Add documentation: Unix.EBADF with unison / unison-fsmonitor - change /proc/sys/fs/inotify/max_user_instances

[Alex-K37]

especially when running Unison, I quite often experienced the following error:

Fatal error: exception Unix.Unix_error(Unix.EBADF, "set_nonblock", "")

Running "strace unison-fsmonitor" yielded:

fcntl(0, F_GETFL)                       = 0x802 (flags O_RDWR|O_NONBLOCK)
fcntl(0, F_SETFL, O_RDWR|O_NONBLOCK)    = 0
fcntl(1, F_GETFL)                       = 0x802 (flags O_RDWR|O_NONBLOCK)
fcntl(1, F_SETFL, O_RDWR|O_NONBLOCK)    = 0
inotify_init()                          = -1 EMFILE (Too many open files)
fcntl(-1, F_GETFL)                      = -1 EBADF (Bad file descriptor)
write(2, "Fatal error: exception Unix.Unix"..., 71Fatal error: exception Unix.Unix_error(Unix.EBADF, "set_nonblock", "")) = 71
exit_group(2)                           = ?
+++ exited with 2 +++

I have fiddled around with 'ulimit -n', without lasting success.

The real solution was to change the following:

sysctl -w fs.inotify.max_user_instances=256 >> /etc/sysctl.d/unison.conf

The original OS vendor setting (OpenSuSE) was at 128. And with Nextcloud and another sync tool running besides Unison my system seemed to have to many inotify resources used.

I would appreciate if you could add a hint to your documentation somewhere, that inotify limits may have to be increased if the before mentioned error is encountered. The issue may be related to #8 .

[Alex-K37]

Posted this as bcpierce00/unison#1019, hopefully the correct place to do this.