03-03-14 - Windows Links

I wrote a deduper in Oodle today. I was considering making the default action be to replace duplicate files with a link to the original.

I wasn't sure whether to use "hard" or "soft" links, so I did a little research.

In Windows a "hard link" means that multiple file names all point at the same file data. It's a bit of a misnomer, it's not really a "link" to the original. There is no "original" or base file - all instances of a hard link are equal peers.

A "soft link" is just an OS-level shortcut. There is an original base file, and the soft links point at it.

Both are ridiculously broken concepts and IMO should almost never be used.

With "hard links" the problem is that if you accidentally edit any of the links, you have editted the master data. If you did not intend that, you may have fucked up something severely.

Hard links are reasonable *if* the files linked are read-only (and somehow actually kept read-only, not just attrib'ed away).

The problem with "soft links" is that the links are not protected; if you rename or move or delete the original file, all the links are broken, and again you may have just severely fucked something up.

The big problem is that you get no warning in either case. Clearly what you want when you try to rename a file which has a bunch of soft links pointing at it is some kind of dialog that says "hey all these links point at this file, do you really want to rename it and break the links? or should I update the links to point at the new name?". Similarly with hard links, obviously what you want is some prompt like "hey if you modify this, so you want these hard links to see the new version or the old version?".

Now obviously you can't solve this problem in general without user prompts. But I believe that a refcounted copy-on-write link would have been a much more robust and safe solution. Open for write should have done a COW by default unless you passed a special flag to indicate you intend to edit shared data.

Even ignoring the fundamental logic of how links work, there are some very bad practical issues for links in windows.

1. Soft links show a file size of 0 in the dir enumeration file info. This breaks the assumption that most apps make that the file size they get from the dir enumeration will be the same as the file size they get if they open that file handle and ask for its size. It can also screw up enumerations that are just trying to skip zero-size files.

Hard link file sizes are out of date. If the file data is modified, only the directory entry for the one that was used to modify the data is updated. All other links still have the old file sizes, mod times, etc.

2. Hard links break the assumption that saving to a file is the same as saving to a temp and then renaming onto the file. Many apps may or may not use the "write to temp then rename" pattern; what you get is massively different results in a very unexpected way.

3. Mod times are hosed. In general attributes are hosed; neither type of link reflects the attributes of the actual file data in the link - until they are opened, then they get updated. Mod times are particularly bad because many apps use them to detect changes, and with links the file data can be changed but the mod time won't reflect it.

Dear lord. So non-robust.