After two days of turning my brain into an unrecognizable slurry slaving over symlink support in Pants' engine, including mentally debugging and programming while rocking my son at 3am in a pitchblack-room, sound-machine full-blast; I am PLEASED to present this very evil test case passes:

pytest.param(
            CreateDigest(
                [
                    FileContent("file.txt", b"four\n"),
                    SymlinkEntry("a/dirlink", "../b/c/d"),
                    SymlinkEntry("b/c/d/link", "../../file.txt"),
                ]
            ),
            # NOTE: No "b/c/d/link", because that'd result in "a/file.txt"
            ("a/dirlink/link", "file.txt"),
            ("a", "a/dirlink", "b", "b/c", "b/c/d"),
            id="symlink-makes-updir-valid",
        ),

Please let me know your favorite evil symlink scenario, and we'll turn the testing up to 11.

Ohh, this is exciting! Some of my "favourite" cases (

lhs -> rhs

is a link called

lhs

pointing to a file

rhs

, i.e. created via

ln -s rhs lhs

)

.
├── broken -> doesnt_exist
├── dir
│   ├── parent -> ..
│   ├── self -> .
│   ├── self_obtusely -> ../dir
│   └── self_very_obtusely -> self/self/self/self/self_obtusely/parent/dir/parent/dir/parent/dir/self
├── escape
│   ├── absolute -> /etc/passwd
│   └── traversal -> ../../../../../../../../../../../../etc/passwd
└── loop
    ├── chain1 -> chain2
    ├── chain2 -> chain3
    ├── chain3 -> chain1
    ├── self -> self
    ├── via_chain -> chain1/file.txt
    └── via_self -> self/file.txt

Most of these are ... going out of my way to be difficult, but: • I imagine the

escape/absolute

one could come in practice, e.g. a Python venv seems to have an absolute symlink to its interpreter. •

readlink -f dir/self_very_obtusely

confirms that it is pointing to

dir/

• Naive tooling may infinite-loop on some of the

loop

ones too.

(Luckly absolute paths are not traversed in this code)