download: ftp: don't use symlink size

There is no way to know if the file is link with current aioftp's LIST
parsing implementation, however we can use custom parser to insert link
info and try to stat link destination.

See-Also: aio-libs/aioftp#169

apt_mirror/download.py

@@ -10,7 +10,7 @@
 from datetime import datetime, timezone
 from email.utils import parsedate_to_datetime
 from enum import Enum
-from pathlib import Path
+from pathlib import Path, PurePosixPath
 from typing import Any, AsyncGenerator, AsyncIterator, Callable, Sequence
 from urllib import parse
 
@@ -672,7 +672,10 @@ async def stream(self, source_path: Path):
                 port=self._url.port or aioftp.DEFAULT_PORT,
                 user=self._url.username or aioftp.DEFAULT_USER,
                 password=self._url.password or aioftp.DEFAULT_PASSWORD,
+                parse_list_line_custom_first=True,
             ) as ftp:
+                ftp.parse_list_line_custom = self.parse_list_line_unix_links(ftp)
+
                 await ftp.change_directory(self._url.path)
 
                 try:
@@ -707,17 +710,42 @@ async def stream(self, source_path: Path):
                 ),
             )
 
+    def parse_list_line_unix_links(self, client: aioftp.Client):
+        def do_parse_list_line(b: bytes) -> tuple[PurePosixPath, dict[str, str]]:
+            info: dict[str, str]
+            path, info = client.parse_list_line_unix(b)  # type: ignore
+
+            s = b.decode(encoding=client.encoding).rstrip()
+            if s[0] == "l":
+                i = s.rindex(" -> ")
+                info["link_dst"] = s[i + 4 :]
+
+            return path, info
+
+        return do_parse_list_line
+
     async def _get_stat(self, ftp: Client, path: Path) -> FTPStat:
         ftp_stat = FTPStat()
 
         try:
             stat: dict[str, str] = await ftp.stat(path)  # type: ignore
 
+            recurse_depth = 5
+            while stat.get("link_dst") and recurse_depth > 0:  # type: ignore
+                try:
+                    stat = await ftp.stat(stat.get("link_dst"))  # type: ignore
+                except StatusCodeError:
+                    # Give up if we can not stat link destination
+                    break
+
+                recurse_depth -= 1
+
             if stat.get("type") != "file":  # type: ignore
                 raise FTPFileMissingException()
 
             try:
-                ftp_stat.size = int(stat["size"])  # type: ignore
+                if not stat.get("link_dst"):  # type: ignore
+                    ftp_stat.size = int(stat["size"])  # type: ignore
 
                 modify, _, _ = stat["modify"].rpartition(".")  # type: ignore
                 if len(modify) == 14:  # type: ignore