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Comment by yjftsjthsd-h | Hacker News Reader

I wonder what happens if you violate the 0 xor 1 rule by hard linking a directory - it is strongly discouraged for very good reasons but was at least historically possible ( https://askubuntu.com/questions/210741/why-are-hard-links-no... comments discuss a bit in between the dire warnings). Or maybe it's not possible on newer systems that have such a cache?

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Apple allows hard linking directories (I believe it is used for Time Machine somehow): https://jameshunt.us/writings/linking-dirs-in-macos/

Is that true on APFS? I know they added it to HFS+ for Time Machine, but I’d assumed it’s no longer necessary (or possible) with the newer filesystem that allows for real snapshots.

It’s no longer possible.

I miss this in the days of APFS actually. As much as it’s potentially dangerous, in a system without bind mounts readily available it really was sometimes the best solution to a problem. Mainly used it to allow various directories that couldn’t be moved to be re-homed, looking at you OneDrive.

I remember back in the Solaris days there was the "link" command that would allow you to make hard links of directories. My memory's bit rotted but I believe fsck would complain about the hard linked directories; I don't remember what it did with it.

Apparently the link[1] command still exists and the man page says "this does not do any error checking"

[1]https://docs.oracle.com/cd/E26505_01/html/816-5166/link-1m.h...

You can do it with FUSE I'm pretty sure

Yes. I've done that (but only on immutable filesystems, where it's not (much of?) a problem).

Being ro helps with deleting directories (by preventing it, but w/e), but cyclical links would be bad regardless. Granted, it's the kind of thing that's probably okay if you know what you're doing well enough to avoid the footguns... but I have no experience, so can't say for sure.

Oh, I didn't have any cycles.

I was exposing a git repository as a DAG of directories via FUSE, and was translating git internal IDs, based on hash, straight to inodes in a one-to-one fashion. Git doesn't allow cycles (unless you break the cryptographic hash function).

That’s a good write up.

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