I had an issue where I was running two different instances of the same server as different users (one for prod, one for dev/test) and only whichever one of them started first could upload temp files. Upon inspection it always tries to write to the temp dir /tmp/plug-1697, no matter what, so we have two programs both trying to write to the same dir. They're run as different users, so whichever is started first starts the dir and owns it, and the other user can't write to it so the upload fails.
Looks like the temp dir name is generated by this code in plug/lib/plug/upload.ex:
On my Linux system, tmp_roots is ["/tmp"], which is fine, and :os.timestamp() is (currently) {1697, 828389, 981738}, which is less fine. The first number is megaseconds, so it will only tick over every 11.5 days.
Annoyingly, neither Erlang nor Elixir appear to provide a binding to the mkdtemp libc function, which is there for exactly this purpose. Making temp files/dirs safely has all sorts of horrible edge cases involved, at least on Unix. It looks like OpenBSD libc implements mkdtemp as just trying random numbers a bunch of times, and musl libc does the same with a worse RNG, so it's pretty cursed but I guess that's life. So getting the suffix from :rand.rand_uniform/1 or :crypto.rand_uniform/2 is probably as good as one can do.
icefoxen
commented
10 months ago
I had an issue where I was running two different instances of the same server as different users (one for prod, one for dev/test) and only whichever one of them started first could upload temp files. Upon inspection it always tries to write to the temp dir
/tmp/plug-1697, no matter what, so we have two programs both trying to write to the same dir. They're run as different users, so whichever is started first starts the dir and owns it, and the other user can't write to it so the upload fails.Looks like the temp dir name is generated by this code in
plug/lib/plug/upload.ex:On my Linux system,
tmp_rootsis["/tmp"], which is fine, and:os.timestamp()is (currently){1697, 828389, 981738}, which is less fine. The first number is megaseconds, so it will only tick over every 11.5 days.Annoyingly, neither Erlang nor Elixir appear to provide a binding to the
mkdtemplibc function, which is there for exactly this purpose. Making temp files/dirs safely has all sorts of horrible edge cases involved, at least on Unix. It looks like OpenBSD libc implementsmkdtempas just trying random numbers a bunch of times, and musl libc does the same with a worse RNG, so it's pretty cursed but I guess that's life. So getting the suffix from:rand.rand_uniform/1or:crypto.rand_uniform/2is probably as good as one can do.