/# ubus call file stat '{"path":"/etc/os-release"}'
{
	"path": "/etc/os-release",
	"type": "file",
	"size": 528,
	"mode": 33188,
	"atime": 1727094886,
	"mtime": 1727094886,
	"ctime": 1727094886,
	"inode": 1124,
	"uid": 0,
	"gid": 0
}
root@OpenWrt:/# ubus call file lstat '{"path":"/etc/os-release"}'
{
	"path": "/etc/os-release",
	"type": "symlink",
	"size": 21,
	"mode": 41471,
	"atime": 1727094886,
	"mtime": 1727094886,
	"ctime": 1727094886,
	"inode": 219,
	"uid": 0,
	"gid": 0
}
root@OpenWrt:/# ubus call file stat '{"path":"/var"}'
{
	"path": "/var",
	"type": "directory",
	"size": 520,
	"mode": 17407,
	"atime": 1730221560,
	"mtime": 1730221753,
	"ctime": 1730221753,
	"inode": 1,
	"uid": 0,
	"gid": 0
}
root@OpenWrt:/# ubus call file lstat '{"path":"/var"}'
{
	"path": "/var",
	"type": "symlink",
	"size": 3,
	"mode": 41471,
	"atime": 1727094886,
	"mtime": 1727094886,
	"ctime": 1727094886,
	"inode": 1264,
	"uid": 0,
	"gid": 0
}
root@OpenWrt:/# ubus call file stat '{"path":"/bin/ash"}'
{
	"path": "/bin/ash",
	"type": "file",
	"size": 405522,
	"mode": 33261,
	"atime": 1727094886,
	"mtime": 1727094886,
	"ctime": 1727094886,
	"inode": 15,
	"uid": 0,
	"gid": 0
}
root@OpenWrt:/# ubus call file lstat '{"path":"/bin/ash"}'
{
	"path": "/bin/ash",
	"type": "symlink",
	"size": 7,
	"mode": 41471,
	"atime": 1727094886,
	"mtime": 1727094886,
	"ctime": 1727094886,
	"inode": 13,
	"uid": 0,
	"gid": 0
}

Tested on: 23.05.5

[stokito]

Do we need all the fields for a symlink? The size of the symlink itself is unusable.
As far I know from a practical sense we may need to know only the link target. And even this may not always needed. So maybe just add the "target" field:

root@OpenWrt:/# ubus call file stat '{"path":"/var"}'
{
	"path": "/var",
	"type": "directory",
        "target": "/tmp/"
	"size": 520,
	"mode": 17407,
	"atime": 1730221560,
	"mtime": 1730221753,
	"ctime": 1730221753,
	"inode": 1,
	"uid": 0,
	"gid": 0
}

That may have a performance penalty of additional readlink syscall but I hope this will be negligible for most use cases.

[systemcrash]

It reuses the same functionality as rpcd stat calls. If you read the manpage, lstat size is the character length of the path or something.

[systemcrash]

See https://www.man7.org/linux/man-pages/man0/sys_stat.h.0p.html

[systemcrash]

ping @jow-

[stokito]

@systemcrash #9 (comment)

[stokito]

To avoid performance penalties we may have it as an option Similarly to the ls -l:

ubus call file stat '{"path":"/var", "link": true}'