Update ReadTarFS to work on cyclic and dangling symlinks

Author: althonos

fs/tarfs.py

@@ -318,25 +318,33 @@ def _directory_entries(self):
         return self._directory_cache
 
     def _follow_symlink(self, entry):
-        """Follow an symlink `TarInfo` to find a concrete entry."""
+        """Follow an symlink `TarInfo` to find a concrete entry.
+
+        Returns ``None`` if the symlink is dangling.
+        """
+        done = set()
         _entry = entry
         while _entry.issym():
             linkname = normpath(
                 join(dirname(self._decode(_entry.name)), self._decode(_entry.linkname))
             )
             resolved = self._resolve(linkname)
             if resolved is None:
-                raise errors.ResourceNotFound(linkname)
+                return None
+            done.add(_entry)
             _entry = self._directory_entries[resolved]
+            # if we already saw this symlink, then we are following cyclic
+            # symlinks and we should break the loop
+            if _entry in done:
+                return None
 
         return _entry
 
     def _resolve(self, path):
         """Replace path components that are symlinks with concrete components.
 
-        Returns:
-
-
+        Returns ``None`` when the path could not be resolved to an existing
+        entry in the archive.
         """
         if path in self._directory_entries or not path:
             return path
@@ -441,17 +449,17 @@ def isdir(self, path):
         _path = relpath(self.validatepath(path))
         realpath = self._resolve(_path)
         if realpath is not None:
-            entry = self._directory_entries[realpath]
-            return self._follow_symlink(entry).isdir()
+            entry = self._follow_symlink(self._directory_entries[realpath])
+            return False if entry is None else entry.isdir()
         else:
             return False
 
     def isfile(self, path):
         _path = relpath(self.validatepath(path))
         realpath = self._resolve(_path)
         if realpath is not None:
-            entry = self._directory_entries[realpath]
-            return self._follow_symlink(entry).isfile()
+            entry = self._follow_symlink(self._directory_entries[realpath])
+            return False if entry is None else entry.isfile()
         else:
             return False
 
@@ -480,12 +488,12 @@ def listdir(self, path):
         elif realpath:
             target = self._follow_symlink(self._directory_entries[realpath])
             # check the path is either a symlink mapping to a directory or a directory
-            if target.isdir():
-                base = target.name
-            elif target.issym():
-                base = target.linkname
-            else:
+            if target is None:
+                raise errors.ResourceNotFound(path)
+            elif not target.isdir():
                 raise errors.DirectoryExpected(path)
+            else:
+                base = target.name
         else:
             base = ""
 
@@ -515,11 +523,16 @@ def openbin(self, path, mode="r", buffering=-1, **options):
         if "w" in mode or "+" in mode or "a" in mode:
             raise errors.ResourceReadOnly(path)
 
-        # check the path actually resolves after following symlinks
+        # check the path actually resolves after following symlink components
         _realpath = self._resolve(_path)
         if _realpath is None:
             raise errors.ResourceNotFound(path)
 
+        # get the entry at the resolved path and follow all symlinks
+        entry = self._follow_symlink(self._directory_entries[_realpath])
+        if entry is None:
+            raise errors.ResourceNotFound(path)
+
         # TarFile.extractfile returns None if the entry is not a file
         # neither a file nor a symlink
         reader = self._tar.extractfile(self._directory_entries[_realpath])

tests/test_tarfs.py

@@ -14,7 +14,7 @@
 from fs.compress import write_tar
 from fs.opener import open_fs
 from fs.opener.errors import NotWriteable
-from fs.errors import NoURL
+from fs.errors import NoURL, ResourceNotFound
 from fs.test import FSTestCases
 
 from .test_archives import ArchiveTestCases
@@ -415,6 +415,53 @@ def test_listdir(self):
         self.assertEqual(sorted(self.fs.listdir("spam")), ["bar.txt", "baz.txt"])
 
 
+class TestBrokenSymlinks(unittest.TestCase):
+
+    @classmethod
+    def setUpClass(cls):
+        cls.tmpfs = open_fs("temp://")
+
+    @classmethod
+    def tearDownClass(cls):
+        cls.tmpfs.close()
+
+    def setUp(self):
+        def _info(name, **kwargs):
+            info = tarfile.TarInfo(name)
+            for k, v in kwargs.items():
+                setattr(info, k, v)
+            return info
+
+        # /foo
+        # /foo/baz.txt -> /foo/bar.txt
+        # /spam -> /eggs
+        # /eggs -> /spam
+
+        self.tempfile = self.tmpfs.open("test.tar", "wb+")
+        with tarfile.open(mode="w", fileobj=self.tempfile) as tf:
+            tf.addfile(_info("foo", type=tarfile.DIRTYPE))
+            tf.addfile(_info("foo/baz.txt", type=tarfile.SYMTYPE, linkname="bar.txt"))
+            tf.addfile(_info("spam", type=tarfile.SYMTYPE, linkname="eggs"))
+            tf.addfile(_info("eggs", type=tarfile.SYMTYPE, linkname="spam"))
+        self.tempfile.seek(0)
+        self.fs = tarfs.TarFS(self.tempfile)
+
+    def tearDown(self):
+        self.fs.close()
+        self.tempfile.close()
+
+    def test_dangling(self):
+        self.assertFalse(self.fs.isfile("foo/baz.txt"))
+        self.assertFalse(self.fs.isdir("foo/baz.txt"))
+        self.assertRaises(ResourceNotFound, self.fs.openbin, "foo/baz.txt")
+        self.assertRaises(ResourceNotFound, self.fs.listdir, "foo/baz.txt")
+
+    def test_cyclic(self):
+        self.assertFalse(self.fs.isfile("spam"))
+        self.assertFalse(self.fs.isdir("spam"))
+        self.assertRaises(ResourceNotFound, self.fs.openbin, "spam")
+        self.assertRaises(ResourceNotFound, self.fs.listdir, "spam")
+
 class TestReadTarFSMem(TestReadTarFS):
     def make_source_fs(self):
         return open_fs("mem://")