the geniuses that maintain the extant Unix systems invented UTF-8 precisely because they value backward compatibility above everything else and won't contemplate breaking stuff for the sake of programmer convenience.

UTF-8 was invented by people who were sufficiently fed up with the shortcomings of Unix that they created a quite incompatible but much improved evolution of it. And it's not just some hack for the sake of backwards compatibility at the cost of other properties, it's a vastly superior design compared to the then existing unicode encodings.

And there is no serious backwards compatibility problem. You just introduce a mount option to enforce utf-8, flip it to default after a say a decade, and people who then still have file systems with pathnames in latin-1 (or other craziness) can then flip it off and get a few extra decades to migrate their stuff. In the meantime the rest of the world just writes software under the assumption that it will be deployed to non-crazy installs, and software becomes magically more reliable and quicker to write.

And it's not like even 1% of software now would handle non-utf-8 filenames robustly anyway (I mean even if you are aware of the problem and diligent about it, there is absolutely no good way to deal with non-utf8 filenames if you need to output them as text for consumption by humans or other programs, which is close to 100% of cases, because at the very least you will need to do so in error messages if there is an IO problem).

Then what happens when you mount it with the utf-8 option on one machine, and without it on another. The one without the utf-8 options writes a filename that breaks utf-8 encoding. You then try to mount it again with the utf-8 option. What happens then?

Not only mount. What happens when you unpack an archive that uses non-fully utf-8 compliant names. Or checkout the history of a git repo with non-utf-8 filenames. Network filesystems are also a source of pain.

And even if you settle for utf-8 you'll still have to deal with differences with filesystems automatically canonicalizing names or being case insensitive.

> What happens when you unpack an archive that uses non-fully utf-8 compliant names.

Exactly what you would want to happen (this is the actual error message you will get, BTW)?

   > (cd /mnt/funkyfilesystem && touch $'\370' && tar cf my-bad-file.tar $'\370')
   > tar x /mnt/funkyfilesystem/my-bad-file.tar
   tar: \370: Cannot open: Invalid or incomplete multibyte or wide character
   tar: Exiting with failure status due to previous errors

Or are you trying to tell me that your life isn't complete without tar or a git checkout silently dropping some nonsense byte sequences into your filesystem?

> And even if you settle for utf-8 you'll still have to deal with differences with filesystems automatically canonicalizing names or being case insensitive.

(Case-preserving you mean? I think truly case-insensitive died with 8.3 DOS filenames). These are also annoying, but a much more minor issue, e.g. you don't need to create a new weird string type just because of them.

Your tar example appear to Work On My Machine. Probably I don't have a funky enough filesystem. Of course if the filesystem itself doesn't support invalid unicode names, there isn't much the os or tools can do.

But hey, if enough tools and applications stop supporting non-utf-8 names, it is possible that in a a few decades you might get what you want as de-facto non-utf8 files will just disappear.

I agree it's something that needs to be dealt with, but an you name a plausible scenario in which this would actually happen and in which the desired behavior would be that you get silently some garbage filename written to your disk?

Let's say you get an EILSEQ on accessing the dirent with readdir and now have to debug how you got some corrupted filename you clearly didn't want in the first place, remount and fix it. How is that worse than having the corrupted filename and not knowing about it before something more insidious happens?

Oh, and BTW: Plan-9, the OS utf-8 was specifically invented for did actually enforce utf-8 for filenames.

> UTF-8 was invented by people who were sufficiently fed up with the shortcomings of Unix that

Uh, not exactly, not quite. UTF-8 was invented by Unix people who were fed up with all the alternative proposals at the time. Those people did not then go and "fix Unix" in any way regarding this. Those people were none others than Ken Thompson and Rob Pike, who were among the creators of Unix -- you could only have gotten closer to "those people" being the creators of Unix by having had Denis Ritchie at that diner table on that fateful day.

> Except that the geniuses who maintain the extant Unix systems still couldn't bring themselves to at least start transitioning pathnames to utf8, [...]

As far as POSIX and Unix `open(2)` and friends go, pathnames are opaque binary with only two special byte values: NUL (0x00, because it's C strings, which are NUL-terminated) and '/' (because that's the path component separator). Any codeset and form that is compatible with that will "work" -- for some value of "work" where if the codeset isn't Unicode in UTF-8 then you'll be sad.

Nothing keeps users / sites from declaring that thou shall use UTF-8 on the filesystem, or, even better, that thou shall use UTF-8 locales only, as the latter is the only simple way to [mostly] get the former.

> At least ZFS (which also incidentally is pretty much the only non-terrible filesystem) has an utf8only flag.

Note that ZFS can't tell if the strings from user-land are in UTF-8. ZFS can only tell if they're not valid UTF-8.